Co‐Occurrence of Preference Functions and Acceptance Thresholds in Female Choice: Mate Discrimination in the Lesser Wax Moth

Abstract

Basic economic models adapted from foraging theory predict that decisions in mate choice may be determined either by Ôbest-of-nÕ preference functions or by sequential rules incorporating acceptance thresholds. However, in some species, more complex determinations that incorporate versions of both protocols are found. To understand the functions of co-occurring protocols, we studied mating decisions in the lesser wax moth, Achroia grisella (Lepidoptera: Pyralidae), an acoustic species in which females prefer males, the advertisement songs of which are delivered at relatively high Ôpulse-pairÕ rates. In addition to this preference, A. grisella females avoid mating with a male, the song of which does not exceed a minimum pulse-pair rate, and they hold to this criterion even when no other singing males are present and regardless of song amplitude. Thus, mating decisions are not simply based on acoustic power (pulse-pair rate · amplitude). We recorded male songs and female responses in an A. grisella population and found that male pulse-pair rates showed a median of 87/s and ranged from 50 to 115/s, while female acceptance thresholds for male song showed a median of 60/s and ranged from 30 to 105/s. The distributions of thresholds were approximately normal and were not significantly skewed toward the right. Male song rates declined slightly with age, but female thresholds remained stable over the adult lifespan. Both the male and female traits showed significant repeatability within individuals. Whereas phylogenetic inference indicates that hearing in pyralid moths originated as a means of avoiding predation by insectivorous bats, the specific distribution of female acceptance thresholds suggests that currently this protocol does not primarily function to preclude inappropriate, and potentially lethal, responses to bat echolocations: pulse rates in the searching-phase echolocations used by either aerial-hawking or substrate-gleaning bats mostly range from 10 to 20/s, and the lack of positive skew in the distribution of thresholds indicates an absence of directional selection from the left. Rather, we infer that thresholds augment preference functions in A. grisella by precluding mating with males which are markedly inferior in a critical song character. In Ethology 111, 609-625 (2005) Ó 2005 Blackwell Verlag, Berlin general, co-occurring protocols may be important where population density fluctuates markedly, as preference functions may be ineffective in preventing mating with inferior males when density is low.

Computational mate choice: Theory and empirical evidence

“…They observed a much stronger preference for large size when alternatives were presented in a paired fashion rather than sequentially, suggesting that females applied comparative decision rules only when simultaneous evaluation was possible. Brandt et al (2005) showed that lesser wax moth females, Achroia grisella, used both absolute and relative criteria to choose their mates: independent of the number of available prospective mates, they rejected mates that call at a rate lower than a fixed threshold, but, when several males called above the acceptance threshold, females chose the one with the highest rate.…”

Testing the fisherian mechanism: examining the genetic correlation between male song and female response in waxmoths

“…In addition to the above preference functions, which describe the way in which females might choose one male over another when given options, threshold values also exist for some of the song characters. A threshold value is most evident for pulse-pair rate, as females do not respond to male song below a certain rate (which ranges from 25 to 70 pulse pairs s -1 among females) regardless of the values of the song's other characters and even when that male is the sole one present (Brandt et al 2005). Female preference functions and threshold values may both impose directional selection on male song characters.…”

“…Moreover, the pulse-pair rate threshold represents a fundamental aspect of a female's behavioral response to male song, individual females vary considerably in this parameter, and it is repeatable (Jang et al 1997). Substantial overlap between the distributions of male pulse-pair rates and female pulse-pair rate thresholds often exists (Brandt et al 2005), indicating that this female response may impose directional selection on the male song character. That is, a certain percentage of males in a population will not even be considered as potential mates by a certain percentage of females.…”

Ultrasonic hearing in moths

“…A. grisella females preferentially oriented towards the calls of males that were louder, delivered sounds with higher rates and more evenly spaced pairs of pulses, and included longer pulse lengths and longer silent intervals within pairs of pulses. A male may have more attractiveness to females, if it sings with higher peak amplitudes and longer asynchrony intervals and for longer nightly periods (Brandt & Greenfi eld 2004;Brandt et al 2005). Signal preference was based on acoustic energy and the power of the male signals (Jang & Greenfi eld 1996).…”