Sarah C. Humfeld

Plasticity of aggressive signalling and its evolution in male spring peepers, Pseudacris crucifer

Abstract Acoustic communication is important for determining and maintaining intermale spacing in breeding aggregations of anurans and insects. Because the number and proximity of signalling males can show extensive temporal and spatial variation, we should expect to find mechanisms that permit males to modify their signalling behaviour in ways that balance the needs to attract females and defend their calling space. We conducted two field playback experiments to investigate the role of plasticity in male aggressive signalling in intermale spacing of spring peepers (Anura, Hylidae). In the first experiment, we found a positive correlation between the amplitude of the advertisement calls of a males nearest neighbour and the stimulus amplitude at which the male first produced aggressive calls. In the second experiment, repeated presentations of advertisement calls above a males aggressive threshold resulted in rapid decreases in aggressive signalling and significant, but temporary, elevations of aggressive thresholds. We suggest that short-term habituation to a neighbours calls could function as a proximate mechanism for plasticity in aggression that would allow males to accommodate nearby callers while also tracking fluctuations in the local density of calling individuals. In a third experiment, we examined female choice as an ultimate-level explanation for plasticity in male aggression. Females preferred advertisement calls to aggressive calls, but this preference was weak and was abolished by a 6-dB reduction in the amplitude of the advertisement call. We suggest that female preferences probably function as only one possible source of selection on plasticity in male aggressive signalling and propose energetic limitations as an additional source of selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Intersexual dynamics mediate the expression of satellite mating tactics: unattractive males and parallel preferences

Male green treefrogs, Hyla cinerea, attract both potential mates and satellite males with their advertisement calls. Males using the satellite mating tactic position themselves silently near a calling male and attempt to intercept females attracted to the caller. Although use of conditional mating strategies may result from competition among males, few studies have considered the importance of female mate-choice preferences on the adoption and expression of alternative mate-acquisition behaviours. In this study, I tested two hypotheses. First, satellite males produce relatively unattractive advertisement signals and hence are at a competitive disadvantage when using the primary tactic. Second, satellite males associate with males producing the most attractive advertisement calls and hence increase the chance that they will intercept a female. Satellite males recorded in the field produced advertisement calls higher in frequency and shorter in duration than those produced by callers. Females consistently preferred the lower-frequency call alternative presented in two-speaker choice experiments, although they showed no preference in regard to call duration. In a second set of two-speaker choice experiments, noncalling males showed phonotactic behaviour similar to that of females, and their acoustic preferences roughly paralleled those of females. This phonotactic response may enable males producing unattractive calls to use the satellite mating tactic in a manner that maximizes potential reproductive fitness.

Temporal order and the evolution of complex acoustic signals

The evolution of complex signals may be favoured by hidden preferences or pre-existing sensory biases. Females of two species of grey treefrogs (Hyla chrysoscelis and Hyla versicolor) were tested with combinations of a conspecific advertisement call and acoustic appendages. Appendages consisted of aggressive calls and segments of advertisement calls from conspecific males and males of three other species and bursts of filtered noise. When a wide variety of these acoustic appendages followed the advertisement call, the resulting compound signal was often more attractive than the same advertisement call alone. When the same appendages led advertisement calls, however, the compound signal was never more attractive and sometimes less attractive. The order effect was especially strong in tests of H. versicolor in which advertisement-call duration was decreased. These results cannot be explained by a general pre-existing bias for extra stimulation per se. Rather, order and other effects may constrain the evolution and subsequent modification of complex and extravagant signals, examples of which have been reported for a wide range of taxa.

Context-dependent plasticity of aggressive signalling in a dynamic social environment

Frog choruses are dynamic social environments in which males must balance the conflicting demands of attracting a female and defending their calling site. The optimal trade-off is probably one that adaptively tracks temporal and spatial changes in the local social environment. Previous studies have indicated that a male frogs propensity to interact aggressively with its nearby neighbours reflects patterns of plasticity that could allow for such tracking. We presently do not know the mechanisms underlying plasticity in aggressive signalling in frogs, nor do we understand how plasticity depends on variation in social context. In this field playback experiment with spring peepers, Pseudacris crucifer, we investigated how differences in the type of signalling interactions (a form of social context) experienced by calling males influence the plasticity of aggression directed towards nearby neighbours in the chorus. We used the habituation–discrimination experimental paradigm to present males with calls simulating a new neighbour that was initially either aggressive (producing a mixture of advertisement and aggressive vocalizations) or nonaggressive (producing only advertisement calls). We found that interacting with an initially aggressive versus nonaggressive neighbour had negligible effects on the plasticity of a males aggressive response, but that subsequent changes in social context (e.g. a switch to aggressive calling by a previously nonaggressive neighbour) greatly influenced a males aggressive responsiveness. We discuss our results in terms of short-term habituation as a mechanism for the plasticity and specificity of aggressive signalling behaviour in frog choruses.

