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Tool use in crocodylians: crocodiles and alligators use sticks as lures to attract waterbirds

The views expressed are those of the author and are not necessarily those of Scientific American.


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Mugger crocodile (Crocodylus palustris) at Madras Crocodile Bank, Tamil Nadu, India, with sticks on its head. What's going on here? Read on. Photo by Vladinir Dinets, from Dinets et al. (2013). Used with permission.

In recent years it has – I really, really hope – become better known that non-bird reptiles (turtles, lizards, snakes, crocodiles, alligators and so on) are not boring dullards, but behaviourally complex creatures that get up to all sorts of interesting things. Play behaviour, complex social interactions, gaze recognition, pair-bonding and monogamy, social hunting, speedy learning abilities and good memories have all been demonstrated across these groups. And another interesting and unexpected bit of complex behaviour has just been published. It’s so interesting that I feel compelled to write about it today. It concerns what seems to be tool use in crocodiles and alligators.

As described by Dinets et al. (2013), Mugger crocodiles Crocodylus palustris in India and American alligators Alligator mississippiensis in the USA have both been observed to lie, partially submerged, beneath egret and heron colonies with sticks balanced across their snouts. Birds approach to collect the sticks for use in nest building and… well, let’s just say that it doesn’t end well for the birds. If the crocodylians really are using the sticks as bait to attract their bird prey, this is tool use, since the sticks are objects that are being employed for a specific function.

American alligator successfully catches Snowy egret (Egretta thula) following stick-displaying behaviour. Photo by Don Specht, from Dinets et al. (2013). Used with permission.

The occurrence of sticks on the crocodylians is not random: stick-displaying behaviour was most frequently observed both in those crocodylians living at rookeries and was exclusively observed during the egret and heron nesting season, being most frequent in late March and April (when the egrets and herons are working hard to find sticks) (Dinets et al. 2013).

The possibility that stick-displaying behaviour results from a random association between rookery-frequenting crocodylians and floating sticks was deemed unlikely by the authors, since floating sticks are extremely rare in the pools concerned, especially at the time of year concerned (partly this is because the local trees – baldcypresses and water tupelos – don’t shed twigs, but also because the nesting birds rapidly remove floating sticks for nest-building). Therefore, deliberate collection and employment of sticks by the crocodylians seems most likely (Dinets et al. 2013): it seems that they are practising baiting behaviour, whereby predators use objects in order to get potential prey to closely approach and hence become easier to catch. Even better, they are seemingly only practicing this baiting behaviour during a specific part of the year.

Green heron using bread as bait to attract fish. From Guido Trombetta's SeaWayBLOG.

Baiting behaviour is already well known for archosaurs. It’s frequently practised by Green herons Butorides virescens: they use feathers, twigs and even berries and bits of bread to attract fish (Norris 1975, Boswall 1983, Walsh et al. 1985, Robinson 1994) [adjacent photo from this article at SeaWayBLOG]. Burrowing owls Athene cunicularia use mammal dung to attract dung beetles (Levey et al. 2004) and gulls of at least two species have been seen using bait to attract finches fish (Henry & Aznar 2006). And it should be noted that this is not the first mention of what seems to be baiting in crocodylians, since Shumaker et al. (2011) anecdotally reported cases in which Saltwater crocodiles C. porosus seemingly used fish fragments to attract birds. What next? Needless to say, we need to be somewhat sceptical about the claimed presence of this behaviour: what’s needed now is for someone to catch it on film and hence demonstrate the behaviour in action. Of course, now that people know that it might occur, they’ll at least be on the lookout for it.

Crocodiles sometimes do strange things. This Orinoco croc (C. intermedius) is eating leaves. Photo by John Brueggen, from Brueggen (2002), taken at St. Augustine Alligator Farm Zoological Park.

As Dinets et al. (2013) note, the apparent presence of this behaviour in two extant crocodilian species raises the possibility that it’s more widespread within the group, and even – given its presence in both crocodylians and birds – that tool use involving bait was practised by extinct archosaurs. A number of surprising and unusual bits of behaviour have been documented in extant crocodylians in recent years (several of which have been covered at Tet Zoo), including fruit eating, leaf eating, adoption of babies, the possible feeding of babies, climbing, co-operative hunting, pair-bonding and monogamy, plus it’s long been known that they have a complex, sophisticated repertoire of vocal and postural communicative signals. These all show that crocodiles, alligators and gharials are complex, adaptable beasts that do many things that we might not consider likely had they not been documented. What’s next? Stay tuned…

Many thanks to Vladimir Dinets for his help. For previous Tet Zoo articles on behaviour in living crocodylians, see…

For for other Tet Zoo articles on crocodiles, see…

Refs – -

Boswall, J. 1983. Tool-using and related behavior in birds: more notes. Avicultural Magazine 89, 94-108.

Brueggen, J. 2002. Crocodilians: fact vs. fiction. In International Union of the Conservation of Nature, Crocodile Specialist Group 1, pp. 204-210.

Dinets, V., Brueggen, J. C. & Brueggen, J. D. 2013. Crocodilians use tools for hunting. Ethology Ecology & Evolution in press doi.org/10.1080/03949370.2013.858276

Henry, P.-Y. & Aznar, J.-C. 2006. Tool-use in Charadii: active bait-fishing by a Herring gull. Waterbirds 29, 233-234.

