Welcome to another of those week-long series of themed posts, produced (ostensibly) to save me from spending time on blogging (other jobs require priority). Previous series have been ankylosaur week and sea monster week. This time round we’re looking at seriously frickin’ weird cetacean skulls although, actually, we’re only going to be looking at odontocetes, as these are (1) the ones I want to write about, and (2) the ones I have neat new photos of. This was, of course, all inspired by the comparative work I recently did on the Tursiops skull. All of the skull photos you’re going to be seeing were kindly provided by Colin McHenry and are used with permission. And we begin with a whale that does, indeed, have a frickin’ weird skull: it’s the Pygmy sperm whale Kogia breviceps…
There are two currently recognised Kogia species (the other one is the Dwarf sperm whale Kogia sima), but genetic evidence suggests that K. sima might actually be two species (Chivers et al. 2005) [K. sima was known as K. simus until recently]. K. breviceps and K. sima differ in that K. sima has a shorter snout, and a much larger, more anteriorly located dorsal fin. K. breviceps lacks functional upper jaw teeth, but K. sima can sometimes have up to three pairs of small maxillary teeth (though I’ve seen pictures – there’s one in Berta & Sumich (1999) – that show K. breviceps with maxillary teeth too). At 3-4 m long, the Pygmy sperm whale is also bigger than the Dwarf sperm whale, which ranges from 2.1-2.7 m [the skull shown in these images is specimen BMNH 1974-874, images © Natural History Museum, London].
The rostrum of Kogia is proportionally the shortest of any cetacean, with some of the bones that usually form the skull roof (the nasals and supraoccipital) being either squashed up and fused, or absent. It seems that, as the rostrum shortened, the bones forming the anterolateral margins of the orbits (the lacrimals, the posterolateral parts of the maxillae, and the anterior parts of the frontals) came to overlap the base of the rostrum, thereby closing up the antorbital notches such that they now remain only as long slits (in a fossil kogiid from the Miocene Pisco Formation of Peru, Scaphokogia cochlearis, the antorbital notches form very long slits, with that on the left side being about equivalent in length to the width of the base of the rostrum). The bones of the mandibles are apparently paper-thin and the thin, sharply pointed teeth (which lack enamel) were regarded by Handley (1966) as being ‘strongly reminiscent of the teeth of pythons’.
The most obvious weird thing about the skull, however, is the presence of a large, rounded, supracranial basin on the skull roof. The presence of a wide, rounded supracranial basin, a condition termed scaphidiomorphy, is common to all physeteroids and is a synapomorphy of the clade (Muizon 1991). The basin houses a unique structure called the spermaceti organ, and while I’d like to discuss ideas on its function I’m not going to as that would easily add 1000 words to what was meant to be a short article.
The supracranial basin of Kogia is strongly asymmetrical and divided along the midline by a septum formed from the asymmetrical premaxillae: the left premaxilla only extends as far as the left naris, while the right premaxilla extends all the way to the vertex, surrounding the right naris entirely and sporting a mid-line crest in the nasal region that then leans over to the left (Schulte 1917, Nagorsen 1985) [you can see these features in the adjacent photo of BMNH 1974-874, here shown in dorsal view]. The right naris is small and sub-rounded while the left one is large and oval. Cranial asymmetry is actually the norm in crown-group odontocetes, only being absent in Pontoporia (the franciscana), though a few other taxa (like Orcinus) are relatively symmetrical compared to their relatives (Ness 1967).
Kogia doesn’t just have a weird skull; it’s one of the weirdest cetaceans all round. Its head is small for the size of the animal, being only 14-16% of total length, and its flippers are proportionally small as well, with unossified phalanges and carpals. Its under-slung mouth has been described by some as shark-like, and it has peculiar ‘false gill’ markings on the side of the head. I have a vague recollection of reading a proposal that it may even try to mimic sharks, but I can’t find this in the literature and wouldn’t take it too seriously anyway. Its neck vertebrae are all fused together and its tail flukes are apparently a lot floppier than those of other cetaceans. I also think it looks pretty scary: the picture below shows the head of a stranded K. sima, reported from Italy by Bortolotto et al. (2003).
