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Petrels: some form-function ‘rules’, and pattern and pigmentation (petrels part III)

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

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Time for more petrels. This article is another introduction, this time to generalities of behaviour and form-function. The previous petrel articles are here and here, and see the list of links below as well.

Several distinct foraging styles are employed by petrels and they’re more diverse in feeding behaviour than most accounts imply. The majority of species are flexible feeders, their diet mostly being determined by the abundance and availability of fish, crustacean and cephalopod prey species.

Aquaflying shearwaters in pursuit of fish. A still from Blue Planet, BBC (c).

Many feed by alighting on the surface to feed on plankton, fish, squid or floating carrion. Surface seizing of prey is fairly common (where the birds reach down to grab prey from the sea surface while in flight), but some species practise aerial pursuit of other seabirds (that is, they’re aerial pirates or kleptoparasites: more on this in a later article). Others dive into the water in pursuit of prey, practising either short, surface dives from a few metres up (a technique termed surface plunging), or longer, more extensive dives in which they pursue prey well beneath the surface (a technique termed pursuit diving). Indeed, some species (some shearwaters and possibly some others) even ‘fly’ down in the water to depths of 10 or even 20 metres and are capable aquaflyers (Habib 2010). I’ll be coming back to aquaflying later on as well. It’s a fairly well known bit of behaviour nowadays, having famously been featured in the 2001 BBC series Blue Planet.

Prions move in a peculiar way across the sea surface, paddling with their feet, holding their bodies lightly against the water, their wings out to the side, and their heads disappearing and reappearing into the water as they scoop up plankton with the bill (Nelson 1980). This form of feeding has been termed hydroplaning.

Ashmole's (1971) classic diagram of seabird feeding techniques (illustrator: Jon Ahlquist). Note the hydroplaning prion and 'pursuit plunging' shearwater. Today we know that a few shearwaters should be included within the 'pursuit diving with wings' (= aquaflying) category.

In one of the neatest papers ever published in ornithology (that’s just my opinion of course), Spear & Ainley (1998) showed how the morphological characters of petrels correlated with their diet and foraging style. Tropical species, they showed, tended to have longer and deeper bills, longer tails, and longer wings with greater surface areas than polar species (Spear & Ainley 1998). Presumably these differences have arisen because tropical species have to cover much larger expanses of ocean in order to find food; furthermore, tropical prey items (like flying fish and flying squid) are typically fast-moving, agile and capable of flight, whereas polar prey are less mobile and tend to be concentrated near the water surface. The presence of longer bills in tropical species might relate to thermoregulatory constraints (it’s better to have small appendages in cold climates).

A polar specialist: the all-white Snow petrel (Pagodroma nivea). Photo by Samuel Blanc, from wikipedia.

Polar species tend to have a relatively low overlap in morphological characters compared to their tropical relatives, though it’s not immediately clear why this is so given that more resources are available to polar seabirds than to tropical ones. [Adjacent photo of Snow petrel Pagodroma nivea by Samuel Blanc ©]  Possibly, polar petrels have evolved alongside a greater diversity of other seabird groups than have tropical ones, and as a consequence the number of niches available to polar petrels may have been relatively low (Spear & Ainley 1998). On the other hand, polar environments may provide seabirds with more niches, and hence more opportunities for specialisation. In contrast, tropical regions are comparatively sparse over large areas: consequently, seabirds that evolve here may have to be generalists.

Mostly blacks, whites and greys, but browns as well: patterns and pigmentation

Tahiti petrel (Pseudobulweria rostrata), image by Aviceda, from wikipedia.

Petrels are mostly patterned in greys, white and blacks. White undersides and dark dorsal surfaces are common and dark wing-tip markings, pale rump bands and facial masks and caps are seen in various species [Tahiti petrel Pseudobulweria rostrata shown here; image by Aviceda]. Some species – gadfly-petrels and prions in particular – have complex pigmentation pattern. The Snow petrel has entirely white plumage [see image above].

Bretagnolle's (1993) diagram of plumage variation seen within tubenoses. This paper is a must-read if you're properly interested in seabird colouration and its adaptive significance.

It may be somewhat surprising to those who know little about seabirds and expect all species to be patterned in grey, white and black to learn that quite a few petrels are solidly black or dark brown. There are black-brown species of gadfly-petrels, shearwaters and Procellaria petrels, and the Bulweria and Pseudobulweria species are wholly dark as well. Some populations of Macronectes (the giant petrels) are wholly dark brown and the Northern fulmar Fulmarus glacialis – generally familiar as a mostly white bird with a grey back and wings – also includes a mostly brown form in parts of its range.

