Gerlinde Höbel

REPRODUCTIVE CHARACTER DISPLACEMENT IN THE ACOUSTIC COMMUNICATION SYSTEM OF GREEN TREE FROGS (HYLA CINEREA)

Abstract Interactions between species can affect the evolution of their sexual signals, receiver selectivity, or both. One commonly expected outcome is reproductive character displacement, whereby adverse consequences of mismating select for greater differentiation of communication systems in areas of sympatry than in areas of allopatry. We found evidence of reproductive character displacement in the acoustic communication system of green tree frogs (Hyla cinerea). The strength of female preferences for the spectral properties of calls that distinguish conspecific calls from those of a closely related congener, H. gratiosa, was greater in areas of sympatry with H. gratiosa than in areas of allopatry. We also found subtle differences in advertisement calls and in the heights of male calling perches when we restricted our comparisons to localities in which H. gratiosa was also breeding (syntopy) with localities where this species was absent. Hyla cinerea and H. gratiosa show only weak genetic incompatibility, but the calls representative of interspecific hybrids were unattractive to females of both parental species. Hybrids might also be at an ecological disadvantage because of different habitat preferences of the two taxa. Thus, selection against production of less fit or less attractive hybrid or backcross offspring are probably the main causes responsible for the differences documented in this paper.

Diversification under sexual selection: the relative roles of mate preference strength and the degree of divergence in mate preferences

The contribution of sexual selection to diversification remains poorly understood after decades of research. This may be in part because studies have focused predominantly on the strength of sexual selection, which offers an incomplete view of selection regimes. By contrast, students of natural selection focus on environmental differences that help compare selection regimes across populations. To ask how this disparity in focus may affect the conclusions of evolutionary research, we relate the amount of diversification in mating displays to quantitative descriptions of the strength and the amount of divergence in mate preferences across a diverse set of case studies of mate choice. We find that display diversification is better explained by preference divergence rather than preference strength; the effect of the latter is more subtle, and is best revealed as an interaction with the former. Our findings cast the action of sexual selection (and selection in general) in a novel light: the strength of selection influences the rate of evolution, and how divergent selection is determines how much diversification can occur. Adopting this view will enhance tests of the relative role of natural and sexual selection in processes such as speciation.

Evolution of advertisement signals in North American hylid frogs: vocalizations as end-products of calling behavior

We studied the advertisement signals in two clades of North American hylid frogs in order to characterize the relationships between signal acoustic structure and underlying behavior. A mismatch was found between the acoustic structure and the mechanism of sound production. Two separate sets of phylogenetic characters were coded following acoustic versus mechanistic criteria, and exploratory treatments were made to compare their respective phylogenetic content in comparison with the molecular phylogeny ( Faivovich et al., 2005 ). We discuss the consequences of the acoustic/mechanistic mismatch in terms of significance of acoustic characters for phylogenetic and comparative studies; and the evolution of vocalizations in North American treefrogs. Considering only the acoustic structure of frog vocalizations can lead to misleading results in terms of both phylogenetic signal and evolution of vocalizations. In contrast, interpreting the acoustic signals with regard to the mechanism of sound production results in consistent phylogenetic information. The mechanistic coding also provides strong homologies for use in comparative studies of frog vocalizations, and to derive and test evolutionary hypotheses.

Nephila clavipes spiders (Araneae: Nephilidae) keep track of captured prey counts: testing for a sense of numerosity in an orb-weaver

Nephila clavipes golden orb-web spiders accumulate prey larders on their webs and search for them if they are removed from their web. Spiders that lose larger larders (i.e., spiders that lose larders consisting of more prey items) search for longer intervals, indicating that the spiders form memories of the size of the prey larders they have accumulated, and use those memories to regulate recovery efforts when the larders are pilfered. Here, we ask whether the spiders represent prey counts (i.e., numerosity) or a continuous integration of prey quantity (mass) in their memories. We manipulated larder sizes in treatments that varied in either prey size or prey numbers but were equivalent in total prey quantity (mass). We then removed the larders to elicit searching and used the spiders’ searching behavior as an assay of their representations in memory. Searching increased with prey quantity (larder size) and did so more steeply with higher prey counts than with single prey of larger sizes. Thus, Nephila spiders seem to track prey quantity in two ways, but to attend more to prey numerosity. We discuss alternatives for continuous accumulator mechanisms that remain to be tested against the numerosity hypothesis, and the evolutionary and adaptive significance of evidence suggestive of numerosity in a sit-and-wait invertebrate predator.

Adaptive plasticity in calling site selection in grey treefrogs (Hyla versicolor)

Calling behaviour is an essential component of gaining access to mates, and calling site selection may be an important component of effective communication. Environmental factors like microclimate, or the presence of competitors and predators often show seasonal or spatial variation, and behavioural plasticity that allows the caller to adjust to this variation may be adaptive. Prompted by the observation of across-season variation in elevated calling site use in our grey treefrog population, we formulated three hypotheses about their calling site choice and tested them using field observations and behavioural trials in the lab. We found that calling site selection is largely determined by local temperature regimes, and suggest that this temperature-based plasticity in calling site selection is adaptive because it allows males to increase their metabolic efficiency and mate attraction effectiveness. The mere presence of heterospecific competitors and predators did not affect calling site selection at the pond, but close proximity to a predator during behavioural trials did reduce calling activity. This suggests that grey treefrog males attend to the presence of predators, that they can assess the degree of risk associated with predator proximity, and that they can adjust calling behaviour adaptively to reduce the chances of being detected by a predator.

