Sex can be dangerous: Acoustically-orienting parasitoids on field crickets (Orthoptera: Gryllidae)

The Orthopterists\u27 Society generously awarded me grants in 1995 and 1997 to conduct research on Teleogryllus oceanicus (Orthoptera: Gryllidae) on the Big Island of Hawaii. Here I report results to date from fieldwork conducted in the past few years

Sex differences in metabolic rates in field crickets and their dipteran parasitoids

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

Environmental and genetic influences on mating strategies along a replicated food availability gradient in guppies (\u3cem\u3ePoecilia reticulata\u3c/em\u3e)

Food availability is expected to influence the relative cost of different mating tactics, but little attention has been paid to this potential source of adaptive geographic variation in behavior. Associations between the frequency of different mating tactics and resource availability could arise because tactic use responds directly to food intake (phenotypic plasticity), because populations exposed to different average levels of food availability have diverged genetically in tactic use, or both. Different populations of guppies (Poecilia reticulata) in Trinidad experience different average levels of food availability. We combined field observations with laboratory “common garden” and diet experiments to examine how this environmental gradient has influenced the evolution of male mating tactics. Three independent components of variation in male behavior were found in the field: courtship versus foraging, dominance interactions, and interference competition versus searching for mates. Compared with low-food-availability sites, males at high-food-availability sites devoted more effort to interference competition. This difference disappeared in the common garden experiment, which suggests that it was caused by phenotypic plasticity and not genetic divergence. In the diet experiment, interference competition was more frequent and intense among males raised on the greater of two food levels, but this was only true for fish descended from sites with low food availability. Thus, the association between interference competition and food availability in the field can be attributed to a genetically variable norm of reaction. Genetically variable norms of reaction with respect to food intake were found for the other two behavioral components as well and are discussed in relation to the patterns observed in the field. Our results indicate that food availability gradients are an important, albeit complex, source of geographic variation in male mating strategies

Sex–specific effects of carotenoid intake on the immunological response to allografts in guppies (\u3cem\u3ePoecilia reticulata\u3c/em\u3e)

Rarely are the evolutionary origins of mate preferences known, but, recently, the preference of female guppies (Poecilia reticulata) for males with carotenoid‐based sexual coloration has been linked to a sensory bias that may have originally evolved for detecting carotenoid‐rich fruits. If carotenoids enhance the immune systems of these fishes, as has been suggested for other species, this could explain the origin of the attraction to orange fruits as well as the maintenance of the female preference for orange males. We used the classic immunological technique of tissue grafting to assay a component of the immune response of guppies raised on two different dietary levels of carotenoids. Individual scales were transplanted between pairs of unrelated fishes, creating reciprocal allografts. Transplanted scales were scored on a six‐point rejection scale every day for 10 days. Five days later, the same pairs of fishes received a second set of allografts and were scored again. Compared with low‐carotenoid‐diet males, high‐carotenoid‐diet males mounted a significantly stronger rejection response to the second allograft but not to the first allograft. High‐carotenoid‐diet females, however, showed no improvement in graft rejection compared with low‐carotenoid‐diet females. To our knowledge, this is the first experimental evidence for sex‐specific effects of carotenoid consumption on the immune system of a species with carotenoid‐based sexual coloration. These results are consistent with the hypothesis that the mate preference for carotenoid coloration is maintained by the benefits to females of choosing healthy mates, but they cast doubt on the idea that the benefits of carotenoid consumption, per se, could account for the origin of the preference. The sex‐specificity of carotenoid effects on allograft rejection in guppies provides indirect support for the general hypothesis that males pay an immunological cost for sexual ornamentation

Variation and Repeatability of Calling Behavior in Crickets Subject to a Phonotactic Parasitoid Fly

Male Teleogryllus oceanicus (Orthoptera: Gryllidae) produce a conspicuous calling song to attract females. In some populations, the song also attracts the phonotactic parasitoid fly Ormia ochracea (Diptera: Tachinidae). I examined the factors affecting calling song by characterizing the calling behavior of caged crickets from an area where the fly occurs. Calling activity (proportion of time spent calling) was repeatable and a significant predictor of female attraction. However, calling activity in the parasitized population was lower than in an unparasitized Moorea population (Orsak, 1988), suggesting a compromise between high activity to attract females and low activity to avoid flies. Calling activity peaked simultaneously with fly searching, so crickets did not shift to calling when the fly is less active. Males harboring larvae did not call less than unparasitized males; however, a more controlled study of the effects of parasitization on calling behavior is needed to evaluate this result. The results are discussed in the context of other studies of the evolutionary consequences of sexual and natural selection on cricket calling behavior