Levey, D. J., Duncan, R. S. & Levins, C. F. 2004. Use of dung as a tool by burrowing owls. Nature 431, 39.

Norris, D. 1975. Green Heron (Butorides virescens) uses feather lure for fishing. American Birds 29, 652-654.

Robinson, S. K. 1994. Use of bait and lures by Green-backed Herons in Amazonian Peru. Wilson Bulletin 106, 569-571.

Shumaker, R. W., Walkup, K. R. & Beck, B. B. 2011. Animal Tool Behavior: the Use and Manufacture of Tools by Animals. The Johns Hopkins University Press, Baltimore.

Walsh, J. F., Grunewald, J. & Grunewald, B. 1985. Green-backed Herons (Butorides striatus) possibly using a lure and using apparent bait. Journal of Ornithology 126, 439-442.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. AndreaCau 3:19 pm 11/30/2013

    It is always very nice to discover new evidence of complex behaviours among reptiles, which – from a palaeontological perspective – shows that it is not necessary to search good analogie for the extinct reptiles among extant mammals. Unfortunately, it is almost impossible for these behaviours to leave fossil evidence (pending new discoveries from some Chinese limestone…): thus it’s prefect for a new series of All Yesterdays artworks ;-)

    “the discovery of this behaviour in two extant crocodilian species raises the possibility that it’s more widespread within the group, and even – given its presence in both crocodylians and birds – that tool use involving bait was practised by extinct archosaurs.”

    The presence in both branches of Archosauria may induce to consider this as an archosaurian trait, although I consider this as an interesting behavioural parallelism, perhaps exapted from some neurological feature shared by all archosaurs. If such behaviour is absent in gavialoids, most neoavians and all non-neoavian birds, it remains more plausible that it evolved several times among archosaurs, and is not an archosaurian synapomorphy.

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  2. 2. naishd 3:39 pm 11/30/2013

    I agree, and didn’t mean to indicate that a behavioural trait like this should be regarded as anything like a ‘synapomorphy’: rather, the discovery seems to show that there’s the potential for tool use of this sort in fossil archosaurs that are similar, in cognitive abilities and behaviour, to the extant taxa that display them.

    As for All Yesterdays, there’s a tool-using non-avialan paravian in All Your Yesterdays. It’s shown engaging in a bit of behaviour so far unique to passerines (and hence likely not present in paravians of this sort). Nice image though.

    Darren

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  3. 3. anzha 4:19 pm 11/30/2013

    Don’t ruin it for me, Andrea!

    I was just mentally painting an image of allosaurs gathering sparse, low level green vegetation into a pile to lure “moderate” sized sauropods along the latter’s migration path during the dry season…

    Talk about an awesome All Yesterday’s picture!

    Will

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  4. 4. RaptorX 4:38 pm 11/30/2013

    “pair-bonding and monogamy”

    Really? I know about monogamous behaviors in skinks and a other lizards, but this is the first I have heard about it in crocodilians. Anybody know where I can read more about that?

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  5. 5. naishd 4:45 pm 11/30/2013

    RaptorX…

    Lance, S. L., Tuberville, T. D., Dueck, L., Holz-Schietinger, C., Trosclair, P. L. 3rd, Elsey, R. M. & Glenn, T. C. 2009. Multiyear multiple paternity and mate fidelity in the American alligator, Alligator mississippiensis. Molecular Ecology. 18, 4508-4520.

    There are numerous online articles discussing this paper (google ‘alligator monogamy’ or such). I think the pdf itself is findable online too..

    Darren

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  6. 6. vdinets 4:53 pm 11/30/2013

    Darren: Thanks for posting this. There are also two upcoming papers about really complex hunting techniques in crocs and gators, like different size classes playing different roles, drives into pre-set ambushes, collectively herding prey into tight bait balls, using legs for prey capture, etc. And now that more people are paying attention to croc behavior, we can expect even more exciting stuff.

    Raptor X: LANCE, S. L., TUBERVILLE, T. D., DUECK, L., HOLZ-SCHIETINGER, C., TROSCLAIR, P. L., ELSEY, R. M. and GLENN, T. C. (2009), Multiyear multiple paternity and mate fidelity in the American alligator, Alligator mississippiensis. Molecular Ecology, 18: 4508–4520. doi: 10.1111/j.1365-294X.2009.04373.x

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  7. 7. SciaticPain 5:29 pm 11/30/2013

    This raises a question that I have been pondering lately and which Cau and Naish touch upon a bit earlier. To what extent, if any, can phylogenetic bracketing be used to infer a behavior in extinct animals?

    For me it certainly seems to fall into the more speculative side of the coin- tool use does not fossilize (except for vain Krackens). But then you have something like nest care among dinosaurs. Correct me if I am overstating this, but nest care among dinosaurs has been increasingly viewed as ranging from the high reptilian to low avian in degree. Is that not an emerging, dare we say consensus view, on dino reproductive behavior that relies heavily on phylogenetic bracketing to infer a behavior? Or maybe nesting, because we have some limited data, is not the best analogy… Maybe I am looking at the question the wrong way- maybe it is more a question of degree of certainty? I don’t know… I would love to hear others thoughts on this.

