There is considerable evidence that males will increase the number of sperm ejaculated in response to sperm competition risk. However, whether they have the capacity to adjust seminal fluid components of the ejaculate has received less attention. Male crickets () have been shown to adjust the viability of sperm in their ejaculate in response to sperm competition risk. Here we show that socially mediated plasticity in sperm viability is probably due, at least in part, to male adjustments in the protein composition of the seminal fluid. Seven seminal fluid protein genes were found to have an increased expression in males exposed to rival calls. Increased expression of these genes was correlated with increased sperm viability in whole ejaculates, and gene knockdown confirmed that at least one of these proteins promotes sperm viability. Our results lend support for recent theoretical models that predict complex responses in male allocation to seminal fluid composition in response to sperm competition risk.

Mounting evidence suggests that nongenetic paternal effects on offspring may be widespread among animal taxa, but the mechanisms underlying this form of nongenetic inheritance are not yet fully understood. Here, we show that seminal fluids underlie paternal effects on early offspring survival in an insect, the cricket Teleogryllus oceanicus, and quantify the contribution of this paternal effect to the inheritance of this important fitness trait. We used castrated males within a full‐sib half‐sib experimental design to show that seminal fluid donors were responsible for variation in the survival of developing embryos to hatching, and in their subsequent survival to adulthood. Increased expression of two seminal fluid protein genes, previously found to be positively associated with sperm quality, was found to be negatively associated with embryo survival. These nongenetic paternal effects hold important implications for the evolution of adaptive maternal responses to sperm competition, and more broadly for the interpretation of sire effects from classic quantitative genetic breeding designs.

A considerable body of evidence supports the prediction that males should increase their expenditure on the ejaculate in response to sperm competition risk. The prediction that they should reduce their expenditure with increasing sperm competition intensity is less well supported. Moreover, most studies have documented plasticity in sperm numbers. Here we show that male crickets exhibit reduced seminal fluid gene expression and accessory gland mass in response to elevated sperm competition intensity. Together with previous research, our findings suggest that strategic adjustments in seminal fluid composition contribute to competitive fertilization success in this species.

Recent interest has focused on the role of reactive oxygen species (ROS) as universal constraints in life-history evolution. Empirical studies have examined the oxidative costs of reproduction for females, with little work conducted on males. The male germline is thought to be particularly susceptible to oxidative damage because the testes, and the sperm themselves, can be prolific producers of ROS. We tested the hypothesis that protection of the male germline from oxidative damage represents a cost of reproduction for males. We fed male crickets, , with one of two experimental diets in which we manipulated the availability of dietary antioxidants, and we induced variation in their expenditure on courtship effort by manipulating access to females. We measured the total antioxidant capacity, levels of ROS production and the amount of oxidative damage to proteins in both testis and thoracic muscle tissues. Dietary antioxidants contributed to positive oxidative balance in both tissue types. Although the testes had greater antioxidant defences than muscle tissue, they also produced considerably higher levels of ROS and sustained higher levels of oxidative damage. Courtship effort had no impact on any measure of oxidative balance. Our data confirm that the male germline is especially susceptible to oxidative stress and that dietary antioxidants can alleviate this oxidative cost of reproduction.

Competitive fertilization success can depend on the relative abilities of competing males to fertilize available ova, and on mechanisms of cryptic female choice that moderate paternity. Competitive fertilization success is thus an emergent property of competing male genotypes, female genotype and their interactions. Accurate estimates of intrinsic male effects on competitive fertilization success are therefore problematic. We used a cross-classified nonbreeding design in which rival male family background was standardized to partition variation in competitive fertilization success among male and female family backgrounds in the field cricket Teleogryllus oceanicus. Male effects were close to zero, supporting previous quantitative genetic designs in which male competitors were assigned at random. In contrast, some 22% of the variance in competitive fertilization success was explained by female effects, suggesting that paternity in this species is influenced strongly by cryptic female choice.