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.

Many animal communication systems have evolved signal flexibility depending on environmental conditions. A common strategy of vocal communication is to increase amplitude in response to increasing noise levels. This phenomenon, known as the Lombard effect, is a widespread trait among mammals and birds. Anurans are a major group with many species that rely heavily on acoustic signals for sexual communication. Although these species often communicate in noisy environments, the presence of the Lombard effect in frogs remains unclear. We exposed male túngara frogs (Physalaemus pustulosus) to different noises with and without playback of conspecific calls. Males increased call amplitude, call rate, and call complexity in response to low-frequency noise (overlapping the species' call range) but not to high-frequency (nonoverlapping) noise. Vocal amplitude increased linearly with noise level demonstrating that túngara frogs exhibit the Lombard effect, and we discuss why different frog species may differ in their control over vocal amplitudes. Furthermore, we found the overall effect of noise to be similar to the effect of conspecific call playback. We speculate that vocal amplitude control may have evolved primarily as a response to increased competition at the cocktail party, similar to the way humans raise their voice when in a heated debate, and subsequently as a strategy to deal with background noise more generally.

Mate choice models derive from traditional microeconomic decision theory and assume that individuals maximize their Darwinian fitness by making economically rational decisions. Rational choices exhibit regularity, whereby the relative strength of preferences between options remains stable when additional options are presented. We tested female frogs with three simulated males who differed in relative call attractiveness and call rate. In binary choice tests, females' preferences favored stimulus caller B over caller A; however, with the addition of an inferior "decoy" C, females reversed their preferences and chose A over B. These results show that the relative valuation of mates is not independent of inferior alternatives in the choice set and therefore cannot be explained with the rational choice models currently used in sexual selection theory.

Sexual selection driven by mate choice has generated some of the most astounding diversity in nature, suggesting that populationlevel 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.

Animals choosing particular display sites often balance sexual and natural selection pressures. Here we assess how physical properties of display sites can alter this balance by influencing signal production and attractiveness of the túngara frog (Physalaemus pustulosus). Males that call from very shallow water bodies (few mm depth) benefit from reduced predation risk, but by manipulating water levels, we show that this comes at a cost of reduced attractiveness to females. Our data show that calling from shallower water reduces a male’s ability to float, limits the inflation of his vocal sac, and consequently reduces signal conspicuousness in terms of amplitude and complexity. Our results demonstrate that display site properties can set limits on signal production and attractiveness and may hence influence signal evolution. Signallers may shift between sites or engineer their display location, which can play a crucial role in signal divergence and speciation, particularly in a rapidly changing world.