Cross-modal facilitation of auditory discrimination in a frog.

Stimulation in one sensory modality can affect perception in a separate modality, resulting in diverse effects including illusions in humans. This can also result in cross-modal facilitation, a process where sensory performance in one modality is improved by stimulation in another modality. For instance, a simple sound can improve performance in a visual task in both humans and cats. However, the range of contexts and underlying mechanisms that evoke such facilitation effects remain poorly understood. Here, we demonstrated cross-modal stimulation in wild-caught túngara frogs, a species with well-studied acoustic preferences in females. We first identified that a combined visual and seismic cue (vocal sac movement and water ripple) was behaviourally relevant for females choosing between two courtship calls in a phonotaxis assay. We then found that this combined cross-modal stimulus rescued a species-typical acoustic preference in the presence of background noise that otherwise abolished the preference. These results highlight how cross-modal stimulation can prime attention in receivers to improve performance during decision-making. With this, we provide the foundation for future work uncovering the processes and conditions that promote cross-modal facilitation effects.

Light and noise pollution interact to disrupt interspecific interactions.

Studies on the consequences of urbanization often examine the effects of light, noise, and heat pollution independently on isolated species providing a limited understanding of how these combined stressors affect species interactions. Here, we investigate how these factors interact to affect parasitic frog-biting midges (Corethrella spp.) and their túngara frog (Engystomops pustulosus) hosts. A survey of túngara frog calling sites revealed that frog abundance was not significantly correlated with urbanization, light, noise, or temperature. In contrast, frog-biting midges were sensitive to light pollution and noise pollution. Increased light intensity significantly reduced midge abundance at low noise levels. At high noise intensity, there were no midges regardless of light level. Two field experiments controlling light and noise levels to examine attraction of the midges to their host and their feeding behavior confirmed the causality of these field patterns. These findings demonstrate that both light and noise pollution disrupt this host-parasite interaction and highlight the importance of considering interactions among species and types of pollutants to accurately assess the impacts of urbanization on ecological communities.

Collateral damage or a shadow of safety? The effects of signalling heterospecific neighbours on the risks of parasitism and predation.

Although males often display from mixed-species aggregations, the influence of nearby heterospecifics on risks associated with sexual signalling has not been previously examined. We tested whether predation and parasitism risks depend on proximity to heterospecific signallers. Using field playback experiments with calls of two species that often display from the same ponds, túngara frogs and hourglass treefrogs, we tested two hypotheses: (1) calling near heterospecific signallers attractive to eavesdroppers results in increased attention from predatory bats and parasitic midges (collateral damage hypothesis) or (2) calling near heterospecific signallers reduces an individual's predation and parasitism risks, as eavesdroppers are drawn to the heterospecifics (shadow of safety hypothesis). Bat visitation was not affected by calling neighbours. The number of frog-biting midges attracted to hourglass treefrog calls, however, rose threefold when played near túngara calls, supporting the collateral damage hypothesis. We thus show that proximity to heterospecific signallers can drastically alter both the absolute risks of signalling and the relative strengths of pressures from predation and parasitism. Through these mechanisms, interactions between heterospecific guild members are likely to influence the evolution of signalling strategies and the distribution of species at both local and larger scales.

Reproductive State Modulates Retinal Sensitivity to Light in Female Túngara Frogs.

Visual cues are often a vital part of animal communication and courtship. While a plethora of studies have focused on the role that hormones play in acoustic communication of anurans, relatively few have explored hormonal modulation of vision in these animals. Much of what we do know comes from behavioral studies, which show that a frog's hormonal state can significantly affect both its visual behavior and mating decisions. However, to fully understand how frogs use visual cues to make these mating decisions, we must first understand how their visual system processes these cues, and how hormones affect these processes. To do this, we performed electroretinograms (ERGs) to measure retinal sensitivity of túngara frogs (Physalaemus pustulosus), a neotropical species whose mating behavior includes previously described visual cues. To determine the effect of hormonal state on visual sensitivity, ERGs were recorded under scotopic and photopic conditions in frogs that were either non-reproductive or hormone-treated with human chorionic gonadotropin (hCG) prior to testing. Additionally, measurements of optical anatomy determined how túngara frog eye and retina morphology related to physiological sensitivity. As expected, we found that both sexes display higher visual sensitivity under scotopic conditions compared to photopic conditions. However, hormone injections significantly increased retinal sensitivity of females under scotopic conditions. These results support the hypothesis that hormonal modulation of neural mechanisms, such as those mediating visually guided reproductive behavior in this species, include modulation of the receptor organ: the retina. Thus, our data serve as a starting point for elucidating the mechanism of hormonal modulation of visual sensitivity.

Behavioral and neural auditory thresholds in a frog.

Vocalizations play a critical role in mate recognition and mate choice in a number of taxa, especially, but not limited to, orthopterans, frogs, and birds. But receivers can only recognize and prefer sounds that they can hear. Thus a fundamental question linking neurobiology and sexual selection asks-what is the threshold for detecting acoustic sexual displays? In this study, we use 3 methods to assess such thresholds in túngara frogs: behavioral responses, auditory brainstem responses, and multiunit electrophysiological recordings from the midbrain. We show that thresholds are lowest for multiunit recordings (ca. 45 dB SPL), and then for behavioral responses (ca. 61 dB SPL), with auditory brainstem responses exhibiting the highest thresholds (ca. 71 dB SPL). We discuss why these estimates differ and why, as with other studies, it is unlikely that they should be the same. Although all of these studies estimate thresholds they are not measuring the same thresholds; behavioral thresholds are based on signal salience whereas the 2 neural assays estimate physiological thresholds. All 3 estimates, however, make it clear that to have an appreciation for detection and salience of acoustic signals we must listen to those signals through the ears of the receivers.