    Duane
    antediluvian salad

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  8. 8. AndreaCau 5:42 pm 11/30/2013

    @Darren: In fact, I noted you used “raises the possibility” instead of a stronger sentence.
    @Anza: I’m a very bad palaeontologist.
    @Duane: The Extant Phylogenetic Bracketing is not just inferring something based on its closest relatives, it’s a protocol for evaluating the type of inference based on both fossil correlates and closest relatives comparison. In absence of something among the fossil traits that may be a correlate of the behaviour, and with such a patchy distribution of the behaviour among extant archosaurs, any inference on fossil taxa is relatively weak and speculative.

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  9. 9. Cameron McCormick 5:55 pm 11/30/2013

    Krackens

    If I may try my hand at Marjanović-ing, the plural indefinite is ‘Kraker’. ‘Kraken’ is the singular definite (“the Kraken”) and ‘Krake’ is the singular indefinite (“a Kraken”). I think the only English-speaker to get this right was Charles Paxton.

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  10. 10. vdinets 5:57 pm 11/30/2013

    AndreaCau: agreed, but it’s so much fun to use in non-scientific publications ;-) Something like “1/14 ofextant crocodilian species and 1/200 of extant bird species are already known to use tools; there were thousands of non-avian dinosaur species; you do the math.”

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  11. 11. SciaticPain 5:57 pm 11/30/2013

    BTW that mugger in the first pic looks especially crafty balancing 4-5(?) sticks on its noggin!!

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  12. 12. Heteromeles 7:13 pm 11/30/2013

    Actually, if anything, the mugger balancing the sticks on its nose is more tricky than it looks. After all, think about balancing three sticks on your nose without using your hands (or think of a dog doing the same thing). How did the mugger align all those sticks so they would stay on its nose as it raised its head out of the water?

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  13. 13. adambritton 7:56 pm 11/30/2013

    This is really very cool Vladimir. I’ve seen saltwater crocs with things on their heads on a regular basis, usually aquatic vegetation. In that case it helps to disguise the shape of their head, which would also be great for convincing waterbirds to ignore you. To us it looks more like a fashion statement! The question for me has always been whether it was an intentional behaviour or not (a disguise, not being a fashionista) . I’ll be keen to read the paper, it’s a compelling theory.

    There’s much more going on with crocs than most have realised, nice to see this starting to get out there.

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  14. 14. Tayo Bethel 8:13 pm 11/30/2013

    Just finished reading a discussion of alligator monogamy, With over 70 percentof the females in a very dense population being monogamous–I wonder if there was a similar result for the males in the same population. Monogamy in crocodilians raises an interesting question, though. The vast majority of bird species are at least seasonally monogamous–life partnerships, as stressed everywhere, are rather uncommon. The question,then, is did monogamy evolve in the avian crown group or, as might be speculated from the behavior of the apparently monogamous alligators, was it a feature common to at least crown group archosaurs?

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  15. 15. JoseD 2:07 am 12/1/2013

    @Naishd

    “American alligator successfully catches Snowy egret”

    Is that just 1 photo or part of a series of photos showing the attack? I ask b/c the latter would be really cool to see.

    BTW, I’m surprised no one’s mentioned “Raptor Red“. That’s where I 1st read about pair-bonding & monogamy in crocs (“In some croc species mother and father join together in helping their young survive the first few dangerous days of life outside the egg”). In any case, the discovery of tool-using crocs may be the most interesting news I’ve heard in a while (& yes, that includes the discovery of new Deinocheirus material). Among other things, it opens up the possibility of tool-use in non-maniraptoran dinos. I already figured that at least some non-avian maniraptorans practiced some kind of tool use (hence my “Anting Alvarezsaurid”: here). Maybe someone w/actual artistic talent (I.e. Not me) should draw a spinosaur practicing baiting behavior. Now that I think of it, maybe someone should also draw a deinonychosaur using rocks to break open large eggs.

    @Vdinets

    “There are also two upcoming papers about really complex hunting techniques in crocs and gators,”

    By who (just so I know what to look for when they come out)?

    “1/14 ofextant crocodilian species…are already known to use tools;”

    If there are 24 extant croc species, doesn’t that mean that 1/12 of them are known to use tools? Just making sure.

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  16. 16. BrianL 4:04 am 12/1/2013

    This is interesting stuff! Now I wonder, has anyone ever tested how crocodilians fare in Skinner boxes or even tried the mirror test on them? I wouldn’t expect them to pass the latter test, but given how we tend to underestimate them and how patient and observant crocodilians must be to hunt the way they do when they’re reasonably intelligent…

    Only barely related as it only concerns tool use of a sort: Despite prehistoric humans and hominines generally being portrayed as fairly shorthaired, shouldn’t we expect them to have sported big masses of head hair and facial hair in males? Certainly modern humans tend to grow just that when their hair isn’t cut or shaven. I suppose we don’t have any clue whatsoever when humans started growing sparsely distributed but very long hair compared to chimpanzees or when we started grooming our hair?

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  17. 17. stargene 4:07 am 12/1/2013

    Pretty amazing crocogators. I am now quite prepared to find that Horner, Bakker or Currie uncover fossilized
    cell phones and pampers in dino rookeries. Along with
    big bifocals.

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  18. 18. AndreaCau 7:40 am 12/1/2013

    Hundreds of species of wasps produce clay pots. Homo produces clay pots. So, the production of clay pots is present in hundreds of metazoans. Thus, the Kraken produced clay pots. ;-)

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  19. 19. vdinets 8:47 am 12/1/2013

    JoseD: unfortunately, there is only one photo. There are now 28 crocodilian spp. (C. niloticus has been split it two, O. tetraspis in three, and M. cataphractus in two). It is possible that C. novaeguineae will also be split eventually, and there might be a crypto croc in the Solomons, so the final number might be 30.

