Environmental effects on sexual size dimorphism of a seed-feeding beetle

Abstract

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.

Behavior and Development of <I>Pseudacteon curvatus</I> (Diptera: Phoridae) Varies According to the Social Form of Its Host <I>Solenopsis invicta</I> (Hymenoptera: Formicidae) in Its Native Range

“…No sexual size dimorphism was found for P. cultellatus either (females, 0.31 Ϯ 0.03 mm, N ϭ 28; males, 0.29 Ϯ 0.03 mm, N ϭ 49; unpublished data). The sexual size dimorphism in arthropods is a result of additional larval instars, longer developmental periods, or higher growth rate in the largest sex (Esperck andTammaru 2006, Stillwell andFox 2007), although it is dependent on temperature and food quality (Parker andJohnston 2006, Stillwell andFox 2007). In the genus Pseudacteon, it is proposed that body size is determined by the host size .…”

Sex Differences in Phenotypic Plasticity Affect Variation in Sexual Size Dimorphism in Insects: From Physiology to Evolution

“…Much of this variation is genetically based and likely due to variation in selection, primarily sexual selection, among species/populations (50, 51). However, recent studies have shown that a considerable amount of intraspecific variation in SSD may be due to differences in phenotypic plasticity between males and females (51, 114, 115, 126). In this review, we examine how sex differences in phenotypic plasticity affect patterns of SSD in insects.…”

Equal temperature–size responses of the sexes are widespread within arthropod species

“…Variation in SSD can result when males and females respond differently to their environment (differential-plasticity hypothesis [18]), and this may be an important contributor to the observed variation in SSD. Thus the degree to which different environmental factors such as juvenile density, food quality or quantity, and temperature elicit contrasting plastic body size responses between the sexes should be informative [12,19]. Here, we specifically focus on sex-dependent differences in T-S responses, as this may ultimately help to elucidate the underlying drivers of both SSD and T-S responses.…”