Spermatophore size

Spermatophore size as determinant of paternity in an arctiid moth (Utetheisa ornatrix). Proceedings of the National Academy of Sciences, USA 91 7081-7084. 

We also have evidence of how the female assesses the size of her mating partners. She appears to do so indirectly, by gauging the size of their spermatophore, for which purpose she may use stretch receptors that female moths are known to have in the chamber (bursa) in which spermatophores are deposited (29). Male Utetheisa can be caused to produce inordinately small spermatophores if they are mated relatively recently beforehand. If such mated males are placed in competition with physically smaller males, whose spermatophores may now be the relatively larger ones, they tend to "lose out" (30). We predict from this that males, in nature, may space their matings days apart. To regain the capacity to produce full size spermatophores takes a male about a week (30). 

We have evidence that the female assesses male size indirectly, by gauging the size of the spermatophore, which in Utetheisa is proportional in size to body size (LaMunyon and Eisner, 1994). If large males are experimentally caused to produce inordinately small spermatophores (as a consequence of having mated shortly beforehand with another female), their sperm will lose out to that of a smaller male, whose spermatophore is then the relatively larger one. 

Sperm selection also provides females with the option of checking on potential liars, males which might have misrepresented themselves in courtship by producing hydroxydanaidal at exaggerated levels relative to their PA content. Unless such males also are able to produce exaggerated-sized spermatophores, they are likely to be found out when they are put to the test in sperm selection. 

The fact that, in Utetheisa, male hydroxydanaidal titer correlates with systemic PA content, and systemic PA correlates with body mass (as well as with spermatophore mass), means that the female, on the two occasions that she exercises mate choice, favors larger males. Body size is at issue in the precopulatory context when she chooses on the basis of hydroxydanaidal titer, and it is at stake after mating when she makes her choice on the basis of spermatophore mass. 

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