2.1 To eat or to mate? sexual cannibalism in mantodea and arachnid  (Page 2/15)

These conditions have created divergent optimum reproductive strategies between the sexes. Males are driven to mate as many times, and with as many females, as possible in order to maximize the number of eggs he fertilizes. Simultaneously, however, females seek to offset the disproportionate costs of reproduction by obtaining a paternal investment from the male, which can come in the form of food, protection, defense of resources, or other energetic expenditures. The female may select mates based on secondary sexual trait indicators of “good providers.” Inducing the male to bear a greater share of the reproductive burden allows the female to store resources for future offspring (reduces her parental investment) (Schneider and Lubin 1998).

Female selection for paternal investment may impose a suite of costs not directly associated with resource provision, such as elaborate male courtship costs (Schneider and Lubin 1998). In species of arachnids and insects, females often guard the ootheca before it hatches, but males do not directly provide parental care. Thus, parental investment and sexual conflict chiefly occur within the context of mating (Schneider and Fromhage 2005).

Life-history parameters such as the timing of sexual maturation and oviposition, expected number of matings, body size, maturity, and generation length are likely to influence the degree and outcome of intersexual conflict (Buskirk et al. 1984). Dynamics of functional morphology and ecology may also limit or enhance mating costs/benefits, impacting intersexual conflict (Andrade 2003).

Several hypotheses explain ways in which sexual cannibalism can occur by natural selection. This chapter first discusses the interpretation of sexual cannibalism as a case of paternal investment : females require substantial nutrients to produce their eggs, and extract food from the male’s own biomass. This “foraging strategy” hypothesis is based on experiments with mantises since male self-sacrifice drastically increases the reproductive yield of the female (Barry et al. 2008). Enhancing female fecundity improves the male’s direct fitness because he can pass on more of his genes through healthy offspring. This chapter also reviews corroborating evidence from redback spider males, who initiate their own cannibalism to maximize reproductive benefit in a hostile environment that opposes repeated mating (Andrade 1996). An outwardly brutal mating interaction may ultimately be adaptive for members of both sexes.

This chapter proceeds to examine the limitations of, and alternatives to, the above hypothesis. Many male spiders and mantises are not complicit in self-sacrifice, indicating that cannibalism is not adaptively favorable to males (Liske and Davis 1987; Wilder and Rypstra 2008). Additionally, sexual cannibalism is prominent in size dimorphic spider species, in which males are too small to substantively feed females. These observations give rise to claims that cannibalism in the context of mating is actually a non-sexual, predator-prey interaction (Wilder and Rypstra 2008b). Alternatively, a sexual selection hypothesis suggests that females discriminately eliminate undesirable mates by devouring them (Elgar and Nash 1988). Overall, the examination of sexual cannibalism reveals the variable influences of species morphology and environmental conditions on a behavior’s evolutionary significance. Also, since both reproductive and mortality considerations are inherent in sexual cannibalism, examining its adaptive value offers intriguing insight into the fitness dynamics of sexual interactions