Scientific literature often touts the many advantages of large body size, but seldom addresses the value of small body size. Yet selection for large size must be counterbalanced by selection for small size, otherwise, all animals would be large. In this paper, we demonstrate female-biased size dimorphism and a strong copulatory advantage for small males in a Jerusalem cricket (JC) (Stenopelmatus) from central California. We selectively paired male and female JCs of diverse body sizes and recorded their ability to copulate. All copulations were successful for males smaller or equal in size to females. In contrast, when the male was 6.1 mm longer than the female, copulation had only a 50% chance of occurring successfully. In general, as the difference between male and female body length increased (i.e., as males became longer than their mates), the probability of successful copulation decreased. These patterns of mating resulted in net selection for small male size and large female size. We also detected positive linear direct selection on male hind leg length, which may explain why male JCs have longer legs than females. The copulatory disadvantage of large males derives from the odd mating behavior of this group, in which males must contort and precisely align their bodies to couple. We believe that this is the first example of small-male advantage based solely on the physical aspects of copulation. In this species, small, not large, males have a copulatory advantage.
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Vol. 17 • No. 2