    BrianL: not to my knowledge, but it’s a neat idea.

    AndreaCau: well, technically speaking, at least 80% of birds use tools, since building nests of sticks is tool use (it usually isn’t considered as such, but it should be). One might argue that some passerine nests are the most complex tools ever made by non-human animals, although some insect nests are also extremely complex.

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  20. 20. JoseD 11:39 am 12/1/2013

    @Vdinets

    My bad. I forgot about O.tetraspis & M.cataphractus being split.

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  21. 21. llewelly 12:32 pm 12/1/2013

    Andrea Cau: ” Unfortunately, it is almost impossible for these behaviours to leave fossil evidence ”

    Almost.

    If memory serves, some corvids engage in tool use that modifies the bill.

    Human use of footwear affects the bones of the feet.

    And so on.

    To me the larger problem would seem to be distinguishing skeletal modification due to tool use from the bewildering variety of other ways (other behaviours, environmantal factors, predation, taphonomy …) the skeleton can be modified.

    Andrea Cau: “The presence in both branches of Archosauria may induce to consider this as an archosaurian trait, although I consider this as an interesting behavioural parallelism, perhaps exapted from some neurological feature shared by all archosaurs. ”

    If the presence of tool using behaviours in multiple branches of Archosauria is due in part to an underlying (neurological, presumably?) synapomorphy, the fact that only a few archosaurs exhibit tool use seems to indicate that the potential of said synapomorphy to evolve into tool use is probably quite modest.

    Nonetheless … the potential of said hypothetical synapomorphy to evolve into tool use could be high enough to justify speculating that some few fossil archosaurs exhibited tool use.

    vdinets: “Something like “1/14 ofextant crocodilian species and 1/200 of extant bird species are already known to use tools; there were thousands of non-avian dinosaur species; you do the math.” “

    My thinking as well … but in addition to presuming said hypothetical synapomorphy, it also presumes the distribution of tool use among crocodilians and birds is representative of fossil archosaurs. As I’m sure you’re aware.

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  22. 22. vdinets 11:54 pm 12/1/2013

    JoseD: one upcoming paper is by Britton et al., the other by yours truly. But neither is accepted yet, so it could be a while. The tree-climbing paper should be out in a couple weeks; just remind me after Dec 15 and I’ll send you a PDF. My email is dinets at gmail.

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  23. 23. Dartian 6:19 am 12/2/2013

    Darren:
    floating sticks are extremely rare in the pools concerned, especially at the time of year concerned (partly this is because the local trees – baldcypresses and water tupelos – don’t shed twigs, but also because the nesting birds rapidly remove floating sticks for nest-building). Therefore, deliberate collection and employment of sticks by the crocodylians seems most likely

    If the sticks are “extremely rare”, where do the crocodiles and the alligators get them from, then? And has it been actually observed what method they use to transport the sticks to the ambush sites without the birds noticing?

    gulls of at least two species have been seen using bait to attract finches

    Finches? Dude, I humbly suggest that you should perhaps look up that paper again… ;)

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  24. 24. naishd 6:23 am 12/2/2013

    Dartian (comment 23): I don’t think it’s known where they get the sticks from, since this hasn’t been observed. Maybe they carry them in from a short distance away.

    Finches: my recollection is that the gulls were catching goldfinches, but I will go check! Or did you just assume that it was a typo for fishes? :)

    Darren

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  25. 25. Jerzy v. 3.0. 6:29 am 12/2/2013

    Great paper, remembers me some cartoons where a cat balances a toast on its nose to catch birds.

    How crocodiles get sticks in the first place (given that sticks around the nesting colonies are, obviously, picked by birds)?

    Interesting simple experiment would be to throw sticks near crocodiles and see what they do.

    About tool use in general: very many birds and mammals sporadically use tools on ad hoc basis. European bluetit and blackbird were also observed once or a few times (out of tens of thousands of hours spent watching these common species). So I don’t think that occasional tool use is somehow special or requiring particular mental skills, as early psychologists and ethologists tended to believe.

    There are also several insects using tools on a regular basis (I remember some wasp using stones to pound sand after closing a nesting burrow, and some bug larva using dead termites as a bait for live termites).

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  26. 26. Dartian 7:29 am 12/2/2013

    Darren (still from the OP; emphasis mine):
    Mugger crocodiles Crocodylus palustris in India and American alligators Alligator mississippiensis in the USA have both been observed to lie, partially submerged, beneath egret and heron colonies with sticks balanced across their snouts.

    How does that hunting tactic work, anyway? Don’t these keen-eyed (and quickly-reacting) birds see the crocodylians? That mugger in the topmost picture, for example, is quite conspicuous at least to a human observer.

    I don’t think it’s known where they get the sticks from, since this hasn’t been observed.

    So in other words, there is no evidence that the crocodylians actually manipulate the tools (by transporting them)?

    my recollection is that the gulls were catching goldfinches

    One gull caught goldfish, the other caught tilapia. No finches involved, alas.

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  27. 27. naishd 7:39 am 12/2/2013

    Wow, goldfish into finches, I am a miracle worker… (cringe).

    As for the hunting technique: birds are dumb and maybe too desperate for sticks.

