February 27, 2012 | 34
Thanks to everyone who had a go at identifying the Baja California whale carcass. Some suggestions were, err, a little far of the mark (fin whale, mesoplodont beaked whale); others were pretty reasonable (pilot whale). So, what do we have? Answer: a toothed cetacean (that is, an odontocete), total length about 3 m, with a large, subtriangular dorsal fin, a blunt head, and a strongly reduced dentition. The photos clearly showed that dentition is absent in the upper jaws, and that the lower jaws have just two or three short, subconical teeth on each side. The carcass is greyish, but it’s difficult to know if this really reflected the colour in life.
One cetacean – and one cetacean alone – matches these listed characters: Risso’s dolphin Grampus griseus (sometimes known as the Grey dolphin or Grey grampus). Well done Cameron McCormick, suferable, vdinets, leecris, Hai-Ren, Dartian, Mark Evans and Phoca, all of whom correctly said Risso’s dolphin. Regarding other possible matches… pilot whales (Globicephala), False killer whales Pseudorca crassidens, Pygmy killer whales Feresa attenuata and Melon-headed whales Peponocephala electra all have both a higher numbers of teeth overall and emergent teeth in the upper jaw, and all are much darker in living condition.
So, the carcass is definitely that of a Risso’s dolphin. A few features normally present in life are not obvious on the carcass. These include a distinctive white, anchor-shaped marking on its chest, and a vertical crease on the forehead. The latter feature is highly distinctive and not seen in other blunt-headed delphinids. Phillips et al. (2003) suggested that it allows the dolphin to create a uniquely angled sonar beam. Unusual specimens with emergent teeth in the upper jaws have been reported on occasion (Kruse et al. 1999) but the typical condition is to have two to seven pairs of teeth in the lower jaws and apparently one or two pairs of unerupted upper jaw teeth.
This is a widespread dolphin, found in tropical, subtropical and temperate oceans and seas worldwide. In contrast to several other globally widespread dolphin ‘species’, molecular variation between populations is low, being about 0.2% (LeDuc et al. 1999). This is consistent with the fact that – in contrast to other globally distributed delphinid ‘species’ – there have not yet been recent proposals to split G. griseus into more than one species.
Risso’s dolphin seems to show a preference for oceanic canyons and continental slope environments and there are indications that it takes advantage of upwelling phenomena and the movement of oceanic fronts up and down continental slopes (Baumgartner 1997, Bearzi et al. 2011) [adjacent photo by Mike Baird]. It often feeds at night. While its ecology and natural history haven’t really been well studied, reliable data show that it’s a cephalopod specialist (Clarke 1986), mostly eating mesopelagic squid and also octopuses. Stomach content data indicates that fish and tunicates are eaten as well, sometimes, in some places. In prey requirements, morphology and habitat choice, Risso’s dolphin is very similar to the Short-finned pilot whale Globicephala macrorhynchus and there are indications that the two avoid competing by segregating (Shane 1995). When they do meet, interactions are aggressive.
Risso’s dolphin is one of the largest delphinids, with some individuals reaching 4 m and 500 kg. Males and females are similar in size and adults of both sexes are always conspicuously marked with numerous white scars across their bodies and tails. Some of these might result from grappling with cephalopods, but the majority are tooth marks made by other individuals. The ubiquity and number of these score marks show that biting or – at least – ‘scoring’ with the mandibular dentition is a commonplace activity in these dolphins, perhaps occurring as part of play and courtship as well as combat. Some individuals are so scarred that they’re white across much of their dorsal surface.
Risso’s dolphin was first recognised scientifically in 1812 when George Cuvier described a skin and skull discovered at Brest in France. He recognised that it represented a new species and named it Delphinus griseus. At about the same time, Cuvier also obtained a description (with drawing) of another distinctive dolphin that stranded near Nice. This report came from a M. Risso, so Cuvier took to referring to the animal as ‘dauphin de Risso’. He named it D. aries, apparently “because the drawing showed that this dolphin had a creased forehead something like the horn boss on a ram’s head” (Watson 1988, p. 254). It’s been near-universally agreed for some decades now that both specimens belonged to the same species, so the vernacular name ‘Risso’s dolphin’ became transferred to D. griseus.
