INTRASPECIFIC VARIATION IN TESTES SIZE AMONG NORTHERN POPULATIONS OF PEROMYSCUS

1. We examined the variation in relative testis size among northern populations of Peromyscus to test if mice from populations with shorter breeding seasons had larger testes than mice from populations with longer breeding seasons. If natural selection on relative testes size in mammals is ultimately due to copulatory frequency and if sperm production is limited, then a decrease in the number of days in which a constant number of matings occurs should promote selection for larger testes. 2. Relative testis size (testis length/body length) significantly varied among 10 populations of P. maniculatus and three populations of P. leucopus. Variation in testes size among P. maniculatus was positively related to length of the breeding season, rather than negatively as we predicted. The positive association was probably due to the increase in young-of-the-year breeding in populations with long breeding seasons. This would promote selection for larger testes due to the increase in the number of receptive females and increased male-male competition. 3. We found no evidence to indicate that differences in mating systems accounted for the observed intraspecific variation in testes size.