    Darren

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  28. 28. bigteethbigbrain 10:15 am 12/2/2013

    For many years I fostered a Caiman (Paleosuchus palpebrosus) – before I tell you my observation of him (Buford was his name) I want to say I took this responsibility very seriously.

    He was a mascot in residence at my firm and we would feed him live rodents. Feeding always was entertaining because Buford could sit motionless for days in his custom built terrarium. We would sometimes feed him in front of guests. Anyway – the point of my story: One day I was standing in front of the glass terrarium and reached into my pocket to give someone some cash to go get me some lunch and “POW!” Buford explodes into an attack!.

    I took a couple of weeks to figure out that the sound of crunching paper excited him – because that was the same sound that his rodent lunch was delivered with – the pet shop always put the rats in a plain brown paper sandwich bag.

    From that time on I had a new respect for Buford – I never thought that a reptile could learn anything.

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  29. 29. llewelly 10:58 am 12/2/2013

    Dartian: “If the sticks are “extremely rare”, where do the crocodiles and the alligators get them from, then?”

    Isn’t it obvious? They climb up into the trees for the sticks ?

    ( hehehe )

    Actually, that may be the biggest weakness in the case for tool use; as far as I can tell, the crocs were not specifically observed obtaining and transporting the sticks.

    Dartian: ” That mugger in the topmost picture, for example, is quite conspicuous at least to a human observer.”

    But there’s also a selection bias operating; the photographer wants a “good” picture of a croc, and thus seeks out an opportunity which shows the croc readily. Maybe said selection bias caused the photographer to select a view which shows a croc that failed to attract a bird. ; )

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  30. 30. pimpmykidney 11:24 am 12/2/2013

    Isn’t it a tad of a stretch to call that tool use? At least in that (in the article) there seems to be no evidence of any kind of ellaboration of the raw materials; birds choosing specific materials to nest would also qualify as bird nesting under the same criterion that seems to be applied here

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  31. 31. pimpmykidney 11:25 am 12/2/2013

    * Correction: I meant, birds using specific materials to help in building nests

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  32. 32. ekocak 11:50 am 12/2/2013

    I haven’t worked much with crocodilians, as the commenter bogteethbigbrain has, but I’ve worked with many many reptiles over the years, and I’m pretty confident in saying I’ve had similar experiences that are more than just pavlovian sorts of feeding responses. Reptile cognition is fascinating stuff, and I think it’s been an incorrect assumption that they’re mindless little surviving machines.

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  33. 33. tuned 3:06 pm 12/2/2013

    Great.
    Soon it will be the hacksaw.
    Be afraid , be VERY afraid!

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  34. 34. Maher 3:15 pm 12/2/2013

    We still have our opposable thumbs.

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  35. 35. David Marjanović 4:17 pm 12/2/2013

    Isn’t it a tad of a stretch to call that tool use? At least in that (in the article) there seems to be no evidence of any kind of ellaboration of the raw materials;

    That would be making tools as opposed to just using them.

    When it dawned on the anthropologists that tool use isn’t limited to humans, they discovered the distinction between using naturally occurring things as tools and making tools, and declared it Very Important Indeed. Later it turned out that toolmaking isn’t limited to humans either, but the distinction has remained; using tools does seem to be more widespread than making them, at least so far.

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  36. 36. vdinets 4:32 pm 12/2/2013

    Dartian: the only way I’ve seen them get those sticks on their snouts was by submerging nearby and then surfacing from directly underneath the sticks. It does take some balancing act to keep the sticks from falling off. But at the height of the nest-building season it’s very difficult to find any sticks just floating around, so they might use some other ways of procuring them that I haven’t observed. The paper is now generating a lot of interest, so hopefully people will now pay special attention to this, and more observations will trickle in.

    This is the problem with studying croc behavior: many things happen so rarely that they can only be observed by chance. For my last few papers, I actually had to solicit observations through various croc-themed groups on Facebook, because a lot of people have singular records that only make sense if combined.

    That mugger in the photo actually looked a lot like a log. As for birds’ awareness of the crocs, there is no simple answer. Young and wintering birds often seem totally unaware of the danger. Resident adults certainly are aware, but still make mistakes. The only birds that seem to be 100% effective at escaping croc/gator/caiman predation are green and striated herons… but perhaps I just never happened to see them being caught.

    Pimpmykidney: there’s a lot of discussion on what can and cannot be called “tool use”. Under currently prevailing use, luring birds with sticks qualifies as “tool use”, but not as “tool making”. If someone observed the crocs breaking twigs off trees, that would be “tool making”.

    I recently published a book Dragon Songs about my research on croc behavior, it has a lot of stuff on all this. Check it out on Amazon (I hope Darren doesn’t mind me promoting it here).

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  37. 37. naishd 4:40 pm 12/2/2013

    Vlad — I certainly don’t mind you promoting the book (Dragon Songs), since any book on crocodylians is worthy of promoting in my view. Indeed, I hope to review the book here in time.

    Darren

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  38. 38. Heteromeles 5:20 pm 12/2/2013

    As a side note, it’s worth remembering that definitions of “tool use” are still regrettably subjective. As pointed out in Bagemihl’s Natural Exuberance, if we include sex toys (primarily mastubatory aids) as the tools they are, and counted the inventoried the species that use sex toys, the number of tool using species would easily double.