In 1828, John Gray decided that D. griseus and a number of other blunt-headed delphinids should be given their own ‘subgenus’, Grampus. This name – once widely used as the vernacular term for various blunt-headed delphinids – seems to derive from a corruption of the French ‘grand poisson’. Gray’s version of Grampus at times included pilot whales, false killer whales and killer whales as well as species later moved to Lagenorhynchus and Cephalorhynchus, and in addition to G. griseus he and others also named the species G. sakamata Gray, 1846, G. richardsoni Gray, 1850 (based on a lower jaw of unknown origin), G. chinensis Gray, 1866 and G. stearnsii Dall, 1873 (also based on a lower jaw). All of these names (and others) are now included in the synonymy of G. griseus (Hershkovitz 1966).
There have been a couple of misguided attempts to come up with new generic names for Grampus. Grayius Scott, 1873 was proposed since Grampus was deemed inappropriate; Iredale and Troughton (1933), similarly, decided that the name Grampus should be applied specifically to Orcinus orca, the Killer whale, and they therefore proposed the new generic name Grampidelphis for Risso’s dolphin. Their arguments were erroneous; Hershkovitz (1961) said “All this and more too painful to recite is sheer fantasy” (p. 549).
Risso’s dolphin is very obviously a true dolphin – that is, part of the delphinoid clade Delphinidae. But where might it belong within this large and morphologically diverse group? Despite its blunt head and lack of a long beak or rostrum*, it has – ignoring confusion from before the 20th century – traditionally not been included within the ‘globicephaline’ group often recognised for pilot whales and other ‘blackfish’. Some authors have included killer whales (Orcinus) within Globicephalinae, and have therefore used the name Orcininae for this group (the latter is Orcininae Wagner, 1846; the former Globicephalinae Gray, 1850). Obviously, globicephalines are mostly black [adjacent pilot whale illustration from here]. Risso’s dolphin isn’t, and I think this partly explains why it hasn’t typically been regarded as a member of the group. But it’s said to exhibit several features of palatal sinus anatomy that are shared with Delphinus and kin (Muizon 1988). Until recently, Risso’s dolphin has, therefore, been included within Delphininae, typically being hypothesised to be closer to Delphinus and Stenella than to the ‘lags’ (= lissodelphinines) or to the ‘stenines’.
* The rostrum is not truly absent in Risso’s dolphin, it’s just very short and not clearly demarcated from the rest of the head.
However, recent molecular studies do not support a delphinine position for Grampus. In the earliest comprehensive analyses of molecular phylogeny within delphinids, LeDuc et al. (1999) proposed several novel, surprising hypotheses, one being that Grampus is not close to the long-beaked dolphins (Delphinus, Stenella and kin*), but is instead a globicephaline, most closely allied with Pseudorca (false killer whale), Feresa (pygmy killer whale), Peponocephala (melon-headed whale) and Globicephala (pilot whales). [Caveat: I’m using the name Globicephalinae here as if it’s branch-based, and thus that any delphinid closer to Globicephala than to Delphinus or Lagenorhynchus is a globicephaline. Some authors seem to use the name as if it’s unique to the ‘blackfish’ clade. Given that there are no explicit phylogenetic definitions anywhere in the literature (so far as I can tell), it’s difficult to know exactly what to do.]
* All of which (Delphinus, Tursiops, Stenella and Lagenodelphis) are potentially ripe for synonymisation according to these authors (LeDuc et al. 1999).
McGowen et al. (2009) recovered evidence for a similar clade, though Orcaella (snubfin dolphins) was now recovered as the sister-taxon to the others and, surprisingly, Steno (the peculiar rough-toothed dolphin) was
positioned in between Orcaella and the remainder recovered as being closer to Grampus and the ‘blackfish’ than was Orcaella (McGowen et al. 2009). Both the long-beaked delphinines and the lissodelphinines were closer to this clade than was Orcinus, the killer whales. Extremely similar results have been reported in other studies (Caballero et al. 2008, Cunha et al. 2011, McGowen 2011, Geisler et al. 2011). In fact we can now say that the position of Grampus within Globicephalinae is well supported and represents the present consensus. Incidentally, Steno was taken away from the globicephalines and put close to the long-beaked delphinines by Cunha et al. (2011).
McGowen et al. (2009) estimated that Grampus diverged from other globicephalines round about 5 million yeas ago (early on in the Pliocene) while Cunha et al. (2011) similarly suggested (based on relaxed molecular clock calibration) a divergence date of 5.5 million years ago. Fossil specimens of Grampus are rare but there’s supposed to be one from the Early Pliocene of Japan and a Pleistocene record from Madagascar.
