Females crickets use song to assess male value

As discussed in class, mate attraction and courtship is influenced by a variety of factors, including operational sex ratio, male mating strategy, and the importance of intrasexual and intersexual selection. In many species of insects the operational sex ratio is skewed towards males due to their relatively less costly gamete production. As a result of the anisogamy, mating patterns tend to include a degree of intrasexual selection among males as they compete for access to females.

In crickets (Gryllus campestris), which exhibit a mobile mating strategy in which females search for mates, males engage in intersexual selection in order to secure and guard females. Females also exert intrasexual selection on their mates by being choosy during mate selection. Males in turn give their mates a small nuptial gift in the form of a spermatophore post-copulation. In addition to this resource, the males produce a variety of auditory signals in order to attract females and trigger mounting.

During the male-male fighting stage of mating, Rillich et al. noted a specific pattern to male aggression, whereby males would face one another and spread their mandibles. This would often escalate to physical fighting whereby they would lock mandibles. While this behavior is common among males, it had never been noted among female crickets until they were brought into the lab. Under lab conditions females were found to engage in aggressive behavior towards one another when placed in the vicinity of a male producing mating songs.

Rillich et al. tested the hypothesis that male calling, courtship, and rival songs signal a male’s resource value to females. They did this by taking a pair of weight match females and placing them in arena where they were subject to one of the three male songs, either directly from a male, or through playback. In the trials, males were either present and sang, present, but muted while playbacks were played, or not present while playbacks were played. The aggressive behavior between females in competition for the male was then measured by determining the percentage of bouts in which any of the following behaviors were exhibited: antennal contact (non-aggressive), antennal fencing, mandible spreading, or interlocked mandibles.

They found that male calling and courtship songs played with and without males present elicited significant increased female aggression. Courtship playbacks alone resulted in 81% of pairings escalating to antennae fencing (as opposed to 22% in the control), 69% escalating to mandible spreading (17% in control), and 39% resulting in physical fighting (3% in control). The fact that females fought for access to the male, and the resource he held, after only hearing his songs suggest that the male song carries information about his value.

In lecture the role of direct benefits during mating was discussed. The cricket spermatophore is a direct benefit to the female, and also a limited resource. Although females assess a male’s value based on a number of cues, including age, scent, condition, and song quality, the results of this study suggest that the visual, tactile, and olfactory cues do not play a significant role in causing female-female aggression, and hence do not play a significant role in female determination of male value. How females are able to quantify a male’s value by his song alone was not made clear by the study. Further research will need to be conducted to determine the auditory differences in songs of males of varying value.

Rillich, J., Edgar, B., Schildberger, K. & Stevenson, P.A. 2009. Female crickets are driven to fight by the male courting and calling songs. Animal Behaviour 77: 737-742.