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  39. 39. Yodelling Cyclist 6:39 pm 12/2/2013

    If we’re broadening tool use this far, caddis fly larvae and hermit crabs have be considered. Then maybe frogs which intertwine their egg strands with plants, thereby using the plants as an anchor. Gibbons will pull fruit towards themselves, modifying their surroundings. Tool use, or perhaps more broadly, manipulation or employment of the surroundings for one’s own purposes, has to be recognised as a continuous spectrum of abilities. Another issue is individual intelligence. Is the stick wearing crocodile that species’ equivalent of Einstein? Is bending a stick the crow equivalent of tensor calculus, or is it the equivalent of tying one’s shoes?

    At some point, some artificial categories may have to be imposed to guide discussion. A class A tool user might manipulate and alter a materials at a chemical level or fashion composites (so fire, smelting, tiring stone to sticks: humans), Class B might fashion or alter crude materials to better serve a mechanical purpose ( crows, chimps), Class C might passively employ unaltered materials (baiting, using shelters, so crocs, octopus). Notably B and C are really different aspects of intelligence. A and B are engineering and understanding of the physical world whilst C is more the understanding of another animals thought process. Bird nesting is still hard to categorise, and should any serious zoologist so consider this rough scheme to be of merit, I would say it requires refinement.

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  40. 40. Yodelling Cyclist 6:47 pm 12/2/2013

    I would also suggest that the plasticity of the behaviour has to be considered. Bird’s nests, although constructed with skill, are fairly uniform within species. Weaver birds ( to my limited knowledge) never say ‘sod this’ and try to bully swifts out of their nooks, for example. Adapting behaviour to the environment should be incorporated into a discussion. This would helpfully make Caddis flies morons once again.

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  41. 41. vdinets 8:23 pm 12/2/2013

    Yodeling Cyclist In Moscow, Russia, where I grew up, crow nests were built almost entirely of cable, wire and cardboard. When modern items of feminine hygiene became widely available in the early 90s, crows immediately learned to tear them apart and use dry parts.

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  42. 42. adambritton 10:22 am 12/3/2013

    Here is unequivocal evidence of tool use in a crocodile. Quite advanced, even.

    http://i.imgur.com/NpJovXA.jpg

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  43. 43. Heteromeles 1:28 pm 12/3/2013

    There’s also a blog posting about birds using cigarette butts in nests to keep down pests.

    Still, you don’t see rock doves making swallow-type nests or taking over hawk nests, so I suspect there’s a mental map involved in nest creation, much as spiders tend to weave specific webs.

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  44. 44. Yodelling Cyclist 2:11 pm 12/3/2013

    vdinets: Yes, although I was unaware of that example, this is the sort of complication I was concerned about. Heteromeles, that was the sort of thing I was striving at.

    My thought was this: are the animals making nests because that’s part of their mental wiring – instinctively making nests as part of the breeding cycle – or is it objective based (I need a spot to raise the young away from predators therefore – I know! – a nest). Repeated construction of the same nest style, but with varying material argues for the former with some modification, but if the nest design were to significantly vary, or the birds noticeably respond to different environments, then we have something a bit different.

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  45. 45. plswinford 3:13 pm 12/3/2013

    A new proverb: “Beware crocodiles bearing sticks!”

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  46. 46. SamsaPDX 5:04 pm 12/3/2013

    Some scientists maintain that the primordial ancestor of the modern journalist may have employed editors, or even possessed the ability to spell correctly for themselves, though obviously this trait did not survive in modern journylians, which favor sylly spellings like “crocodylian.”

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  47. 47. naishd 5:09 pm 12/3/2013

    Oh dear, some people don’t know much about crocodylomorph nomenclature, do they? I will direct the honourable commenter to the Tet Zoo article here (in short, the spelling ‘crocodylian’ is specifically used for the crown-group of Crocodylomorpha, the group traditionally termed ‘Crocodilia’. In other words, ‘crocodylian’ has a precise zoological meaning).

    Darren

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  48. 48. SamsaPDX 1:39 am 12/4/2013

    Well, that is certainly enlightening if a bit confusing. Thank you, Darren. I stand corrycted.

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  49. 49. Yodelling Cyclist 6:53 am 12/4/2013

    SamsaPDX: you’re new here, right? ;-)

    Don’t worry, those of us in the cheap seats get very well acquainted with googling the technical terminology as we go. Very educational. I made the mistake of trying to correct Darren in my first comment. Turned out I was talking out of my Jehol.

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  50. 50. Jerzy v. 3.0. 7:25 am 12/4/2013

    I think a good viral video (or BBC documentary) could be made by throwing some sticks in alligator-infested heron nesting colony. Would alligators actually collect sticks and use them as a bait?

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  51. 51. ekocak 9:36 am 12/4/2013

    @YodelingCyclist
    When I first came to TetZoo (around 2006 I think), I had no idea what a clade was. Good times.

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  52. 52. Heteromeles 9:37 am 12/4/2013

    @Jerzy: Why bother with the bird colony? You could do the study pretty well using sticks at an alligator display in a zoo, and preferentially feeding the alligator that balanced the most sticks on its nose. Then you could study alligator learning in a semi-controlled environment too, by watching how the practice spread, who the alligator learned to manipulate the sticks, and which animals picked it up most quickly.

    Do zoo alligators get bored? This might give them something to do.

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  53. 53. TeaLovingDave 9:43 am 12/4/2013

    vdinets: I have been unable to find any information on a split of M.cataphractus – is this a cutting-edge development?

