August 2, 2011
Open letter to be posted on

Céline H. Frère
School of Biological, Earth and Environmental Sciences
University of New South Wales
Sydney, New South Wales 2052

Dear Céline H. Frère:

I read with interest Inbreeding Tolerance and Fitness Costs in Wild Bottlenose Dophis, Céline H. Frère et al. Proceedings of the Royal Society B vol. 277 no. 1094 September 7. 2010 pages 2667-2673.  You describe the degree of inbreeding and the resultant effects on fitness in the population of some two to three thousand dophins in Shark Bay in Western Australia, finding that inbreeding is present in excess of what random mating would produce and that it reduces fitness in that it decreases the number of calves a female would have and bring to an age of ten years and increases the time it takes for calves to mature.

You point out the artilce by Helgason in 2008 which indicates that inbreeding to a degree increases fitness in that among Icelanders it increases fertilty both in the first and second generation.  Thus there must be an optimal popualtion size that would assure just the right amount of inbreeding for maximum fitness. 

You may be intereted in knowing that this has been proven not to be unique to humans.  There is an artilce (On the Regulation of Populations of Mammals, Birds, Fish, and Insects.  Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hone, Mark Pagel SCIENCE VOL 309 22 JULY 2005 page 609) that has shown in over 1,000 serial field counts of animals that this is a general rule; it is a law of biology.  Yet we are so indoctrinated with the notion that inbreeding is bad and genetic diversity is good that it is hard to keep the balanced truth in mind. 

You reasonably ponder why these dolphins should tolerate, nay seem to seek out, inbreeding and sustain the resulting fitness loss.  I do have a suggestion.  The Helgason article points out that whereas couples they describe as “second cousins or closer” have a higher fertility than “third cousins or closer,” when it comes to grandchildren, the third cousin or closer couple has the greater reproductive success. 

So as the young female swims about seeking or being sought by kindred males, she will mate with nearer kin as well as more distant.  Since the nearer kin are more likely to impregnate her on any single encounter she is likely to have a greater degree of inbreeding than if pregnancies followed the same pattern as matings. 

There is little choice in the matter.  Assuming that fertility declines with decreasing kinship at the same rate as documented in the Iceland study an in the one creature actually graphed out in the Sibley study, two thousand is simply too many for a stable population.  If all the dolphins in Shark Bay mated at random their average fertility would be well below replacement and there it would remain until the population was much smaller, 500 in the Sibley study and averaging about 256 I should recon in the Helgason study.  Nor is there any assurance it would stop there.  That would be the equilibrium level.  As the population plummeted the remaining dolphins would still not be any more closely related than the 2,000.  The population would still have to equilibrate over a number of generations, each continuing at below replacement, before there would be any hope of recovering fertility. 

So whatever their mechanism, the dolphins are doing just what needs to be done to keep the population alive.  The reduced fitness among individuals is a price that is being paid to have the population as a whole survive.


M. Linton Herbert MD 

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