Temperature and host species affect nuptial gift size in a seed‐feeding beetle

Abstract

1 In many insects species, males contribute large nutritional gifts to females during mating, generally as seminal fluids (ejaculates) or spermatophores. These nuptial gifts can affect both male and female fitness, and can mediate selection on male body size. However, it is unclear how environmental variables, such as temperature and diet, affect gift size and the consequences of gift size for male and female fitness. 2 We examine how temperature and rearing host affect male nuptial gift size (both total ejaculate size and the proportion of a male's mass allocated to his seminal fluids), and the relationship between gift size and female reproduction, in two populations of the seed-feeding beetle Callosobruchus maculatus. 3 Males reared at lower temperature (20 °C) produced substantially larger ejaculates than males reared at higher temperatures (25, 30 and 35 °C). However, males allocated a smaller proportion of their body mass to their ejaculate at the lowest temperature compared with other temperatures. This effect of temperature on male allocation to their ejaculates mirrored the effect of temperature on female body size – male ejaculate size remained a relatively constant proportion of their mate's body mass across temperatures. 4 Rearing host also affected male ejaculate size but the magnitude and direction of the host effect differed between populations. 5 Rearing temperature affected the relationship between male body mass and ejaculate size. Temperature also affected the relationship between female body mass and fecundity. The relationship between male body mass and ejaculate size was significantly lower when beetles were reared on cowpea than when beetles were reared on azuki or mung. 6 We found no evidence that male body size or nuptial gift size affected female fecundity in either population of C. maculatus. We thus propose that the effect of nuptial gift size on male fitness is through a reduction in female mating frequency and thus increased paternity for males producing larger nuptial gifts.