Male X. rufovillosumdonate on average 13.5% of their body weight to the female during copulation. The weight transferred is significantly greater in heavier males and declines in subsequent matings. When virgin females are offered a choice of several mates, they tend to mate with a heavy male. Females appear to be selective during courtship, refusing to mate with light males. Artificial weights attached to males increase the frequency of acceptance as mates by females, but not significantly. The evolutionary origins and significance of a large paternal investment and apparent female selection for heavy mates are discussed.