August 31, 2013
to be posted on if there is no objection

D. Lucas
Department of Zoology
University of Cambridge
Downing Street
Cambridge CB2 3EJ

Dear Dr. Lucas:
I applaud your article.  (D. Lucas and T. H. Clutton-Brock The Evolution of Social Monogamy in Mammals SCIENCE vol. 341 no. 6145 August 2, 2013 page 526) I have a thought to contribute.  Let me drastically oversimplify your conclusions: when females are solitary, males will bond with one female; when females are in groups males will attempt to amass harems.  You make an excellent case for there having been some ancestral condition of solitary females and roaming males which then evolved into one or the other of the two more or less stable strategies.

The elephant in the room is that when a mating pool gets too large fertility declines.  There is ample evidence for this, which I have collected as best as I have been able:
In other words Malthus was wrong twice; he said population expands exponentially until food runs out while food supply increases only linearly.  Humans have, at least for now, been able to play catch up with the food supply for most of the world, but his first point is still respected in spite of the evidence I offer a link to.  So far as I know there is only one paper that has ever actually done the experiment and proved Malthus wrong.  I attach it.  I have heard rumors that an experiment was done with rodents, possibly using the name “ratopia,” but have not been able to find that one. 

To make another oversimplification – and your work deserves better – we shall divide environments into “rich,” which can sustain high densities of a mammal in question and “poor,” where mammals are perforce solitary, and assume that the other factors you list for solitude are not our interest.  In the rich environment a random mating strategy will produce a very large social pool, which will entail a loss of fertility.  An animal with such a strategy will be swamped by an animal that is able to control mating pool size without sacrificing the number of females.  The obvious choice then is a harem strategy.  This is consistent with your drift.

I have read within the past year the assertion that the development of the large brain in early hominids was not in response to the opportunity to develop technology but because of an advantage of being able to have a more sophisticated social order.  Alas I have lost that reference.  But you can see how it works; people are gregarious yet usually have the “social monogamy” or pair bonding usually found among solitary mammals.  This has been a successful alternative to harem formation as a way of having a large social pool without a large mating pool.

As you point out, birds are different.  I have no idea how they have worked out an optimal flock size using a modest brain size while humans only learned to live in bands, villages or groups of kindred after having developed a brain that takes – what? – 20% of available nutrition.

Please let me know what you think.


M. Linton Herbert  

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