Variation in the rates of senescence among populations may be due to difference in extrinsic mortality. Previous research on the effects of extrinsic mortality on senescence has focused on factors such as predation and resource availability. However, resource quality, independent of resource availability, may influence rates of extrinsic mortality. We examined the effects of resource quality on lifespan in the bean beetle (Callosobruchus maculatus). The bean beetle is an ideal model system for examining the effect of resource quality on lifespan, as it undergoes larval and pupal stages in the endosperm of legumes, and adults do not feed. We allowed female bean beetles to lay eggs on either of three bean species (mung, adzuki, or black-eyed pea) that vary in quality. To control for the effects of resource quantity, only beans with one egg were used. We recorded emergence mass, sex, and lifespan of the emerged adults.
Results/Conclusions
For all host types, females were significantly larger at emergence and lived significantly longer than males. As a result, we analyzed the data separately for the two sexes. For females, adults emerging from adzuki beans were significantly smaller than those emerging from either black-eyed peas or mung beans. However, adults that emerged from mung beans lived significantly longer than those emerging from either adzuki beans or black-eyed peas. Yet, when controlling for differences in mass, host type did not significantly affect lifespan. Similar to females, males that emerged from adzuki beans were significantly smaller than those emerging from either black-eyed peas or mung beans. In contrast to females, males emerging from black-eyed peas had the shortest lifespan. The median lifespan of males emerging from adzuki beans and mung beans were not significantly different. The effect of host type on lifespan in males did not change when the effects of mass were removed. Our results suggest that the effects of resource quality on extrinsic mortality differ between males and females. Resource quality appears to be more important in males, perhaps because they are smaller. As a result, how variation in resource quality may lead to differences in rates of senescence among populations may depend on which sex is considered.