Is there any consistency to this diversity of colours and patterns? That’s hard to answer. It might be ideal, for purposes of camouflage from prey and predators, for a seabird to be pale ventrally and greyish or bluish dorsally. However, numerous sometimes incompatible selection pressures mean that organisms do not necessarily evolve in an ‘optimal’ direction when it comes to pigmentation (Endler 1978, Burtt 1981). Seabird colouration seemingly incorporates sexually selected display traits, so there are evolutionary pressures to be showy and distinctive.

Masked shrike, by Claudia Becher, from wikipedia.

Plumage colours may also reflect the need for social cohesion (species that feed in groups need to be able to see conspecifics from a long way off), they may play a role in thermoregulation, and birds may sometimes use feather colour to reflect or absorb sunlight. Dark masks, for example, may help reduce glare and enable predators to see better in strongly lit environments. That last idea is mentioned in the ornithological literature with respect to the dark masks of peregrines and other raptors, and it’s even been suggested that raccoons and other mammals might benefit from their masks in the same way. A recent experimental study on shrikes provides support for the hypothesis, since Masked shrikes Lanius nubicus with dark facial masks were more effective hunters (being far better able to hunt facing the sun) than ones with artificial pale masks (Yosef et al. 2012). [Masked shrike image by Claudia Becher.]

Differential erosion of feathers according to melanin content. At top: the unpigmented primary tips of a Herring gull (Larus argentatus) are worn away while the dark remainder persists. At bottom: brown feather segments in the Snow bunting (Plectrophenax nivalis) are worn away during the year, meaning that these feathers appear to change from brown to black. From Strauch (1991).

The role of melanin in protecting feathers from abrasion and UV damage may also mean that feathers, or their tips, are dark for reasons unrelated to ecology or behaviour. Some petrel (and other seabird) species may also be coloured to mimic others (more on this later). Feeding style and prey type may also have some control over pigmentation, and diurnal and nocturnal species are obviously going to be under different pressures when it comes to camouflage. And do the selection pressures that act on nesting (often burrow-dwelling) petrels have as much, or even more, influence on their pigmentation than the pressures they experience while foraging and avoiding predators at sea? Remember that some petrels may be visiting their nest burrows for more than seven months out of the year. Species are also sometimes patterned or coloured the way they are because that’s what their ancestors were like – that is, their patterns and colours aren’t necessarily adaptive. Some biologists argue that exaptation is a very dodgy idea since it fails to account for (often under-studied) adaptational explanations for structures and behaviours. There may be some truth in this, but a historical, phylogenetic perspective demonstrates that exaptation must have occurred in many lineages.

Some of the graphs from Bretagnolle (1993), here showing the results of correspondence analyses between colouration, diet and feeding style in petrels and other tubenoses.

Bretagnolle (1993) looked specifically at the adaptive significance of tubenose pigmentation and concluded that no one factor had a dominant effect. However, foraging style and group size seemed to be the most important controlling factors. Small species that feed in groups (like prions and some gadfly-petrels) seemingly tend to be cryptically coloured in whites and greys, but dark upperparts may also aid concealment against the sea surface when aerial predators (like larger tubenoses and skuas) need to be avoided. Prominent countershading (like that present in many shearwaters) was suggested to correlate with the underwater pursuit of fish and the avoidance of aquatic predators.

These proposed explanations are somewhat speculative and, as I said above, the incompatibility of various of the selection pressures acting on pigmentation may mean that some (or many, or most) of these birds don’t really possess optimal coloration for their lifestyle. Bretagnolle (1993) also suggested that tubenoses might differ from the majority of other seabird groups in pigmentation since they tend to practise continuous feeding (that is, they forage constantly across the whole duration of their time at sea, rather than commuting to an area of prey abundance). In other words, they might not follow the same rules as other seabird groups.

And, yes, more on petrels to come. For previous Tet Zoo articles on seabirds, see…

Refs – -

Ashmole, N. P. 1971. Avian Biology Volume 1. Academic Press, London.

Bretagnolle, V. (1993). Adaptive Significance of Seabird Coloration: The Case of Procellariiforms The American Naturalist, 142 (1) DOI: 10.1086/285532

Burtt, E. H. 1981. The adaptiveness of animal colors. BioScience 31, 81-102.

