Recall the recent article about ‘extreme cetaceans’? Well, here’s the second one in the series.
Spectacled porpoise. Porpoises – the seven* species of the delphinoid family Phocoenidae – are small, short-beaked cetaceans that mostly live fairly cryptic lives in shallow coastal seas (this description applies to the living species: some fossil porpoises were comparatively large and long-beaked). The species that typifies the group – the Harbour porpoise Phocoena phocoena – is greyish (except for its white belly), has a low, triangular dorsal fin and is not especially charismatic.
* I’ve followed recent taxonomic decisions and am recognising two species within Neophocaena (N. phocaenoides and N. asiaeorientalis).
But other porpoises are rather different, and here we’re going to look at a far more flamboyant species, namely the Spectacled porpoise P. dioptrica of the cool and cold waters of the sub-Antarctic and Antarctic seas. This is a very poorly known species, and one of the things said about it most often is that just about nothing is known about it. It’s a 20th century discovery, its scientific debut occurring in 1912.
This species is remarkably pigmented relative to other Phocoena porpoises, being black dorsally, white ventrally, and with dark circles around its eyes. There have actually been a bunch of competing ideas on its exact appearance over the years, authors and artists disagreeing with respect to where the boundary between its dark and white areas are, what colour its flippers and tail flukes are, and so on. It’s distinct enough from the other Phocoena species that some authors have preferred to keep it in its own genus (Australophocoena), but this isn’t fashionable at the moment due to molecular data on its phylogenetic position. The suggestion has even been made that its pattern and colouring give it the ability to mimic killer whales and thus avoid predation. Cool idea, buuuut…. unlikely given that porpoises are so distinct from killer whales in size and surely in vocalisations and in the echolocatory signature that predatory cetaceans use when evaluating potential prey.
The Spectacled porpoise isn’t just remarkable for its pigmentation, however, but also for its shape, and in particular for its dorsal fin. This is ‘normal’ in some individuals, but disproportionally large – strangely so – in some individuals where it looks like an out-sized rounded flag projecting upwards and backwards at a size about twice or three times that you might predict. Like the keels, humps and unusual dorsal fins of some spinner dolphins (see the previous article in this series), this is a sexually dimorphic feature that’s especially exaggerated in mature males. Its presence is therefore presumably a sociosexual indicator of age and sexual status. Another odd thing about the dorsal fin (albeit one not unique to this species within porpoises as a whole) is that there are tiny tubercles along the leading edge (Evans et al. 2001), albeit seemingly not in all individuals. Dorsal fin tubercles are actually known for all porpoises – they’re weird and interesting and I’ll try to remember to come back to them in another article.
This giant dorsal fin isn’t a newly discovered feature – it was reported and illustrated as far back as 1916 (Bruch 1916) – but it hasn’t ben commented upon as often as it might, especially given that it’s one of the most pronounced expressions of sexual dimorphism in cetaceans. Indeed, as Ellis (1983) noted, “only the killer whale manifests such a difference in the dorsal fin” (p. 198); sexual dimorphism of the dorsal fin is known in other porpoises, but isn’t as extreme as it is here (Torre et al. 2014). Apart from the fact that it’s obvious, and looks fairly absurd in the older males that have it, we don’t know much about this fin or its function. Maybe it’s ‘just’ a visual signal of sex, maturity and (perhaps) health and condition. Maybe – recall the comments in the previous article about dorsal fins functioning as thermal windows – it also plays an important physiological role. Whatever it does, it makes this an ‘extreme’ cetacean; an animal that looks surprising, weird and flamboyant.
Finally — there’s a adoption scheme for the Spectacled porpoise. Adopt one yourself and aid in the conservation of this poorly known species.
More in this series soon. Here’s some of the stuff on cetaceans that exists in the archives (as always, much of the material at TetZoo versions 2 and 3 has been ruined by the removal of images, though remember that much or all of this is archived at Wayback Machine)…
On identifying a dolphin skull, July 2008 (all images now missing)
Seriously frickin' weird cetacean skulls: Kogia, shark-mouthed horror, July 2008 (all images now missing)
Scaphokogia!, July 2008 (all images now missing)
Cetacean Heresies: How the Chromatic Truthometer Busts the Monochromatic Paradigm, April 2015 (but now lacking all images)
Whale Watching in the Bay of Biscay, August 2019
Extreme Cetaceans, Part 1, September 2019
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Refs - -
Bruch, C. 1916. El macho de Phocaena dioptrica Lah. Physis, 2461-2462.
Evans, K., Kemper, C. & Hill, M. 2001. First records of the Spectacled porpoise Phocoena dioptrica in continental Australian waters. Marine Mammal Science 17, 161-170.
Natalie, R., Goodall, P. & Brownell, R. L. 2018. Spectacled Porpoise. In Würsig, B., Thewissen, J B. M. & Kovacs, K. (eds) Encyclopedia of Marine Mammals, Academic Press, pp. 912-916.
Sekiguchi, K., Olavarría, C., Morse, L., Olson, P., Ensor, P., Matsuoka, K., Pitman, R., Findlay, K. & Gorter, U. 2006. The spectacled porpoise (Phocoena dioptrica) in Antarctic waters. Journal of Cetacean Research and Management 8, 265-271.
Torre, J., Vidal, O. & Brownell, R. L. 2014. Sexual dimorphism and developmental patterns in the external morphology of the vaquita, Phocoena sinus. Marine Mammal Science 30, 1285-1296.