*Grace Day*
Sexual selection, a derivation of natural selection in which some individuals out-reproduce others because they are more attractive mates, takes place in all species. In order to understand sexual selection, simply ask yourself, would you rather marry Ryan Gosling or the writer of this paper? Here many would choose Gosling and therefore he would have a much likelier chance of out-reproducing the author of this very article.
Through evolutionary history, the strength, direction, and form of intra- and intersexual selection has been shown to affect the outcome of a given sexual trait. Female mate choice and male-male competition are well-known processes that drive the engine of sexual selection. In order to study the combined effect of intra- and intersexual selection on a given sexual trait, sensory ecologists studied frogs of the genus Eupsophus inhabiting the temperate forests of Chile and Argentina. The rosy ground frog (Eupsophus roseus) is a favorable model organism for this study because their croaks, known as advertisement vocalizations, provide a dual function; attracting females and maintaining separation among males. The dual function of these calls in the rosy ground frog provides the opportunity to determine how inter- and intrasexual selection interact on a given sexual trait.
In the study, they sought to look at three variables; the preference of females for specific variables in the advertisement calls, determine which of the signal values also attracted conspecific males, and lastly to evaluate how inter- and intrasexual selection (female attraction and conspecific male attraction) interact in selecting the given characteristics of a sexual trait.
In intersexual selection (female choice), they found that females prefer lower frequency calls and this preference could lead to a negative selection (the selective removal of alleles that are possibly harmful or not beneficial). In addition, past studies have found that females prefer lower frequency calls because they are correlated to larger male body size. However, in intersexual selection (male-male competition), there was no clear differentiation in responses. This is possibly due to the varying competitive abilities of the males (i.e. size, fighting skills, attractiveness) which was not taken into account in this study. Therefore, each male would react differently to advertisement calls depending on their competitive ability. Two current hypotheses that may explain this phenomenon are the satellite male hypothesis and the aggressive displacement hypothesis. The satellite male hypothesis states that unattractive males gather near attractive males in an attempt to intercept females. On the other hand, the aggressive displacement hypothesis claims that males congregate to engage in calling contests. Further studies should look to evaluate the two hypotheses by associating the competitive abilities of males with the calls that are most preferred by the females.
Finally, after considering the effect of inter- and intrasexual selection on the characteristics of advertisement calls, they found that female choice has a greater impact than male-male competition. In the end, there is a negative directional selection on the frequency and stabilizing selection on the amplitude ratio (between the second and third harmonic). This is shown in Figure 1 taken from Moreno-Gomex et al. 2015).
Figure 1. Distribution of acoustic variables in advertisement calls by males (F2=2nd harmonic, AR=Amplitude ratio between 2nd and 3rd harmonic). (a) frequency; (b) amplitude ratio. Female preference for a certain frequency and amplitude ratio are shown in (c) and (d). Male nonpreference for those variables are shown on (e) and (f). Lastly the relative importance of inter- and intrasexual selection for frequency (g) and amplitude ratio (h) in three scenarios; intersexual > intrasexual (solid line), intersexual=intrasexual (dashed line), intersexual < intrasexual selection (dotted line).
This study found that males created a decrease in the potential effect of a female’s preference on the characteristics of the advertisement call showing that it is crucial to look at both forms of sexual selection in future studies. Therefore although females showed a clear preference for certain characteristics in a male’s call, it is also important to note the interaction between the call and other conspecific males that will affect the mating success of the advertising male.
References:
1. Moreno-Gomez, F.N., Bacigalupe, L.D., Silva-Escobar, A.A., Soto-Gamboa, M. (2015) Female and male phonotactic responses and the potential effect of sexual selection on the advertisement calls of a frog. Animal Behavior 104(2015) 79-86.
