Ann V. Hedrick

The evolution of sexual dimorphism in animals: Hypotheses and tests

Three major hypotheses, based upon mechanisms of sexual selection, intersexual food competition and reproductive role division, have been advanced to explain the evolution of sexual dimorphism in body size and morphology of animals. Genetic models suggest that all of the hypotheses are plausible, and empirical studies demonstrate that each of the three mechanisms operates in natural populations. However, problems arise in testing hypotheses for the evolution of sexual dimorphism: more than one mechanism may be operating simultaneously, and the demonstrated occurrence of a mechanism does not indicate that it actually results in selection for dimorphism. A recent statistical technique offers a solution to these problems and provides a promising new approach to the study of sexual dimorphism, in which researchers can assess the relative importance of each mechanism in present-day selection for sexual dimorphism within a species.

Female preferences for male calling bout duration in a field cricket

SummaryThe calls of male field crickets attract sexually receptive females. In Gryllus integer, males differ from one another in their durations of uninterrupted calling (calling bout lengths). Tape recordings of the calls of 50 wild-caught males revealed that 14 males spent most of their calling time in short bouts (Fig. 1A), 18 in both short and long bouts (Fig. 1B), and 18 in long bouts (Fig. 1C). Re-recordings of 32 males after 3 weeks showed that calling bout lengths of individual males are stable with time (age) (Fig. 2). Three phonotaxis experiments investigated whether calling bout lengths of males affect female preferences. They demonstrated that (1) females can discriminate among conspecific males on the basis of calls alone; (2) females are preferentially attracted to males with long calling bout lengths; and (3) calling bout length is the specific factor responsible for preferential attraction. These results precisely identify a criterion that females use to discriminate among potential mates of their own species.

Crickets with extravagant mating songs compensate for predation risk with extra caution

Modern models for the evolution of conspicuous male mating displays assume that males with conspicuous displays must bear the cost of enhanced predation risk. However, if males can compensate behaviourally for their increased conspicuousness by acting more cautiously towards predators, they may be able to lower this cost. In the field cricket Gryllus integer, males call to attract females, and differ in their durations of uninterrupted trilling (calling–bout lengths). Differences among males in calling–bout lengths are heritable, and females prefer males with longer calling bouts. In this study, males with longer, more conspicuous songs behaved more cautiously than males with shorter songs on two different tests of predator avoidance. They took longer to emerge from a safe shelter within a novel, potentially dangerous environment, and they ceased calling for a longer time when their calls were interrupted by a predator cue. Thus, these males appear to compensate behaviourally for their more conspicuous mating displays. Additionally, latencies to emerge from a shelter in the novel environment were consistent over time for both individual males from the field and males that had been reared in the laboratory, indicating that the differences in latency among males may be heritable.

Parasitism, predation and the evolution of animal personalities

Trade-offs between behavioural traits promoting high life-history productivity and mortality may fuel the evolution of animal personalities. We propose that parasites, including pathogens, impose fitness costs comparable to those from predators, and influence the adaptiveness of personality traits associated with productivity (PAPs). Whether personality traits are adaptive or not may also depend on individual immunological capacity. We illustrate this using a conceptual example in which the optimal level of PAPs depends on predation, parasitism and host compensation (resistance and tolerance) of parasitisms negative effects. We assert that inherent differences in host immune function can produce positive feedback loops between resource intake and compensation of parasitisms costs, thereby providing variation underlying the evolution of stable personalities. Our approach acknowledges the condition dependence of immune function and co-evolutionary dynamics between hosts and parasites.

Levels of predation and genetically based anti-predator behaviour in the spider, Agelenopsis aperta.

Abstract Two populations of the spider, Agelenopsis aperta, experience markedly different levels of avian predation. Although there is no measurable level of avian predation on this species inhabiting a grassland habitat, estimated bird predation on A. aperta in a riparian (woodland) habitat rangesfrom 15 to 86% of the local population per week. Population variability in anti-predator behaviour was tested in an experiment using large amplitude vibrations in the web to simulate a predator. Individuals from the respective populations were tested at their web-sites in the field and second-generation individuals reared in the laboratory were tested in the laboratory for their responses to this stimulus. In both the field and laboratory, significant population differences were observed in the latency to return to a foraging mode following retreat into the protection of the web-funnel. The population exposed to predation pressure by birds exhibited the longer latency to return. Within this population, moreover, latency to return to a foraging mode covaried at an individual level with latency to respond to prey encountering the web-sheet.