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  54. 54. Jerzy v. 3.0. 10:03 am 12/4/2013

    @Heteromeles
    I was hinting that it is unknown if crocodilians consciously select sticks. This may be just a side effect of crocodilians waiting under bird nests, and not bothering to shake off falling sticks.

    But yes, zoo alligators get bored. At least zoo people believe so and use environmental enrichment for reptiles. Normally it has to do with making a bit more difficult for them to get their meal.

    On a side, playing with sticks is a form of play of cormorants and darters. I saw it once myself, pretty amusing.

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  55. 55. Heteromeles 2:48 pm 12/4/2013

    @Jerzy, That’s one heck of an accident, if those sticks on the mugger pictured fell exactly that way and just so happened to balance themselves on its nose. Admittedly, I don’t know how the croc would go about balancing those sticks on its own nose, but the structure looks intentional to me.

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  56. 56. vdinets 3:53 pm 12/4/2013

    TeaLovingDave: yes, the paper has just been accepted. Interestingly, there might be some behavioral differences: one sp. climbs trees a lot, the other one doesn’t (that’s in another paper that also has just been accepted).

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  57. 57. metingun2 7:38 pm 12/4/2013

    Well ; the behavior is actually `better camouflage trick to `reinforce ` appearance of a trunk of tree with the `small branches` protruding from / to finish off the body color and shape of crocodile -look like a floating tree stump – in the water , rather than a tool to lure the nesting birds .
    Fish and frog and other water living creatures are frequently hunted by the birds by landing on the floating tree trunks , and the birds are landing on the crocodiles to pick up a tree branch for `nesting purpose` is little bit a stretch and unrealistic assumption and the birds would always prefer to pick the available ` dry `and` lighter branches` and `various sizes` to their specifications at the shores (land) instead of wet ones from the water try to rip them apart from the stem of a tree ( Crocodile carrying stick /look alike a floating stem ) ..
    The bottom line is : the behavior is more like a `CAMOUFLAGE` to `look alike` as floating tree for landing on and to feed from the water – for birds – rather than a `TOOL/BAIT` for the birds to pick up the floating wet stick from the water for nesting purposes … This assumption rather `underestimates `the intelligence of birds when they are `nesting` they will always prefer land to pick up a dry and lighter branch rather than water .

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  58. 58. metingun2 4:47 am 12/5/2013

    I must also add that the preferential aiming to the branches to grab for landing purpose rather than the stem(crocodile`s back) is `natural and usual behavior` for the birds because ; A-It is physically better and secure to grab when landing rather than skidding at the back B- The Branch is located at the higher level from the water surface than the back of crocodile C – Branch location of a floating stem gives better wider visualization of the water surface for hunting from the water to a landing bird rather than the back location of the camouflaged crocodile ( large stem of floating tree look alike ).

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  59. 59. Jerzy v. 3.0. 9:06 am 12/5/2013

    @Heteromeles (@55)
    In most animals it is obvious that tool manipulation is intentional. But crocodilians often resurface with water plants and another rubbish stuck on their backs. It is not certain at all that this is intentional.

    Sometimes a heap of plants makes a caiman more conspicous, other times it walks on land with plants still on the back etc.

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  60. 60. vdinets 12:42 pm 12/5/2013

    metingun2: these issues are addressed in the paper, have a look: http://dinets.info/DinetsBrueggen&Brueggen.pdf

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  61. 61. Heteromeles 2:15 pm 12/5/2013

    @59: Jerzy, I’d agree in the case of the photo, if two of the three twigs weren’t balanced on the one below them. That could be chance, but in this particular case, it looks intentional to me.

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  62. 62. Zee Zee 3:55 am 12/6/2013

    As an artist I know nothing about Archosaurians, Crocydilians, etc, however I read the article and comments with great interest and it occurred to me that if one of these creatures were rootling about on the bottom it might raise a few sticks which might land on a snout and said creature might surface with these sticks still there, and if birds are as lazy as most humans…..well photo number 2 tells us the ending.

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  63. 63. DGudd4523 9:08 am 12/6/2013

    The inference that can be drawn from this paper is limited. There were only 11 sightings total over a year and yet the behavior is considered tool use and presumably adaptive? If sticks are really limited then a simple experimental addition of sticks could have shown if stick limitation caused the miniscule amount of sightings (< 1 / month). The authors didn't even quantify vegetation.

    Given the effective population size of crocodiles and alligators, there would have to be a very large selection coefficient for this to evolve in the population. An extra meal or two does not constitute strong selection and according to basic population genetics this trait could not evolve given the ecological constraints.

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  64. 64. ekocak 1:31 pm 12/6/2013

    Just saw this study got a mention on CNN.com:

    http://www.cnn.com/2013/12/05/us/crocodile-lures/index.html?hpt=hp_t3

    Although honestly the best part is the comments degrading instantly into a hilarious political fight.

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  65. 65. vdinets 9:56 am 12/7/2013

    DGudd4523: N=11 only for Louisiana part of the study. In florida there were hundreds of sightings over the years. As for Louisiana, gators there are much less numerous and much more shy. Of the 11 sightings, 8 were made near colonies during the nest-building season, and they represented about 15% of all alligator sightings at these locations at that time.

    As for the evolution of this behavior, note that the numbers of wading birds in the sotheastern US today are still only a fraction of what they before the plume-hunting era. Also, gators don’t eat as much as mammals, and in winter they don’t eat much at all, so an egret in early spring is a really good reward. There are plenty of rare predatory behaviors: brown bears have been observed hunting sea otters by swimming up to them just three times, AFAIK. If you can’t explain how these behaviors evolved, it doesn’t mean they don’t exist.

