Inbreeding is costly for dolphins: study

Thursday, 15 April, 2010
Bob Beale

New evidence of the costs of inbreeding among the bottlenose dolphins of East Shark Bay, in Western Australia, has been found as part of a long-running study.

Inbred females tend to have a lower calving success and those with inbred calves must spend longer weaning their offspring, according to a new paper in the journal Proceedings of the Royal Society B by an international team of researchers led by Dr Celine Frere, of the UNSW School of Biological, Earth and Environmental Sciences.

Inbred females are less successful than non-inbred females at raising a calf to the age of three years, and females with inbred calves produce fewer calves over their lifetime because they spend longer weaning each calf. They note, however, that the calf mortality rate at Shark Bay - estimated at 42% - is uncertain because the bay is aptly named and has a large shark population.

The Shark Bay dolphins number at least 2,000 to 3000 - one of the largest documented populations of inshore bottlenose dolphins - and have been closely observed by scientists since the mid-1980s.

This latest paper drew on accumulated observations of behaviour and genetic studies - using biopsy samples - of 41 mothers and 58 mother-calf pairs. Surprisingly, the study found a high degree of inbreeding.  About 14% of calves appeared to be offspring of matings between half-bother and half-sister.

The reasons why inbreeding occurs within the dolphin population are unknown, but the researchers note that it is most common among younger females and may be related to their inexperience in avoiding sexual coercion by groups of males.

"Inbreeding can be looked at from a cost-benefit point of view," says Associate Professor Bill Sherwin, one of the authors of the study. "There's a cost resulting producing offspring with lower survival rates but for the young females there's also a cost in trying to avoid inbreeding: the female might be harmed.

"For those older females who avoid inbred matings better than the young ones, we do not know how they recognise related males.  Male calves tend to leave their mother after weaning, and some time later, she will have another calf, possibly a daughter.  So there is not much opportunity for a female calf to get to know her half-brothers.  Calves usually do not reach sexual maturity until the age of 11 or more , so a question arises about how a  female would know which male dolphin was a close relative and therefore to avoid it."

"We also still don't know how dolphins recognise close relatives. In other mammals, including humans, it seems that scent may be an important signal in that respect, and particular genes are known to be involved.  We hope to study whether that is also true of dolphins in the near future."

Media contacts:
Bill Sherwin - 02 9385 2119
UNSW Faculty of Science media liaison - Bob Beale 0411 705 435
Celine Frere - is now at the University of Queensland