Absolute Versus Relative Measurements of Sexual Selection: Assessing the Contributions of Ultrasonic Signal Characters to Mate Attraction in Lesser Wax Moths, Achroia grisella (Lepidoptera: Pyralidae)

Abstract

When females choose a mate among a group of signaling males concentrated in a small area, a male's mating success is often determined not only by his absolute attractiveness but by the attractiveness of his neighbors as well. Multivariate analyses of sexual selection measurements based on absolute values of predictor variables are then misleading, because such analyses assume that the fitness of a given individual is not influenced by others. We addressed this problem of relative fitness in sexual selection by developing two adjustments of the predictor variables in the multivariate analyses by including group means in addition to absolute values and by using relative values, deviations from group means. In the lesser wax moth, Achroia grisella (Lepidoptera: Pyralidae), aggregated males produce ultrasonic signals attractive to females in the vicinity of honey bee colonies. Playback experiments showed that females prefer signals whose pulses are louder, longer, delivered at a faster rate, and include lengthy silent gaps within pulse pairs (long asynchrony intervals). To measure sexual selection on ultrasonic signals, attractiveness was assessed by observing the number of females orienting toward given males in four‐choice trials. The various signal characters of the males were computed from ultrasound recordings. Multivariate regression and nonparametric analyses revealed that peak amplitude, asynchrony interval, and pulse rate were the direct targets of selection. Nonlinear stabilizing and correlational selection were also detected. Comparisons of results from multivariate analyses based on absolute values and on the two relative adjustments indicated that the quality of a four‐male group did not affect the strength or direction of linear selection but changed the strength of nonlinear selection. Both relative adjustments improved the prediction of male attractiveness. Three‐dimensional surface plots generated by the nonparametric regression analyses showed that attractiveness increased monotonically with any combination of the signal characters. This graphical technique also showed that female choice in A. grisella was based on a relative, as opposed to a threshold, decision rule. Overall, male attractiveness for the population remained consistent between testing days. However, attractiveness values for some individual males changed between successive testing days. In these cases, the change in attractiveness was related only to the change in peak amplitude.

Coevolution of costly mate choice and condition-dependent display of good genes

“…Perhaps the most important and readily tested assumption is that of relative preferences for mates (Zuk et al 1990;Hoikkala & Aspi 1993;Jang & Green® eld 1998). In some lek systems, one or a small number of males sometimes gain most of the matings.…”

Experimental Evidence for Multivariate Stabilizing Sexual Selection

“…Several other univariate studies (e.g., Ryan and Keddy-Hector 1992;Polakow et al 1995;Ritchie 1996;Murphy and Gerhardt 2000) indicate that preferences for intermediate values of call frequency and various temporal structure measures may likewise result in stabilizing selection on these properties. However, analyses need to account for the simultaneous effects of selection on all relevant aspects of the phenotype (Jang and Greenfield 1998;Ryan and Rand 2003), particularly when there are genetic or phenotypic correlations among traits. The components of call structure are often phenotypically correlated in grasshoppers (Klappert and Reinhold 2003), field crickets (Scheuber et al 2003;J.…”

“…Although tests for nonlinear sexual selection in a range of taxa have applied multivariate methods (Jang and Greenfield 1998;Blows et al , 2004Ryan and Rand 2003), none of these studies have found clear evidence of multivariate stabilizing selection. Ryan and Rand (2003) constructed artificial calls to explore the relationship between call properties and fitness in the Tú ngara frog (Physalemus pustulosus) unconstrained by phenotypic correlations among call properties.…”

Evaluation of amplitude in male song: female waxmoths respond to fortissimo notes

“…Playback experiments in which females were attracted to loudspeaker broadcasts of digitized song recordings indicate that phonotaxis thresholds may be as low as 50-60dBSPL [peak amplitude (Brandt et al, 2005) and unpublished data]. Studies of several A. grisella populations showed that females prefer male songs with pulses of greater peak amplitude that are delivered at a faster rhythm, and that include longer asynchrony intervals within the pulse pairs (Jang and Greenfield, 1996;Jang and Greenfield, 1998). A major portion of female evaluation of male song may be based on acoustic power, defined here as the product of mean peak amplitude and pulse pair rate: playback experiments using synthetic signals suggested that females were equally attracted to songs with low peak amplitude pulses delivered at a rapid pulse pair rate and to songs with high peak amplitude pulses delivered at a slow pulse pair rate, with acoustic power held constant in both songs (Greig and Greenfield, 2004).…”