Dip listening or modulation masking? Call recognition by green treefrogs (Hyla cinerea) in temporally fluctuating noise.

Despite the importance of perceptually separating signals from background noise, we still know little about how nonhuman animals solve this problem. Dip listening, an ability to catch meaningful 'acoustic glimpses' of a target signal when fluctuating background noise levels momentarily drop, constitutes one possible solution. Amplitude-modulated noises, however, can sometimes impair signal recognition through a process known as modulation masking. We asked whether fluctuating noise simulating a breeding chorus affects the ability of female green treefrogs (Hyla cinerea) to recognize male advertisement calls. Our analysis of recordings of the sounds of green treefrog choruses reveal that their levels fluctuate primarily at rates below 10 Hz. In laboratory phonotaxis tests, we found no evidence for dip listening or modulation masking. Mean signal recognition thresholds in the presence of fluctuating chorus-like noises were never statistically different from those in the presence of a non-fluctuating control. An analysis of statistical effects sizes indicates that masker fluctuation rates, and the presence versus absence of fluctuations, had negligible effects on subject behavior. Together, our results suggest that females listening in natural settings should receive no benefits, nor experience any additional constraints, as a result of level fluctuations in the soundscape of green treefrog choruses.

HELMINTH DIVERSITY OF GREEN TREEFROGS (HYLA CINEREA) IN THEIR EXPANDED GEOGRAPHICAL RANGE.

There has been a growing interest in characterizing the parasite faunas of species populations as they expand their geographical ranges as a result of climate change. Expanded-range populations often exhibit lower parasite diversity than historical-range populations, and reduced parasitism may, in part, be attributable to expanded-range populations escaping their native range parasites. The present study compares the helminth faunas of green treefrogs (Hyla cinerea) from 4 historical and 4 expanded-range populations to determine whether these latter populations have undergone parasite escape. Results of this study found relatively high degrees of similarity in species composition among helminth assemblages within historical or within range-expansion locations, with marked differences in the composition of helminth faunas between historical and expanded-range populations of these frogs. Because green treefrogs from expanded-range locations exhibited significant decreases in helminth species diversity compared with those from historical sites, they appear to be escaping levels of parasitism typically experienced by these frogs in their native range. Most notably, there was a decrease in the abundance of helminths with direct life cycles and the absence of trematode assemblages with indirect life cycles among expanded-range populations of H. cinerea. The low prevalence of trematode assemblages among historical populations of green treefrogs could limit these parasites' ability to be introduced and propagated in expanded-range locations. However, the lack of trematode assemblages among populations of H. cinerea in its expanded range may also be due to the absence or limited availability of other aquatic hosts that are required to complete the life cycles of these parasites. The reduction in helminth diversity among expanded-range populations of green treefrogs lends some credence to the notion that individuals at the front of a range expansion often invest less energy in reproduction and in doing so allocate more energy to dispersal and other life-history traits, including resistance to parasites. There may, however, be other explanations for differences in parasite species diversity between historical and expanded-range populations of H. cinerea. Because many of the helminths reported from this study are host generalists of amphibians whose recruitment and transmission among intermediate and paratenic hosts are known to be constrained by water and/or soil moisture conditions, we cannot ignore the role of both local amphibian diversity and local abiotic factors in influencing helminth diversity between the 2 population types of green treefrogs. These latter factors would decrease the role of parasite escape or energy trade-offs in driving helminth diversity among populations of H. cinerea and instead would suggest that local conditions play a more prominent role in structuring their helminth communities.

Hormonal modulation of phonotaxis and advertisement-call preferences in the gray treefrog (Hyla versicolor).

Hormonal levels fluctuate during the breeding season in many anurans, but the identity of the hormones that modulate breeding behavior and their effects remain unclear. We tested the influence of a combined treatment of progesterone and prostaglandin on phonotaxis, the key proceptive reproductive behavior of female anurans. First, we found that female gray treefrogs (Hyla versicolor) treated with progesterone and prostaglandin exhibited phonotaxis to synthetic male advertisement signals significantly more often than animals treated with ringers vehicle or uninjected controls. Responsive females had greater levels of plasma progesterone and estradiol compared to both control groups, suggesting that these steroids may be promoting phonotaxis. Second, we found that the selectivity of hormonally-induced phonotaxis in H.versicolor was similar to that observed in freshly captured breeding animals. Females made the same choices between acoustic signals after hormone treatments in tests of frequency, call rate and pulse rate, compared to their responses without treatment immediately after collection from the breeding chorus. The preference for a longer call was, however, significantly weaker after hormonal induction of phonotaxis. Hormonally primed females were also less likely to respond in any test and took longer to respond than did freshly collected females. Consequently, our study shows how progesterone-prostaglandin induced phonotaxis in female treefrogs influences both the quality and quantity of phonotaxis, relative to that exhibited by naturally breeding females.

A Comparison of Helminth Faunas of Cope's Gray ( Hyla chrysoscelis ) and Green ( Hyla cinerea ) Treefrogs in Areas of Recent Niche Overlap.

This study compares the helminth faunas between Cope's gray treefrogs ( Hyla chrysoscelis ) and green treefrogs ( Hyla cinerea ), in areas where they have recently overlapped due to range expansion by H. cinerea , to determine whether or not 2 species of frogs with a high degree of similarity in many of their life history traits also exhibit similarities in the composition of their helminth assemblages. Results of this study did not find significant differences in helminth species diversity when sympatric and allopatric populations of the same species of frog were compared. There was, however, a significant difference in helminth diversity among sympatric populations of H. chrysoscelis and H. cinerea , and this difference was in large part attributable to the significantly higher abundance of the gastrointestinal nematode Cosmocercoides variabilis among H. chrysoscelis . Additional studies will be required to determine whether the observed patterns are due to differences in arrival time, perch locations within the chorus, or parasite-mediated competition.

Progesterone and prostaglandin F2α induce species-typical female preferences for male sexual displays in Cope's gray treefrog (Hyla chrysoscelis).

Endocrine systems play critical roles in facilitating sexual behavior in seasonally breeding vertebrates. Much of the research exploring this topic has focused on the endocrine correlates of signaling behavior in males and sexual proceptivity in females. What is less understood is how hormones promote the expression of the often complex and highly selective set of stimulus-response behaviors that are observed in naturally breeding animals. In female frogs, phonotaxis is a robust and sensitive bioassay of mate choice and is exhibited by gravid females during the breeding season. In stark contrast, females exhibit low phonotactic responsiveness outside the breeding season, but the administration of hormones can induce sexual proceptivity. Here we test the hypothesis that manipulation of a minimal set of reproductive hormones-progesterone and prostaglandin F2α-are capable of evoking not only proceptive behavior in non-breeding females, but also the patterns of intraspecific selectivity for male sexual displays observed in gravid females tested during the breeding season. Specifically, we investigated whether preferences for faster call rates, longer call durations, and higher call efforts were similar between breeding and hormone-treated females of Cope's gray treefrog (Hyla chrysoscelis). Hormone injections induced patterns of selective phonotaxis in non-breeding females that were remarkably similar to those observed in breeding females. These results suggest that there may be an important contribution of hormonal pleiotropy in regulating this complex, acoustically-guided sexual behavior. Our findings also support the idea that hormonal induction could be used to evaluate hypotheses about selective mate choice, and its underlying mechanisms, using non-breeding females.