Molecular patterns of differentiation in canyon treefrogs (Hyla arenicolor): evidence for introgressive hybridization with the Arizona treefrog (H. wrightorum) and correlations with advertisement call differences

Detection of genetic and behavioural diversity within morphologically similar species has led to the discovery of cryptic species complexes. We tested the hypothesis that US populations of the canyon treefrog (Hyla arenicolor) may consist of cryptic species by examining mate‐attraction signals among three divergent clades defined by mtDNA. Using a multi‐locus approach, we re‐analysed phylogenetic relationships among the three clades and a closely related, but morphologically and behaviourally dissimilar species, the Arizona treefrog (H. wrightorum). We found evidence for introgression of H. wrightorum’s mitochondrial genome into H. arenicolor. Additionally, the two‐clade topology based on nuclear data is more congruent with patterns of call variation than the three‐clade topology from the mitochondrial dataset. The magnitude of the call divergence is probably insufficient to promote isolation of the nuclear DNA‐defined clades should they become sympatric, but further divergence in call properties significant in species identification could promote speciation in the future.

Pre-existing sensory biases in the spectral domain in frogs: empirical results and methodological considerations.

In many species of anurans, advertisement calls excite only one of the two inner-ear organs. One prediction of the pre-existing bias hypothesis is that signal innovations that additionally excite the “untapped” organ will be more behaviorally effective than normal calls. However, recent studies have shown that females of three species with single-peaked calls that stimulate only the basilar papilla (BP) preferred single-peaked synthetic calls with a frequency typical of conspecific calls to two-peaked calls that also stimulated the amphibian papilla (AP). We report that in spring peepers (Pseudacris crucifer) that also produce single-peaked calls, females did not show a preference in choices between single-peaked and two-peaked synthetic calls. Thus, the addition of energy exciting the AP had a neutral effect on signal attractiveness. Together, these results are unsupportive of the pre-existing bias hypothesis. An alternative hypothesis is that positive fitness consequences of responding to sounds providing extraordinary spectral stimulation are required for a novel call to become established as a mate-attracting signal. Testing these ideas requires a taxonomically broader examination of responses to sounds with novel spectral complexity, and attention to some methodological details will improve the comparability of such studies.

Near-synchronous calling in the hip-pocket frog Assa darlingtoni

Abstract Sustained perfect synchrony of signal production by animals is unrealistic, but even near-synchronous signalling is extremely rare. Near-synchronous signalling has been documented in some orthopteran insects and fireflies, and one kind of frog. This study provides observations and analyses of sustained bouts of impressive near-synchronous calling by a terrestrial breeding frog from Australia, the hip-pocket frog (Assa darlingtoni). Males called in scattered clusters of several individuals from the rainforest floor of northern New South Wales. In eight of nine pairs of semi-isolated males, there was sustained near-synchronous calling in bouts consisting of 16–20 calls and lasting 5–10 min. There was extensive overlap of the pulsed calls, and calls of a lagging male began overlapping that of a leading male after 2–5 pulses of a leader’s call note. In five pairs, one male’s calls were usually in the leading position; in three pairs, leadership frequently switched between males. In the latter interactions, males frequently skipped a call and produced its next call or calls in the leading position. This tactic has been interpreted as sexual competition between neighbours in orthopterans, but further research is required to rule out possible alternative hypotheses.

Behavioral and phylogenetic differentiation in a potential cryptic species complex, the canyon treefrog

Detection of genetic and behavioral diversity within morphologically similar species has led to the discovery of cryptic species complexes. We tested the hypothesis that the canyon treefrog (Hyla arenicolor) may consist of cryptic species by examining mate-attraction signals among highly divergent lineages defined by mitochondrial DNA (mtDNA). Unexpectedly, calls exhibited little variation among the three U.S. lineages despite large mtDNA sequence divergences. We re-analyzed intraspecific and interspecific phylogenetic relationships by sequencing both mitochondrial and nuclear genetic markers among populations and a closely related, but morphologically and behaviorally different species, the Arizona treefrog (H. wrightorum). Discordance between mitochondrial and nuclear datasets suggests multiple instances of introgression of H. wrightorums mitochondrial genome into populations of H. arenicolor. Furthermore, intraspecific population structure based on nuclear markers shows better congruence with patterns...