    Also, you assume that this is a genetically encoded behavior, but it doesn’t have to be. It could be easily transmitted by learning, and invented repeatedly following some lucky incidents. If you think alligators are not sufficiently smart for this, I’d like to see it experimentally proven :-)

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  66. 66. DGudd4523 4:22 pm 12/8/2013

    Any low effective population size will reduce the maintenance of either genetic or culturally transmitted traits. Even if wading birds had higher population sizes before, they are still limited as they are almost at the top of the food chain compared to other birds.

    Nonetheless, “a really good reward” is insufficient and unsatisfactory in terms of quantifying the benefit. Given the dramatic decrease in population size of wading birds, crocodylians must have compensated by shifting dietary preferences and losing a meal represents a meager fitness loss.

    There is a very large difference between maintaining tool use and opportunistic hunting. To suggest the two are comparable is unfounded. Tool use requires cognitive processing of both the environment and prey. A predator trying to capture a non-typical prey item requires either misidentification or starvation, or some combination.

    Even if the behavior is cultural, your own arguments in the article suggest it could not be reinvented. If sticks were rare it would be even more unlikely to be reinvented and given the low effective population size in Louisiana, would still require massive fitness benefits to maintain it. It could be transmitted by learning, please provide evidence that alligators attend to this behavior.

    I didn’t say alligators weren’t sufficiently smart, I said the data was insufficient to claim tool use. Since I am not the one making a novel claim, I do not need to provide evidence. Despite the fact I do not need to provide evidence to say the claim is insufficient given the lack of vegetation quantification or experimentation, I still argued that the behavior could not be maintained given basic population genetics theory. I’ll await your journal article that overturns five decades of theory.

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  67. 67. vdinets 6:12 pm 12/10/2013

    DGudd4523: What does effective population size have to do with this? The entire Red Wolf population comes from a handful of captive-bred individuals, still they are doing fairly well in the wild and are totally capable of hunting deer; no loss of any particular hunting behavior has been documented.

    Alligators in Louisiana are observed floating with sticks significantly more often around egret rookeries, and significantly more often at the time of egret nest building. I think it demonstrates that they float with sticks in order to attract and catch birds; there are observations of them catching birds in this way. If you have an alternative explanation, I’d really like to hear it, and see some supporting evidence. Do you really think that a stick on the nose of a 9-foot alligator makes it better camouflaged?

    I don’t understand what you mean by “documenting the vegetation”. The vegetation is Taxodium and Nyssa; none of them regularly sheds twigs at any time of year.

    I am not claiming that this behavior is either cultural, genetically inherited, or intermediate. I simply don’t know. If you’d like to find out, a simple study of captive-raised gators can answer the question. I don’t have the opportunity to run such a study at the moment, but you are welcome to.

    If you think this behavior doesn’t fit some theory (I personally don’t see any contradiction, but you say you do, that’s fine), you can look for possible explanations or a better theory, but claiming that it doesn’t exist because it doesn’t fit a theory is outside the scientific method.

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  68. 68. AMorris 8:39 am 12/11/2013

    Maybe I’m wrong but I seem to remember that egrets nest over the water. I presume the previous year’s nests fell apart and many of the branches and twigs probably fell from the trees and landed in water thereabouts. Many of these would likely be adequate nesting material and egrets would likely gather them also even if wet as wet sticks are more malleable and can be deformed enough to wedge into place. Wet sticks would not line the nest but frame it. Not that I know much about these nests but I presume egrets line their nests with soft and dry material like grasses or leaves or even feathers. Most of the sites I read trying to ascertain the lining materials said nothing about them but a few referenced “small pieces of plant material” in one way or another. There wouldn’t have to be a huge amount of useable material falling into the water every year, just enough for some of the gators to use. Rejected sticks might make it into the water as well and any stick might be looked at during this competitive stick-hunting part of the year by egrets looking to build a nest.

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  69. 69. vdinets 7:52 am 12/12/2013

    AMorris: nests begin to fall apart when the chicks are about to fledge. I don’t know what the typical situation is, but in the year when I did the observations there was a hurricane in the fall, so there were no old nests left by the time the new nest-building season began.

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  70. 70. AMorris 6:33 pm 12/12/2013

    Ah thanks vdinets but I imagine a great many of these sticks would still be around perhaps even sunk below the surface of the water. I’m just saying that the gators probably don’t have to go far for them and probably don’t need too many of them either. Likely most gators do not use the stick method and pushy ones could probably defend the ones they find. I actually think this is a fascinating idea.

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  71. 71. Jerzy v. 3.0. 8:22 am 12/13/2013

    I think if this is consious tool use, then only few individual crocodylians learned it. Most alligators live away from bird colonies.

    Anyway, it would be easy and interesting to provide sticks in such places, and see what crocodilians do.

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  72. 72. vdinets 11:38 pm 12/13/2013

    AMorris and Jerzy: There are lots of good questions for follow-up studies, and I hope some students will get interested in the subject. Actually, I know one who did :-) I am not planning any croc research at the moment myself (unless I suddenly get one of the grants I’ve applied for), but I expect a lot of interesting stuff to be discovered in the next few years – there is now a whole bunch of very observant people studying croc behavior. Stay tuned…

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