Socially mediated plasticity of chorusing behavior in the gladiator frog Hypsiboas rosenbergi

Competition for mates often occurs in social settings composed of many displaying males. While this poses some special challenges for communication, the proximity of other males may also provide information that chorus participants may use to adaptively adjust their calling behavior to the local level of competition. Conducting behavioral observations in natural choruses as well as playback experiments with focal males, I investigated socially mediated plasticity in the calling behavior of a neotropical gladiator frog, Hypsiboas rosenbergi. In natural choruses, male call rate was negatively correlated with the distance to the nearest calling neighbor, suggesting that the presence and distance of competing males influences call rate in this species. In a playback experiment that tested for the effect of the presence and competitiveness of rivals, H. rosenbergi males proved able to quickly notice changes in their acoustic environment and to respond to those changes in a graded fashion. As competition increased (either by increasing the number or attractiveness of the simulated rivals), males raised their calling rate; as competition was reduced, males lowered their calling rate. This indicates that males can perceive changes in their social environment and modify their calling behavior in ways that reflect the level of competition in the chorus. This socially mediated plasticity in calling behavior may help males to economize their energy reserves.

Memory of prey larders in golden orb-web spiders, Nephila clavipes (Araneae: Nephilidae)

Nephila clavipes spiders accumulate prey larders on their webs. We conducted a field experiment to ask if the spiders search for larders that have been pilfered (experimentally mimicking the potential effect of kleptoparasites), and to ask if the spiders vary their search efforts according to the size of the larder. All spiders searched for larders removed from their web, and spiders that lost larger larders (i.e., consisting of more prey items) searched for longer intervals. We thus suggest that N. clavipes form memories of the size of the larders they have accumulated, and that they use those memories to regulate recovery efforts when the larders are pilfered. The content of those memories may include discrete prey counts or the accumulation of a continuous variable correlated with counts, such as the total mass of captured prey. We discuss the adaptive significance of this ability in the framework of costs related to kleptoparasites and the ecology of food hoarding.

Describing mate preference functions and other function-valued traits

Mate preferences are important causes of sexual selection. They shape the evolution of sexual ornaments and displays, sometimes maintaining genetic diversity and sometimes promoting speciation. Mate preferences can be challenging to study because they are expressed in animal brains and because they are a function of the features of potential mates that are encountered. Describing them requires taking this into account. We present a method for describing and analysing mate preference functions, and introduce a freely available computer program that implements the method. We give an overview of how the program works, and we discuss how it can be used to visualize and quantitatively analyse preference functions. In addition, we provide an informal review of different methods of testing mate preferences, with recommendations for how best to set up experiments on mate preferences. Although the program was written with mate preferences in mind, it can be used to study any function‐valued trait, and we hope researchers will take advantage of it across a broad range of traits.

Wood frogs (Lithobates sylvaticus) use water surface waves in their reproductive behaviour

The ability to sense water surface waves has been described in only a few species, but across a wide taxonomic range. Water surface waves are typically used to localize prey or to avoid predators, and in some cases also for sexual communication. Here we add to the sparse knowledge of the use of this sensory modality by reporting observational and experimental evidence that wood frogs (Lithobates sylvaticus) respond to water surface waves generated by conspecifics; that there are pronounced differences in response between males and females; and that they use surface waves in a behavioural context not previously reported for anuran reproductive behaviour: sexual eavesdropping. Because the water waves that elicit the described responses are incidental by-products of calling and locomotion behaviour, we consider this an example of sexual eavesdropping rather than sexual communication. Males quickly and accurately approach a surface wave source, thus aiding in mate acquisition which in this species is mainly achieved by scramble competition. By contrast, females move away from a surface wave source. This may help them avoid sexual harassment by mate-searching males. Because it assures that only the fastest, strongest, and potentially fittest males can amplex them, it may also be a strategy for indirect mate choice by females.

Effect of temporal and spectral noise features on gap detection behavior by calling green treefrogs

Communication plays a central role in the behavioral ecology of many animals, yet the background noise generated by large breeding aggregations may impair effective communication. A common behavioral strategy to ameliorate noise interference is gap detection, where signalers display primarily during lulls in the background noise. When attempting gap detection, signalers have to deal with the fact that the spacing and duration of silent gaps is often unpredictable, and that noise varies in its spectral composition and may thus vary in the degree in which it impacts communication. I conducted playback experiments to examine how male treefrogs deal with the problem that refraining from calling while waiting for a gap to appear limits a males ability to attract females, yet producing calls during noise also interferes with effective sexual communication. I found that the temporal structure of noise (i.e., duration of noise and silent gap segments) had a stronger effect on male calling behavior than the spectral composition. Males placed calls predominantly during silent gaps and avoided call production during short, but not long, noise segments. This suggests that male treefrogs use a calling strategy that maximizes the production of calls without interference, yet allows for calling to persist if lulls in the background noise are infrequent.