Maternal Investment, Female Rivalry, and a Fallacy

In the digger wasp Sphex ichneumoneus it has been observed that when two females fight over a joint investment the one that has invested least is most likely to give up the fight. This behaviour has been interpreted as a commitment to the "Concorde fallacy", since both females are fighting for the same resource, viz. their joint investment. I argue here that such an interpretation is incorrect. The case of a conflict between the strategies of two different individuals is not equivalent to the case of a conflict between two alternative strategies of one individual. It holds for any organism that to the extent that loss of investment implies loss of future reproduction (e.g. because the total capital that can be invested is fixed) one should expect defence strength to increase with the amount invested.