Intrinsic and extrinsic factors interact to mediate switches of mating tactics.

Sarah Conditt Humfeld, University of Missouri-Columbia.

Male mating behavior provides a fascinating example of the variation found among and within individuals. In many species, males can use different behaviors to obtain matings with females (e.g. displaying or fighting versus sneaking, mimicking females, or acting as a satellite, attempting to mate with females attracted to other males). I am interested in determining the conditions under which individuals make decisions about which mating behavior to use.

Alternative reproductive behaviors are usually considered to be a consequence of male-male competition for a limited number of females, in which only a subset of males can successfully employ the dominant mating tactic. Animals which produce a sexual display and switch between alternative mating behaviors over a short time period in response to local environmental conditions provide an opportunity to study the interplay of intrinsic (physiology) and extrinsic (communication environment) factors that mediate this decision. Male green treefrogs (Hyla cinerea) produce acoustic signals to attract females; however, some silent satellite males attempt to intercept females attracted to nearby calling males. Females select mates on the basis of the quality of their advertisement calls. The primary objective of my research is to determine how sexual selection, in the form of male-male competition and female mate choice, mediates the probability and conditions under which males adopt alternative mating tactics.

Satellite males appear to have intrinsic constraints on their competitive ability. These males tend to be smaller and skinnier than calling males. Condition may be more important than size in mating tactic decisions because satellite males that resumed calling were in better condition, but not larger than, males that remained satellites. Field observations and laboratory experiments indicate that males in poor condition produce less intense and higher frequency advertisement calls at a lower rate than similarly sized males in good condition. If these call characteristics are important for male-male competition or female choice, males in poor condition may adopt the satellite tactic because they are poor acoustic competitors.

Male frogs form aggregations where they produce advertisement calls. This chorusing behavior results in acoustic competition among neighbors to be detected and preferred by females. I conducted a field playback experiment in which I simulated the presence or a small or large competitor (high or low frequency call) at different distances (variable call intensity) and observed the calling and mating behaviors of a calling male to determine whether relative calling behavior mediates the decision to switch mating tactics. Males may switch to a satellite strategy because they are difficult to detect by females; although males that stopped calling did not produce softer calls, their calls were closer in frequency to the artificial stimulus. Males may also become satellites because their calls are unattractive; calling males unable to maintain high call rates switched to the satellite tactic, as did males with spectral components higher in frequency than those of the stimulus.

Playbacks of competing males along cannot indicate whether mating tactic switches are mediated via male-male competition or female choice. To determine whether female mate choice preferences might be important in mating tactic switches, I conducted a series of two speaker choice experiments to determine female preferences for several call characteristics. Females strongly preferred calls produced at higher rates and calls with lower-than-average frequency. Males switched tactics when they were competing with another male that was more attractive to females than themselves.

Condition-dependent sexual signaling and intersexual dynamics are important factors in mating tactic decisions. My results provide a foundation for future studies of conditional mating tactic adoption. What is the proximate role of sex and stress hormones on mating tactic decisions? How are changes in body mass are reflected in the frequency of the call? What are the effects of the chorus (background noise levels, frequency of satellites) on mating tactic decisions? Finally, I am interested in whether or not there is genetic variation for the behavioral switchpoints I observed in this population.