Endler, J. A. 1978. A predator’s view of animal color patterns. In Hecht, M. K., Steere, W. C. & Wallace, B. (eds) Evolutionary Biology, Volume 11. Plenum, New York, pp. 319-364.

Habib, M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society 99, 687-698.

Nelson, B. 1980. Seabird: Their Biology and Ecology. Hamlyn, London.

Spear, L. B. & Ainley, D. G. 1998. Morphological differences relative to ecological segregation in petrels (family: Procellariidae) of the Southern Ocean and tropical Pacific. The Auk 115, 1017-1033.

Strauch, 1991. Feathers. In Brooke, M. & Birkhead, T. (eds) The Cambridge Encyclopedia of Ornithology. Cambridge University Press (Cambridge), pp. 20-26.

Yosef, R., Zduniak, P. & Tryjanowski, P. 2012. Unmasking Zorro: functional importance of the facial mask in the Masked Shrike (Lanius nubicus). Behavioral Ecology doi: 10.1093/beheco/ars005

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 He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at! 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. Halbred 5:45 pm 03/19/2012

    All this aquaflying talk makes me wonder if it was an intermediate “step” in penguin evolution.

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  2. 2. naishd 8:57 pm 03/19/2012

    Excellent topic for discussion, what with all the exciting work on fossil penguins and their biology. I’ll wait to see what others say but, for now, note that petrels (and other tubenoses) don’t tell us anything >directly< about penguin origins, since the two groups do not share an immediate ancestry (part of my reason for saying this is that there was a time in the recent past when an especially close relationship was mooted between the two groups: in fact, during the 1990s some authors even regarded tubenoses as being paraphyletic with respect to penguins).


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  3. 3. Dartian 2:38 am 03/20/2012

    Dark masks, for example, may help reduce glare and enable predators to see better in strongly lit environments. That last idea is mentioned in the ornithological literature with respect to the dark masks of peregrines and other raptors, and it’s even been suggested that raccoons and other mammals might benefit from their masks in the same way.

    Raccoons aren’t really diurnal, though. Thus, at least in their case, the facial mask is unlikely to be an adaptation to “strongly lit environments”.

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  4. 4. naishd 5:10 am 03/20/2012

    Dartian: ok, sure. But the point is “it’s even been suggested” :) Having said that, the suggestion I recall reading is that the raccoon mask combats glare shining off water, and that this applies to moonshine occurring at night.


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  5. 5. naishd 5:12 am 03/20/2012

    PS – in any case, raccoons are sometimes/often diurnal, right?


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  6. 6. Dartian 6:49 am 03/20/2012

    the point is “it’s even been suggested”

    Yes, but people do suggest the darndest things. ;)

    I recall reading is that the raccoon mask combats glare shining off water, and that this applies to moonshine occurring at night

    Hmm. Reference?

    raccoons are sometimes/often diurnal, right?

    They sometimes are, yes. But they are also among the quite few terrestrial mammals that are truly colour-blind (i.e., they have monochromatic vision). This strongly suggests that raccoons have become specifically adapted to scotopic habitats, so I think it’s safe to say that raccoons may be considered primarily nocturnal animals. Incidentally, diurnal procyonids such as coatis have dichromatic vision, which is the most common type of vision in mammals (Jacobs & Deegan, 1992).

    Jacobs, G.H. & Deegan, J.F. 1992. Cone photopigments in nocturnal and diurnal procyonids. Journal of Comparative Physiology A 171, 351-358.

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  7. 7. naishd 6:56 am 03/20/2012

    Yeah, I’ve already tried to find a reference mentioning this (glare avoidance relevant at night as well as in daylight) and have failed (though here’s one authoritative source). Nevertheless, despite the monochromatic vision, raccoons are still fairly day-active, albeit “more nocturnal than diurnal”, as it says in Walker’s Mammals of the World.


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  8. 8. Dartian 7:09 am 03/20/2012

    here’s one authoritative source

    Posting links like that is a way to win the internet. ;)

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  9. 9. llewelly 8:36 am 03/20/2012

    “… in any case, raccoons are sometimes/often diurnal, right?”

    I’ve seen an awful lot of raccoons, but never during the day, unless they were dead.

    Of course I usually saw them in urban environments, and always in areas where they are invasives. That might affect their behavior.

