Data from: Sibling rivalry: males with more brothers develop larger testes

When females mate with multiple partners in a reproductive cycle, the
relative number of competing sperm from rival males is often the most
critical factor in determining paternity. Gamete production is directly
related to testis size in most species, and is associated with both mating
behavior and perceived risk of competition. Deer mice, Peromyscus
maniculatus, are naturally promiscuous and males invest significantly more
in sperm production than males of P. polionotus, their monogamous
sister‐species. Here, we show that the larger testes in
P. maniculatus are retained after decades of enforced monogamy in
captivity. While these results suggest that differences in sperm
production between species with divergent evolutionary histories can be
maintained in captivity, we also show that the early rearing environment
of males can strongly influence their testis size as adults. Using a
second‐generation hybrid population to increase variation within the
population, we show that males reared in litters with more brothers
develop larger testes as adults. Importantly, this difference in testis
size is also associated with increased fertility. Together, our findings
suggest that sperm production may be both broadly shaped by natural
selection over evolutionary timescales and also finely tuned during early
development.