Hybrids, morphs, mutants or just Dolphins with a dark-side?

Dan Brown

Ever encountered a dolphin with a dark-side? Careful scrutiny of pods of Common Dolphins occasionally reveals dark individuals, but what are they?

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.

Common Dolphin: A classic individual showing a creamy-peach thoracic patch.

Common Dolphin: A classic individual showing a creamy-peach thoracic patch.

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:

http://www.bbc.co.uk/news/uk-scotland-highlands-islands-29541605

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.

Common Dolphin: melanistic individual showing the dark thoracic patch, darker pectoral fins, and a darker lower flank stripe

Common Dolphin: melanistic individual showing the dark thoracic patch, darker pectoral fins, and a darker lower flank stripe

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).

Common Dolphin: A melanistic individual between Harris & St Kilda

Common Dolphin: A melanistic individual between Harris & St Kilda

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.

Striped Dolphin: A small, compact, highly energetic dolphin, found in deep oceanic waters

Striped Dolphin: A small, compact, highly energetic dolphin, found in deep oceanic waters

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!

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