Gita R. Kolluru

Sex differences in metabolic rate (MR) can result from dimorphism in the performance of energetically demanding activities. Male crickets (Teleogryllus oceanicus) engage in costly calling and aggressive activity not performed by females. Consistent with this difference, we found higher maximal MR, factorial scope, and fat content in males than females. T. oceanicus song is also costly because it attracts the parasitoid fly Ormia ochracea. Parasitized crickets had reduced maximal MR consistent with a metabolic cost to harboring larvae. This cost was greater for females, either because females invest more heavily into reproduction at the expense of metabolic capacity, or because males are under stronger selection to respond to infection. Little is known about O. ochracea outside of its auditory system and parasitic lifestyle. We observed greater resting MR in male flies, possibly reflecting a sex difference in the requirement for metabolic power output, because male flies perform potentially costly mating behavior not seen in females. We found a positive relationship between larval density within a cricket and pupal resting MR, suggesting that crickets in good condition are able to both harbor more larvae and produce larvae with higher resting MR. These results reveal a complex interplay between the metabolism of crickets and their fly parasitoids.

Food availability can influence the optimal allocation of time and energy among alternative behaviors such as foraging, courting, and competing for mates. If populations differ consistently in food availability, selection may cause geographic divergence in allocation strategies. At the opposite extreme, a norm of reaction may evolve such that food intake influences the allocation strategy of individuals in the same way in all populations. Between these two extremes, food intake reaction norms may diverge genetically among populations. For example, at sites where food is scarce, selection may strengthen the effect of food intake on behavior, whereas at sites with abundant food, selection may be weak or even oppose plasticity. We tested these ideas by raising male guppies from streams differing in food availability in a common laboratory environment on either low or high food levels, and then observing them in the presence of male competitors (from the same population and diet group) and receptive females. Males from low-food-availability streams spent more time foraging than males from high-food-availability streams, independent of food intake. Compared with males raised on the high food level, males raised on the low food level spent more time foraging and were less aggressive towards other males. Courtship display rate increased with food intake but only in males from low-food streams. In contrast, males from high-food streams showed greater plasticity with respect to male-male aggression. These results generally support the resource availability/behavioral tradeoff hypothesis while also revealing a surprising degree of ontogenetic complexity in a relatively simple system.

Some populations of the field cricket Teleogryllus oceanicus are parasitized by the phonotactic fly Ormia ochracea. Flies locate crickets by their song and deposit larvae onto them. The larvae develop inside the cricket for 1 week before killing the host upon emergence. The reproductive compensation hypothesis predicts that parasitized crickets should increase their reproductive effort during the initial stages of infestation to offset the loss of fitness resulting from their shortened life span. An alternative hypothesis predicts that parasitized crickets will decrease reproduction, either because they are unable to reproduce or because selection acting on the parasitoid favors decreased host reproduction. In laboratory experiments, parasitized male crickets had reduced reproductive effort (spermatophore production, calling, mating activity, and mass allocated to reproductive tissue) compared to unparasitized males. Parasitized males fed ad libitum showed no evidence of allocating a greater proportion of their resources to reproduction. Parasitized and healthy males did not differ significantly in resting or maximal metabolic rates, although this may have been due to the substantial contribution of larval respiration to the metabolic rate of the host—parasitoid complex. These results are consistent with previous studies and suggest that T. oceanicus males parasitized by O. ochracea do not increase their reproductive effort. We discuss potential reasons that crickets do not increase reproductive effort in response to fly larvae and address difficulties in demonstrating altered life-history patterns in response to parasitism.