The scent of dominance: female field crickets use odour to predict the outcome of male competition

Pheromones may convey information about mate quality and social status. In the field cricket Gryllus integer, females mount the males for copulation, such that males cannot coerce females to mate. We examined whether virgin G. integer females preferred the scent of potentially dominant males to that of subordinate males. First, we collected pheromones by confining males on filter paper. Next, we offered filter paper from each of two size-matched males and control paper to females that had never been exposed to males, and measured the time spent by the female on each kind of paper. Finally, dominance status of the males in each size-matched pair was determined by pitting the two males against one another in agonistic contests. When offered filter paper from subsequently dominant versus subsequently subordinate males, females spent more time on the paper from the dominant male than the subordinate male, and much less time on control paper. Thus, pheromones may inform female G. integer about a males potential to achieve dominant social status. Male pheromones were also associated with the females tendency to mount a male. In contrast to cockroaches, where females prefer the scent of subordinate males (presumably to avoid risk of injury), female crickets prefer the scent of potentially dominant males and are more likely than males to wound their mating partners.

Sex differences in the repeatability of boldness over metamorphosis

Consistent individual differences in boldness have been identified in many species and can have important effects on fitness. In most animals, juveniles face different costs and benefits of risk-taking behavior than do adults. Furthermore, profound changes in hormones, morphology and environment often occur when juveniles become adults. Therefore, the boldness of individuals might change with ontogeny. In field crickets, adult males call to attract sexually receptive females, and male calling increases predation risk. We measured the repeatability of boldness (latency to emerge from a safe refuge) in both male and female crickets. Each cricket was tested once as a small nymph and once as an adult. We found that boldness was repeatable across metamorphosis in females, but not in males. Males became less bold with maturation, a result that we predicted because of the risk associated with calling for mates. We also found that in general, nymphs were bolder than adults and that individuals spent more time immobile in response to a predator cue when they were nymphs, versus when they were adults.

Detection of the Spider Predator, Hololena Nedra By Naïve Juvenile Field Crickets (Gryllus Integer) Using Indirect Cues

Summary In many species, prey detect predators using chemical cues, which may be based on the predator’s previous diet. Furthermore, the predator’s previous hunting strategy and diet may affect the prey’s behavioural strategies to avoid predation. Juvenile Gryllid field crickets face predation by spiders, but the responses of juvenile crickets to spiders are unknown. We studied whether naive juvenile field crickets can detect a predatory spider, Hololena nedra, using chemotactile cues (silk, secretions and excreta). We also studied the effect of the predator’s diet on the juvenile field cricket’s anti-predator behaviour. Cricket nymphs avoided the chemotactile spider cues when the spiders were previously fed with crickets, but not when spiders were not fed crickets. Our study suggests that the ability to detect the indirect cues of a potential predator may be an important factor determining the anti-predator responses of insects towards spiders.

Environmental condition-dependent effects on a heritable, preferred male trait

Models for the evolution of female mating preferences suggest that preferred male traits may be condition dependent. In the field cricket, Gryllus integer, a preferred male trait (calling-bout

Temperature preferences of male field crickets ( Gryllus integer) alter their mating calls.

Abstract. Temperature affects the mating displays of many ectothermic animals, yet almost no information exists on the temperature preferences of ectotherms while they are displaying for mates. This study investigated the preferences of displaying male field crickets (Gryllus integer) for microhabitats of different temperatures. G.integer males attract sexually receptive females by calling from cracks in the ground. We collected data from the field on the temperature of male calling sites (cracks in the ground), on the amount of herbaceous cover (which affects crack temperature) surrounding calling sites, and on the temporal properties of male calls at different temperatures. Laboratory experiments demonstrated that males prefer warmer sites and confirmed that temperature influences mating calls. We conclude that males of this ectothermic species prefer to call for mates from warmer sites, and that microhabitat choice on the basis of temperature affects their mating calls, and potentially their reproductive success.