Kogia does some weird behavioural stuff. K. sima has been reported to release a reddish anal fluid (presumably faeces) which then spreads out in the water to form a cloud of about 100 m sq, and to then hide in the middle of the cloud (Scott & Cordaro 1987). This is of course irresistibly comparable to what cephalopods do, which is all the more ironic given that kogiids are cephalopod predators. They’ve also been reported to be surprisingly aggressive, ramming boats and leaping towards them when trapped in seine nets. This is somewhat reminiscent of what Physeter – the big sperm whale – does, though its size of course means that its boat-ramming activities are better documented. Incidentally, it may not be a coincidence that sperm whales big and small have a spermaceti organ, and indulge in head-ramming. More on this another time. As for feeding, Kogia species use rapid jaw-opening, powerful retraction of the tongue and expansion of the throat to generate negative pressures when suction-feeding (Bloodworth & Marshall 2005). I’ve been mentioning suction-feeding on and off at Tet Zoo for ages now and, again, will elaborate on it at some other time.
Incidentally, I personally quite like the name Kogia. It seems that our predecessors did not share this opinion however. Writing in 1854, W. S. Wall ‘regretted that a barbarous and unmeaning name like Kogia should have been admitted into the nomenclature of so classical a group as the cetacea’. Richard Owen agreed with Wall and used the name that Wall proposed, Euphysetes Wall, 1851, over Kogia Gray, 1846, writing of Kogia in 1870 that ‘I have that confidence in the common-sense and good judgment of my fellow countrymen and labourers in philosophical zoology which leads me to anticipate a tacit burial and oblivion of the barbarous and undefined generic names with which the fair edifice begun by Linnaeus has been defined’. This was all recounted by Gill (1871) who, while using the name Kogia because of its nomenclatural priority, recognised ‘the justness of the criticisms upon it’ (p. 740). Oh, come on, the name ‘Kogia‘ is really not that bad.
Refs – –
Berta, A. & Sumich, J. L. 1999. Marine Mammals: Evolutionary Biology. Academic Press, San Diego.
Bloodworth, B. & Marshall, C. D. 2005. Feeding kinematics of Kogia and Tursiops (Odontoceti: Cetacea): characterization of suction and ram feeding. The Journal of Experimental Biology 208, 3721-3730.
Bortolotto, A., Papini, L., Insacco, G., Gili, C., Tumino, G., Mazzariol, S., Pavan, G. & Cozzi, B. 2003. First record of a dwarf sperm whale, Kogia sima (Owen, 1866) stranded alive along the coasts [sic] of Italy. In 31st Symposium of the European Association for Aquatic Mammals. Tenerife.
Chivers, S. J., LeDuc, R. G., Robertson, K. M., Barros, N. B. & Dizon, A. E. 2005. Genetic variation of Kogia spp. with preliminary evidence for two species of Kogia sima. Marine Mammal Science 21, 619-634.
Gill, T. 1871. The sperm whales, giant and pygmy. The American Naturalist 4, 725-743.
Handley, C. O. 1966. A synopsis of the genus Kogia (pygmy sperm whales). In Norris, K. S. (ed) Whales, Dolphins and Porpoises. University of California Press (Berkeley & Los Angeles), pp. 62-69.
Muizon, C. de 1991. A new Ziphiidae (Cetacea) from the Early Miocene of Washington State (USA) and phylogenetic analysis of the major groups of odontocetes. Bulletin du Muséum national d’Histoire naturelle (4e sér.) 12, 279-326.
Nagorsen, D. 1985. Kogia simus. Mammalian Species 239, 1-6.
Ness, A. R. 1967. A measure of asymmetry of the skulls of odontocete whales. Journal of Zoology 153, 209-221.
Schulte, H. von W. 1917. The skull of Kogia breviceps Blainv. Bulletin of the American Museum of Natural History 37, 361-404.
Scott, M. D. & Cordaro, J. G. 1987. Behavioral observations of the dwarf sperm whale, Kogia simus. Marine Mammal Science 3, 353-354.