If you’ve ever paid much attention to Common Dolphins you may have been lucky enough to see a dark individual amongst a pod. But what are they?? There are a number of documented records of these dolphins and back in mid-September I was lucky enough to bump into one on the sailing from Harris to St Kilda.
Much like Mallards, Gulls, chickens, & bonobos, dolphins will have a go at humping just about anything! There are a few instances of hybridisation amongst captive Dolphins and recently the first wild instance in the UK of a Risso’s x Bottlenose Dolphin off the Western Isles:
Clymene Dolphin is also likely to have arisen as a species through hybridisation, and there has been talk that these dark Common Dolphins are also hybrids but their structure doesn’t point to any other species influence.
It seems most likely that these animals are simply melanistic individuals. Given the rarity of this morphotype it would probably be incorrect to call them morphs (in generally >1% of a population has to show the features associated with being a morph rather than a result of a mutation).
In the case of this individual and other observed in the North Atlantic (eg http://cotf5.blogspot.com.es/2014/01/day-4-blow.html) the dark colouration is often restricted to the thoracic patch, which is normally a creamy-peach colour. Other variations including notably pale individuals have been recorded with animals around New Zealand well documented.
This colouration can often lead to confusion with the similar Striped Dolphin, however, structure and behaviour should be enough to rule this species out. Striped Dolphin is a compact and highly energetic dolphin and generally found in deeper oceanic water. Given good enough views the distinctive lateral blaze is also characteristic.
Next time your crossing Biscay, or out in the Atlantic keep an eye open for aberrant dolphins, the more information we have on them the better!