Exploitations of Sexual Signals by Predators and Parasitoids

Signals used to attract mates are often conspicuous to predators and parasites, and their evolution via sexual selection is expected to be opposed by viability selection. Many secondary sexual traits may represent a compromise between attractiveness and avoidance of detection. Although such signal exploitation appears to be widespread, most examples come from species that use acoustic or olfactory mating signals, and relatively few cases of visual signal exploitation can be substantiated. Because males are usually the signaling sex, they are more at risk from predators or parasitoids that locate prey or hosts by sexual signals; this differential selection on the two sexes can affect the intensity of sexual selection on male ornamental traits. The notable exception to male signaling and female attraction occurs in pheromone-producing insects, particularly lepidopterans, which show an opposite pattern of female odor production. Exploitation of such sex pheromones is relatively rare. We discuss reasons for the reversal in sex roles in these species and its implications for signal exploitation. Changes in signals that appear to be adaptations to avoid predation include the use of different signal modalities, changes in signaling behavior, loss of signals, and alteration of signal characteristics such as pitch. Selection pressure from signal exploiters could lead to the production of a novel signal and thus facilitate speciation. Relatively little work has been done on adaptations on the part of the exploiting species, but such adaptations could indirectly influence the mating system of the predator or parasitoid. Signal exploitation is also expected to be a fruitful source of examples of coevolution. Finally, plants emit attractants analogous to secondary sex characters in animals, and may also be vulnerable to signal exploitation

Parasitism patterns and the size-fecundity relationship in the acoustically orienting dipteran parasitoid \u3cem\u3eOrmia ochracea\u3c/em\u3e

Female parasitoids are expected to distribute offspring among hosts in a manner that maximizes fitness. Several theoretical and empirical studies have focussed on clutch-size patterns in hymenopteran parasitoids. In contrast, dipteran parasitoids, which differ from hymenopterans in potentially important ways, have received little attention. The phonotactic tachinid fly Ormia ochracea has been intensively studied for its effects on host crickets, and has recently been the subject of studies of its own reproductive biology. This work suggests a negative relationship between clutch size and progeny fitness (consistent with hymenopterans), but no adjustment of clutch size to host size (different from hymenopterans). However, the repeatability of these patterns and the relationship between fly size and fitness remain to be demonstrated. We examined clutch sizes of O. ochracea larvae in the cricket Teleogryllus oceanicus. Most clutches were smaller than a cricket can support to pupation. Smaller clutches yielded larger offspring and larger wild-caught flies had higher fecundity, supporting the idea that small clutches yield higher fitness. However, although parasitised male crickets were slightly larger than unparasitised males, there was no correlation between cricket size and the number of larvae. Potential reasons for this departure from the patterns found in hymenopteran parasitoids are discussed

Genetic and social control of male maturation in \u3cem\u3ePhallichthys quadripunctatus\u3c/em\u3e (Pisces: Poeciliidae)

Age and size at maturity can have significant fitness consequences. Selection often favors early-maturing individuals because of their higher survival to maturity and greater relative contribution to population growth rate, but it may also favor delayed maturation if fitness increases with age or size at maturity. Males of several poeciliid fishes exhibit variation in age and size at maturity primarily controlled by a sex-linked gene called the P-locus. Wild-caught Phallichthys quadripunctatus males show a bimodal size distribution, which is often associated with a P-locus polymorphism in other poeciliids. We conducted two experiments to evaluate the inheritance of male age and size at maturity and the influence of social environment (presence of mature or juvenile males during development) on these traits. We specifically tested the hypothesis that male age and size at maturity in P. quadripunctatus are governed by a single Y-linked locus, and modified by the social environment. Although our results imply both a genetic and an environmental component to the dimorphism in maturation, both large and small males were able to sire both large and small sons, allowing us to reject the hypothesis that age and size at maturity in this species are controlled by a single, Y-linked locus. Our data do not conform adequately to any of the genetic mechanisms described to date for maturation polymorphism in poeciliids. We suggest alternative mechanisms that may operate in P. quadripunctatus

The effects of resource availability on alternative mating tactics in guppies (\u3cem\u3ePoecilia reticulata\u3c/em\u3e)

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