Specimen of the Week 197: The Common Dolphin Skull
By Will J Richard, on 20 July 2015
Hello! Will Richard here, putting fingers to keyboard once again to bring you the next instalment of specimen of the week. And this week I am going to make things easy for myself. I’ve had enough of subspecies versus species, questionable (mis)identifications, taxing taxonomy and chaotic cladistics. So this time I’m keeping it simple. A cut and dried case: the common dolphin. What could be clearer?
This week’s specimen of the week is…
**The Common Dolphin Skull**
1) Which dolphin is this dolphin?
Delphinus is a genus comprising two species of common dolphin: the short-beaked (Delphinus delphis) and the long-beaked (Delphinus capensis). The long-beaked common dolphin tends to be larger, reaching approximately nine feet when compared to the short-beaked common dolphin’s six to eight. They also tend to have longer “beaks” (or rostra) with “up to 60” interlocking teeth. This is vastly different to the short-beaked common dolphins’ “between 50 and 60”. The literature is full of qualifiers: “typically, usually, normally”. It seems that even the experts cannot always be sure.
2) So… which dolphin is this dolphin?
Both species share the characteristic grey and cream colouration, dark on top and pale below, and are found throughout the tropical and cooler temperate waters of the Atlantic and Pacific Oceans. There are also distinct subpopulations in a variety of enclosed seas (including the Mediterranean). As a rule of thumb, short-beaked common dolphins live further out to sea (up to thousands of kilometres) whereas long-beaked common dolphins tend to stick to shallower waters around the shore (within 180 kilometres). Though, as short-beaked common dolphins spend a lot of their time round the shore too, this is by no means water-tight when it comes to making an identification.
3) Seriously… which dolphin is this dolphin?
Although historically lumped together and after over 20 failed proposals, the two were eventually split in 1994 once the morphological and genetic data finally agreed. Some taxonomists still propose a third species (D. tropicalis) though others maintain that this is merely a subspecies of D. capensis. What is certain is that the long-beaked common dolphin exhibits a higher genetic diversity than any found in the more widespread short-beaked. Whether this merits splitting further is a matter of debate.
4) Never mind
Whatever the species, common dolphins hunt using echolocation, eating small fish, squid and octopuses. They feed communally and, if food stocks allow, can form groups of up to ten thousand. Refreshingly, this genus does seem to live up to its name. The global population of D. delphis alone is believed to exceed four million and numbers appear stable. These “common” dolphins are actually common.
Each dolphin consumes between eight and nine kilogrammes of prey a day. To scale this up, the largest conglomerations get through between 80 and 90 tonnes and globally (if we take four million as our total population) common dolphins eat somewhere in the region of between 32,000 and 36,000 tonnes of small fish and cephalopods every 24 hours. While this sounds huge, it is equivalent to only about 14% of humanity’s total wild fish catch, estimated at 91.3 million tonnes last year (2014).
Common dolphins reach sexual maturity at between 12 and 15 years of age and usually give birth to a single, metre-long calf after a gestation period of about a year. Among the 68,000 things in the Grant’s collection we also have a common dolphin foetus (don’t ask which species!). Given that ours is only 9.5 centimetres (nose to tail), and relatively undeveloped in its features, it must be an example of an immature individual, not long into the developmental process.
5) Who cares about species anyway?
Given that many of our specimens are knocking on for two hundred years it is perhaps not surprising that none of our documentation makes reference to a species identified in 1994. Even the IUCN list the long-beaked common dolphin (D. capensis) as data deficient, and much of the literature refers to their biology as “assumed similar to D. delphis”: a fair assumption, though obviously not ideal. If we were going to play the odds, it is more than likely that our specimens are of the more common D. delphis. A pretty inconclusive conclusion. And anyway, there might be another species along in a few years…
Rosel, P. E., Dizon, A. E., Heyning, J. E. (1994) Genetic analysis of sympatric morphotypes of common dolphins (genus Delphinus); Marine Biology, 119: 159-167
Natoli, A., Cañadas, A., Peddemors, V. M., Aguilar, A., Vaquero, C., Fernández-Piqueras, P. and Hoelzel, A. R. (2006) Phylogeography and alpha taxonomy of the common dolphin (Delphinus sp.); Journal of Evolutionary Biology, 19: 943–954.
Pusineri, C., Magnin, V., Meynier, L., Spitz, J., Hassani, S. and Ridoux, V. (2007) Food and feeding ecology of the common dolphin (Delphinus delphis) in the oceanic northeast Atlantic and comparison with its diet in neritic areas.; Marine Mammal Science, 23: 30–47.
Food and Agriculture Organization of the United Nations (2014) The State of World Fisheries and Aquaculture
Will Richard is Visitor Services Assistant at the Grant Museum of Zoology