Some speculations about sinus form, snout shape and pigmentation
What does this mean for delphinid evolution? It suggests several interesting things. One is that pterygoid sinus morphology – the sole anatomical character complex used to ally Grampus with long-beaked delphinines – is seemingly less reliable than classically thought. That might not be surprising given evidence from elsewhere in the tree of life that the development of sinuses and air-sacs is labile and opportunistic. I think in any case that the complex sinus system of Grampus is only superficially similar to that of long-beaked delphinines: when I look at the sinus configurations illustrated by Fraser & Purves (1960), the Grampus condition looks like an elaborate version of the simpler condition present in pilot whales, and not especially like that of Tursiops, Stenella and Delphinus.
Another interesting thing concerns snout shape (and note here that, by ‘snout shape’, I refer to the condition in the living animal: that is, where the melon and its associated soft tissues envelop much of the length of the bony rostrum). Consider that porpoises and monodontids (narwhals and belugas) are close relatives of delphinids and, as we’ve just seen, most recent phylogenetic studies find killer whales and lissodelphinines (Cephalorhynchus and the ‘lags’ and so on) to be outside the globicephaline + long-beaked delphinine clade.
Notably, all of these groups are short-snouted, or at best ‘medium-snouted’ (though, long-beaked fossil porpoises do confuse the picture somewhat). Could it be, therefore, that short snouts were ancestral for delphinids and widespread during most of their history, that globicephalines were primitively short-snouted (that is, they did not evolve from long-snouted ancestors), and that long snouts were only evolved at the base of the delphinine lineage? On balance, I think it’s safest to assume that modest, mid-length snouts were the norm for most of delphinid history. Globicephalines might not be as unusual (in being blunt-headed) as people used to think, but they’re still modified relative to the ancestral condition in having an especially wide bony rostrum (this is most evident in pilot whales and false killer whales) and an enlarged, anteriorly prominent melon.
Finally, what about pigmentation? Morphological character sets on extant species often neglect to include or code pigmentation data, but – while undeniably labile – it clearly carries a phylogenetic signal (and tests involving coat patterns in cats and other groups often produce encouraging results). As is obvious from the vernacular term ‘blackfish’, globicephalines are unusual among delphinids in being mostly black. If Grampus and Orcaella are part of Globicephalinae [see caveat above], but outside the ‘blackfish’ clade (Pseudorca, Feresa, Peponocephala and Globicephala), then we might infer that the greyish Grampus and Orcaella represent the primitive condition. In fact (time for some rampant speculation), Grampus is born dark and becomes paler with age, only retaining the darker pigmentation on its fins, flukes and dorsal fin (and sometimes on the chin as well). Could the dark overall colouration in the blackfish perhaps be a paedomorphic feature?
Also interesting is that Grampus has a wide, white area on its chest, joined by a thin ‘stem’ to another wide, white area on its belly. The same, ‘anchor-shaped’ configuration is present in pilot whales, false killer whales, pygmy killer whales and melon-headed whales: in these taxa the ‘stem’ is longer, the abdominal field is smaller, and the ‘anchor’ is sometimes greyish or off-white rather than pure white [adjacent pilot whale image by Barney Moss]. Nevertheless it’s conceivable that this is a shared derived character.
As usual, what was meant to be a quick, couple-hundred-words-article saying “Yes, it was a Risso’s dolphin” evolved in something with a bit more substance. I hope you learnt something!
For links to ALL previous Tet Zoo cetacean articles (there are quite a few of them), please see…
Refs – -
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Bearzi, G., Reeves, R. R., Remonato, E., Pierantonio, N. & Airoldi, S. 2011. Risso’s dolphin Grampus griseus in the Mediterranean Sea. Mammal Biology 76, 385-400.
Caballero, S., Jackson, J., Mignucci-Giannoni, A. A., Barrios-Garrido, H., Beltrán-Pedreros, S., Montiel-Villalobos, M. G., Robertson, K. M., Baker, C. S. 2008. Molecular systematics of South American dolphins Sotalia: sister taxa determination and phylogenetic relationships, with insights into a multilocus phylogeny of the Delphinidae. Molecular Phylogenetics and Evolution 46, 252-268.
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