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  10. 10. David Marjanović 9:25 am 03/20/2012

    dichromatic vision, which is the most common type of vision in mammals

    Not just placentals?

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  11. 11. Jerzy New 9:33 am 03/20/2012

    I just love the way this blog discussions smoothly go from petrels to raccoon masks :-)

    Keep going on!

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  12. 12. naishd 10:25 am 03/20/2012

    … and maybe we could go from here to dichromatic vision in mammals to plush toys? (this only makes sense if you follow the link in comment 7). Of course, there’s also the evolution of penguins (comment 1). I didn’t want to spoil the fun by talking about Waimanu or the newly unveiled Kairuku, the paper on which is very relevant to penguin origins and body shape evolution…

    Ksepka, D. T., Fordyce, R. E., Ando. T. & Jones, C. M. 2012. New fossil penguins (Aves, Sphenisciformes) from the Oligocene of New Zealand reveal the skeletal plan of stem penguins. Journal of Vertebrate Paleontology 32, 235-225.


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  13. 13. Dartian 10:28 am 03/20/2012

    Not just placentals?

    I was speaking of mammals as a whole, but yes, dichromacy is common in marsupials too. Whether it is more common than trichromacy is another question and one that hasn’t yet been settled – we haven’t properly investigated enough marsupial species yet! But based on what we do know, (all?) New World opossums are dichromats, just like most placental mammals are. In Australasian marsupials, trichromacy seems to be fairly widespread, but, as I said, exactly how widespread remains to be established (Ebeling et al., 2010).

    Ebeling, W., Natoli, R.C. & Hemmi, J.M. 2010. Diversity of color vision: not all Australian marsupials are trichromatic. PLoS ONE 5(12), e14231.

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  14. 14. Jerzy New 11:13 am 03/20/2012

    So what mammals besides primates are trichromats?

    And are dichromatic mammals blind to red light (it seems dark to them) or do they simply don’t recognize it from green?

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  15. 15. Heteromeles 11:24 am 03/20/2012

    Of course, we’re ignoring the fact that raccoons hunt largely by scent and touch. Remember those amazing forepaws? Why are we focusing on their eyes again?

    I’ll suggest an alternative hypothesis: lets assume that melanin makes both hair and feathers tougher. Let’s also assume that the face is harder to groom than most other parts of the body. A raccoon, for example, can’t lick its eyes clean like a gecko, and neither can a tubenose. Instead, they have to rub them clean, or use claws. Eyes are also a focus for bugs and parasites, so keeping the clean is vital.

    The advantage to dark areas around eyes is that it makes the hairs tougher, so that they can withstand the grooming necessary to keep the eyes clean. It’s probably not a huge advantage (or everyone would have the mask), but given the necessity for caring for that region, it’s something that gets selected for, if the pigment mask shows up.

    The other thing I’d add is that dark masks are common enough that there’s probably a common mutation that produces that pattern.

    As for birds, how many masked tubenoses feed at night as well?

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  16. 16. pmurphy98 1:25 pm 03/20/2012

    I remember reading something (can’t place the source unfortunately) about studies done to determine the color of fossilized feathers in some non-avian theropods which found similar dark-tipped feathers. Not really sure of the implications, but it does seem to make the issue all the more interesting.

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  17. 17. Dartian 1:43 pm 03/20/2012

    So what mammals besides primates are trichromats?

    Only marsupials, it seems. And as for the primates, there is much variation among them in that respect. See here for a neat summary.

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  18. 18. naishd 6:33 pm 03/20/2012

    Pmurphy98 (comment 16): there are quite a few studies on the role of melanin in feather-strengthening, one of the best known being…

    Bonser, R. H. C. 1995. Melanin and the abrasion resistance of feathers. Condor 97, 590–591.

    And it’s not a new idea…

    Averill, C. K. 1923. Black wing tips. Condor 25, 57–59.

    Carney et al. (2012), in their work on the pigmentation of the isolated Archaeopteryx feather, noted this strengthening role and inferred that it might have been important for Archaeopteryx.

