Nineteen Years of Consistently Positive and Strong Female Mate Preferences despite Individual Variation.

Sexual selection driven by mate choice has generated some of the most astounding diversity in nature, suggesting that population-level preferences should be strong and consistent over many generations. On the other hand, mating preferences are among the least repeatable components of an individual animal's phenotype, suggesting that consistency should be low across an animal's lifetime. Despite decades of intensive study of sexual selection, there is almost no information about the strength and consistency of preferences across many years. In this study, we present the results of more than 5,000 mate choice tests with a species of wild frog conducted over 19 consecutive years. Results show that preferences are positive and strong and vary little across years. This consistency occurs despite the fact that there are substantial differences among females in their strength of preference. We also suggest that mate preferences in populations that are primarily the result of sensory exploitation might be more stable over time than preferences that are primarily involved in assessing male quality.

Harmonic calls and indifferent females: no preference for human consonance in an anuran.

The human music faculty might have evolved from rudimentary components that occur in non-human animals. The evolutionary history of these rudimentary perceptual features is not well understood and rarely extends beyond a consideration of vertebrates that possess a cochlea. One such antecedent is a preferential response to what humans perceive as consonant harmonic sounds, which are common in many animal vocal repertoires. We tested the phonotactic response of female túngara frogs (Physalaemus pustulosus) to variations in the frequency ratios of their harmonically structured mating call to determine whether frequency ratio influences attraction to acoustic stimuli in this vertebrate that lacks a cochlea. We found that the ratio of frequencies present in acoustic stimuli did not influence female response. Instead, the amount of inner ear stimulation predicted female preference behaviour. We conclude that the harmonic relationships that characterize the vocalizations of these frogs did not evolve in response to a preference for frequency intervals with low-integer ratios. Instead, the presence of harmonics in their mating call, and perhaps in the vocalizations of many other animals, is more likely due to the biomechanics of sound production rather than any preference for 'more musical' sounds.

Psychophysics and the evolution of behavior.

Sensory information allows animals to interpret their environment and make decisions. The ways in which animals perceive and measure stimuli from the social and physical environment guide nearly every decision they make. Thus, sensory perception and associated cognitive processing have a strong impact on behavioral evolution. Research in this area often focuses on the unique properties of the sensory system of an individual species, yet certain relevant features of perception and cognition generally hold across taxa. One such general feature is the proportionally based translation of physical stimulus magnitude into perceived stimulus magnitude. This process has been recognized for over a century, but recent studies have begun to consider how a law of proportional psychophysics, Weber's law, exerts selective force in behavioral evolution.

Signal perception in frogs and bats and the evolution of mating signals.

Psychophysics measures the relationship between a stimulus's physical magnitude and its perceived magnitude. Because decisions are based on perception of stimuli, this relationship is critical to understanding decision-making. We tested whether psychophysical laws explain how female túngara frogs (Physalaemus pustulosus) and frog-eating bats (Trachops cirrhosus) compare male frog calls, and how this imposes selection on call evolution. Although both frogs and bats prefer more elaborate calls, they are less selective as call elaboration increases, because preference is based on stimulus ratios. Thus, as call elaboration increases, both relative attractiveness and relative predation risk decrease because of how receivers perceive and compare stimuli. Our data show that female cognition can limit the evolution of sexual signal elaboration.

Complexity increases working memory for mating signals.

Females often prefer to mate with males who produce complex signals. It is not clear why they do so. Females might prefer complexity if it predicts mate quality, or signals might evolve complexity to exploit females' sensory or cognitive biases. We tested whether complexity increases active time, the period over which a signal influences a receiver's response to that signal. Mating signals are often ephemeral, yet their active time has largely been ignored. Here we demonstrate that signal complexity influences active time in túngara frogs. Male advertisement calls consist of frequency sweeps (whines) followed by 0-7 high-frequency bursts (chucks). Females preferentially approach complex (whines with chucks) over simple (whines alone) calls but do not consistently prefer greater complexity, so the function of multiple chucks has been uncertain. We found that females remember which speaker previously broadcast complex calls when choosing between simple calls broadcast after a delay. This effect occurred for calls with multiple chucks, but not with single chucks. Neither motivation nor orientation behavior differed with chuck number, suggesting that results are due to differential memory. Thus, female memory could favor the evolution of increasing signal complexity through sexual selection.