Abstract

The outcome of post-copulatory sexual selection is determined by a complex
set of interactions between the primary reproductive traits of two or more
males and their interactions with the reproductive traits of the female.
Recently, a number of studies have shown the primary reproductive traits
of both males and females express phenotypic plasticity in response to the
thermal environment experienced during ontogeny. However, how plasticity
in these traits affects the dynamics of sperm competition remains largely
unknown. Here, we demonstrate plasticity in testes size, sperm size and
sperm number in response to developmental temperature in the bruchid
beetle Callosobruchus maculatus. Males reared at the highest temperature
eclosed at the smallest body size and had the smallest absolute and relative
testes size. Males reared at both the high- and low-temperature extremes
produced both fewer and smaller sperm than males reared at intermediate
temperatures. In the absence of sperm competition, developmental temperature
had no effect on male fertility. However, under conditions of sperm
competition, males reared at either temperature extreme were less competitive
in terms of sperm offence (P2), whereas those reared at the lowest temperature
were less competitive in terms of sperm defence (P1). This suggests
the developmental pathways that regulate the phenotypic expression of
these ejaculatory traits are subject to both natural and sexual selection: natural
selection in the pre-ejaculatory environment and sexual selection in
the post-ejaculatory environment. In nature, thermal heterogeneity during
development is commonplace. Therefore, we suggest the interplay between
ecology and development represents an important, yet hitherto underestimated
component of male fitness via post-copulatory sexual selection.