    Carney, R. M., Vinther, K., Shawkey, M. D., D’Alba, L. & Ackermann, J. 2012. New evidence on the colour and nature of the isolated Archaeopteryx feather. Nature Communications 3, Article number: 637 doi:10.1038/ncomms1642


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  19. 19. SRPlant 3:53 am 03/21/2012

    It’s a pity that darts manufacturers (before the invention of plastic flights) were unaware of the melanin/toughness connection. As far as I remember (They were boozy days…) dart flights were often made with the more fragile white turkey feathers as oppose to black or bronze (I read somewhere that modern battery turkeys are slaughtered at too young an age for their feathers to be stiff enough to be made into flights at all). Perhaps the lack of melanin was an evolutionary adaptation of the dart to make itself visible through the fog of cigarette smoke – and thus increase its chances of survival into the next leg.
    The poor stuffed galliform would often be the prize in the Christmas darts competition and, being plucked, kept the relative toughness of its feathers a mystery to any scientists that may have been talking turkey at the end of the evening.

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  20. 20. Dartian 8:00 am 03/21/2012

    The advantage to dark areas around eyes is that it makes the hairs tougher, so that they can withstand the grooming necessary to keep the eyes clean.

    But that doesn’t explain why dark facial markings/masks also occur frequently in vertebrates that have neither hair nor feathers (e.g., fish, frogs, and snakes).

    Also, the more structurally tough the hair or the feathers are, the better is the cover and the physical protection that they’ll offer ectoparasites, no?

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  21. 21. Jerzy New 8:27 am 03/21/2012


    Facial masks in racoons are I think for species signalling. As somebody who spent much time looking at fuzzy indistinct shapes in forest understory at night, I can understand that nocturnal mammals have problem knowing if they are looking at the face or rump of their fellow.

    It is interesting, because other carnivores like tigers have spots around their eyes which are designed to hide eyes, to keep prey from noticing the predator looking at it.

    When you look at carnivores, it seems that species which are nocturnal plus hunt prey which is either slow or underground (raccoons, badgers, polecats) have facial “bandit-masks”. Those diurnal or hunting mobile prey (foxes, felids, genets, weasels) have not.

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  22. 22. Heteromeles 9:14 am 03/21/2012

    @Dartian: Yes and no. Melanin is pretty common as a durable chemical for a lot of things, including fungal spores. One could make the same argument for skin. The counterargument isn’t that it makes the hair tougher, it’s that other sensitive tissues aren’t similarly dark. Really though, I was getting tired of a bunch of biologists discussing dark marks as an anti-glare advantage for nocturnal foragers, and that needed to be broken up.

    @Jerzy: That’s the more likely explanation to me: big swaths of color provide a “cartoon face” that makes species recognition easier. Jonathan Kingdon had some fun with that concept back in the 80s, if you’ve ever seen his abstracted paintings showing how it worked (e.g. in Island Africa).

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  23. 23. Dartian 10:08 am 03/21/2012

    One could make the same argument for skin. The counterargument isn’t that it makes the hair tougher, it’s that other sensitive tissues aren’t similarly dark.

    But you specifically suggested that dark masks may have evolved in order to “withstand the grooming”. I just pointed out that that can’t explain the presence of dark masks in species which, for anatomical reasons, rarely or never groom their faces (fish and snakes, in particular).

    I was getting tired of a bunch of biologists

    Where did that come from?

    “Island Africa”

    Yeah, that’s a lovely volume. Probably my second most favourite Jonathan Kingdon book (nr. 1 being Lowly Origin).

    Back to possible explanations of facial mask functions: Newman et al. (2005) have suggested that they do indeed function mainly as signals – but not directed at conspecifics. These authors noted that among mammalian carnivores, conspicuous facial markings mainly occur in mid-sized species which either have an aggressive disposition (e.g., badgers, raccoons) or possess chemical defences (e.g., skunks, polecats), and that facial masks may therefore be warning signals meant to deter larger predators from attacking. It’s an interesting idea, though I’m personally not quite prepared to buy it.

    Newman, C., Buesching, C.D. & Wolff, J.O. 2005. The function of facial masks in “midguild” carnivores. Oikos 108, 623-633.

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  24. 24. Jerzy New 11:06 am 03/21/2012

    I am not buying it very much. Raccoons and polecats have not especially strong bite compared to similar carnivores. European badgers have strong jaws indeed, but wolves, apparently, have no difficulty in killing badgers. And cacomistles and ringtails are really fragile.

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  25. 25. John Scanlon FCD 11:20 am 03/21/2012

    Sorry this isn’t petrel-related (or melanin-related), but worth bringing up on an active thread (I don’t know how long it’ll stay up):

    …huh, I was assuming it was a new post, but actually over a year old. Slightly funny, at least.

    About melanin masks, though: brings to mind a paper I read when it was almost new, Pough (1978).

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  26. 26. Heteromeles 4:40 pm 03/21/2012

    Raccoons seem almost social in some circumstances. For example, I’ve seen one raccoon throw an arm out to stop another raccoon from wandering out in front of an oncoming car (Berkeley raccoons are very well educated). Because of that, the black mask as social display does seem reasonable to me. My real question is whether a mask is to distinguish raccoons from non-raccoons, or whether the mask is so variable in shape that it helps distinguish individuals. I’d bet the later, but does anyone have any real insight?

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  27. 27. Heteromeles 4:49 pm 03/21/2012

    @Dartian, thanks for the clue-in about Lowly origin. I haven’t seen it yet.

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  28. 28. naishd 5:40 pm 03/21/2012

    On dark facial masks, let us not forget…

    Ficken, R. W., Matthiae, P. E. & Horwich, R. 1971. Eye marks in vertebrates: aids to vision. Science 173, 936-939.

    It’s mostly about lines and stripes, but does say “Some mammals have such patches which almost certainly act as reducers of glare.” (p. 938).

    Badgers: I reckon that facial stripes are aposematic – a warning to other predators that badgers can defend themselves with a ferocious bite (mustelids have insanely strong bites for their size: the strongest of any mammal group, actually). Which other predators? I am reliably informed that badgers are predated upon by leopards where the two occur together. Not all badgers have those facial stripes, of course.


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  29. 29. David Marjanović 6:40 pm 03/21/2012

    Please keep going. I’m learning a lot. :-)

    mustelids have insanely strong bites for their size: the strongest of any mammal group, actually

    …surely with the exception of the Tassie devil?

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  30. 30. naishd 6:47 pm 03/21/2012

    Yeah, I should revise that statement. But not right now, too tired. Check this out if you can…

    Christiansen, P. & Wroe, S. 2007. Bite forces and evolutionary adaptations to feeding ecology in carnivores. Ecology 88, 347-358.


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  31. 31. Spugpow 9:23 pm 03/21/2012

    Surely with the exception of Thylacoleo.

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  32. 32. Dartian 2:55 am 03/22/2012

    Raccoons and polecats have not especially strong bite compared to similar carnivores

    Well, like I said, with polecats the idea is that it’s their scent, not their bite, that deters predators. As for raccoons, your statement isn’t true for all raccoons. The North American raccoon Procyon lotor does indeed have a rather modest bite force. But the similar-sized South American crab-eating raccoon Procyon cancrivorus has a bite force comparable to that of the Eurasian badger Meles meles (though it’s weaker than that of the North American badger Taxidea taxus) (Christiansen & Adolfssen, 2005).

    wolves, apparently, have no difficulty in killing badgers

    I’m not going to put too much effort into defending a hypothesis that I don’t really subscribe to myself, but, just for the record: keep in mind that Newman et al‘s. hypothesis doesn’t necessarily require that facial masks offer protection against the largest potential predators. It’s already a big help if they deter larger predators from attacking. For example, a badger’s facial mask may not be enough to prevent a wolf from attacking it, but it might well be enough to make (say) a lynx or a golden jackal hesitate.

    About melanin masks, though: brings to mind a paper I read when it was almost new, Pough (1978).

    Have you seen Kwiatkowski & Burt (2011)? Their results suggest that dark facial masks in snakes (or in New World pitvipers, at least) are not associated with diel activity period. Instead, it seems that dark masks are more common in terrestrial species, whereas arboreal species seem to secondarily lose them during evolutionary history.

    thanks for the clue-in about Lowly origin”

    You’re welcome; personally, I consider that book one of the best semi-popular accounts of human origins to have been published in recent years. Its main strength is that it provides something that’s usually lacking from such accounts: a solid biogeographical context (based on Kingdon’s extensive personal experience of Africa’s fauna and flora) to human evolution.

    badgers are predated upon by leopards where the two occur together

    By tigers, too.

    Christiansen, P. & Adolfssen, J.S. 2005. Bite forces, canine strength and skull allometry in carnivores (Mammalia, Carnivora). Journal of Zoology, London 266, 133-151.

    Kwiatkowski, M.A. & Burt, D.B. 2011. Evolutionary losses of facial stripes in New World pitvipers. Biological Journal of the Linnean Society 104, 923-933.

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