Midbrain auditory sensitivity in the spring peeper (Pseudacris crucifer): correlations with behavioral studies

1. We derived audiograms from recordings of multiunit activity in the torus semicircularis of 10 males and 6 females of the spring peeper from central Missouri, USA. We used free-field stimulation with tone bursts that had temporal properties similar to typical advertisement calls and that ranged in frequency from 500-6000 Hz. 2. Audiograms from different electrode positions in the same animal had the same general shape. There was no evidence of tonotopy. 3. Audiograms showed two regions of maximal sensitivity: a low-frequency region (500-700 Hz); and a high-frequency region (2000-4000 Hz). Absolute thresholds and frequencies of maximum sensitivity varied considerably from individual to individual. 4. Audiograms derived from all individuals of each sex indicated that in the high-frequency region, corresponding to the frequency range of advertisement calls, males were more broadly tuned than females. However, tuning in both sexes was relatively weak, and the data predict relatively little selectivity in behavioral responses over the entire range of variation in frequency of the advertisement call in local populations. 5. The results are discussed in terms of behavioral experiments with both males and females from the same populations in central Missouri. We show that merely summarizing the audiograms based on estimates of minimum thresholds of a population or species may mask significant individual differences in tuning. Moreover, most behavioral studies are conducted at playback levels considerably above threshold. For these reasons, behavioral selectivity is not always accurately predicted by inspection of "average" audiograms.     

Influence of wetland hydroperiod on diversity and abundance of metamorphosing juvenile amphibians

Numbers of successfully metamorphosing juvenile amphibians were tabulated at three wetlands in South Carolina, U.S.A. using terrestrial drift fences with pitfall traps. A relatively undisturbed Carolina bay was studied for eight years, a partially drained Carolina bay for four years, and a man-made borrow pit for three years. Annual production of juveniles at the undisturbed Carolina bay ranged from zero to 75,644 individuals of 15 species. Fewer individuals of fewer species typically metamorphosed at the borrow pit than at the undisturbed bay, with the least numbers at the partially drained Carolina bay. Both total number and species diversity of metamorphosing juveniles at each site each year showed a strong positive correlation with hydroperiod, i.e., the number of days a site contained standing water that year. Data for one common anuran species and the most common salamander species were analyzed separately by multiple regression, in addition to the community analyses. For the mole salamander, Ambystoma talpoideum, hydroperiod was a significant predictor of the number of metamorphosing juveniles, but the number of breeding females was not. For the ornate chorus frog, Pseudacris ornata, the number of breeding females was a significant predictor of the number of metamorphosing juveniles, but hydroperiod was not. Variation in the dates of wetland filling and drying interacts with other factors to determine amphibian community structure and diversity. Either increasing or decreasing the number of days a wetland holds water could increase or decrease the number and species diversity of amphibians in and around a wetland.     

Disease hotspots or hot species? Infection dynamics in multi‐host metacommunities controlled by species identity,not source location

Pathogen persistence in host communities is influenced by processes operating at the individual host to landscape‐level scale, but isolating the relative contributions of these processes is challenging. We developed theory to partition the influence of host species, habitat patches and landscape connectivity on pathogen persistence within metacommunities of hosts and pathogens. We used this framework to quantify the contributions of host species composition and habitat patch identity on the persistence of an amphibian pathogen across the landscape. By sampling over 11 000 hosts of six amphibian species, we found that a single host species could maintain the pathogen in 91% of observed metacommunities. Moreover, this dominant maintenance species contributed, on average, twice as much to landscape‐level pathogen persistence compared to the most influential source patch in a metacommunity. Our analysis demonstrates substantial inequality in how species and patches contribute to pathogen persistence, with important implications for targeted disease management.     

Temperature‐driven shifts in a host‐parasite interaction drive nonlinear changes in disease risk

Climate change may shift the timing and consequences of interspecific interactions, including those important to disease spread. Because hosts and pathogens may respond differentially to climate shifts, however, predicting the net effects on disease patterns remains challenging. Here, we used field data to guide a series of laboratory experiments that systematically evaluated the effects of temperature on the full infection process, including survival, penetration, establishment, persistence, and virulence of a highly pathogenic trematode (Ribeiroia ondatrae), and the development and survival of its amphibian host. Our results revealed nonlinearities in pathology as a function of temperature, which likely resulted from changes in both host and parasite processes. Both hosts and parasites responded strongly to temperature; hosts accelerated development while parasites showed enhanced host penetration but reduced establishment (encystment) and survival outside the host. While there were no differences in host survival among treatments, we observed a mid‐temperature peak in parasite‐induced deformities (63% at 20 °C), with the lowest frequency of deformities (12%) occurring at the highest temperature (26 °C). This nonlinear effect could result from temperature‐driven changes in parasite burden owing to shifts in host penetration and/or clearance, reductions in host vulnerability owing to faster development, or both. Furthermore, despite strong temperature‐driven changes in parasite penetration, survival, and establishment, the opposing nature of these effects lead to no difference in tadpole parasite burdens shortly after infection. These findings suggest that temperature‐driven changes to the disease process may not be easily observable from comparison of parasite burdens alone, but multi‐tiered experiments quantifying the responses of hosts, parasites and their interactions can enhance our ability to predict temperature‐driven changes to disease risk. Climate‐driven changes to disease patterns will therefore depend on underlying shifts in host and parasite development rates and the timing of their interactions.     

Eight novel tetranucleotide and five cross-species dinucleotide microsatellite loci for the ornate chorus frog (Pseudacris ornata)

We describe the cloning and characterization of eight novel tetranucleotide microsatellite loci in the ornate chorus frog (Pseudacris ornata). We also screened 26 loci from GenBank that were isolated from other Pseudacris species and obtained consistent product from five of these dinucleotide loci. All loci are polymorphic. In our sample of 26 frogs from a natural population, polymorphism ranged from 1 to 22 alleles per locus with expected heterozygosities ranging from 0 to 0.958. These loci enable high-resolution studies of P. ornata. Moreover, cross-species amplification success suggests they will also be useful for other chorus frog species.     

REINFORCEMENT IN CHORUS FROGS: LIFETIME FITNESS ESTIMATES INCLUDING INTRINSIC NATURAL SELECTION AND SEXUAL SELECTION AGAINST HYBRIDS

Maladaptive hybridization is hypothesized to be an important force driving the evolution of reproductive isolation between closely related species. Because the magnitude and direction of selection can vary across a life cycle, an accurate understanding of the ubiquity of reinforcement requires fitness to be estimated across the life cycle, but the literature is surprisingly depauperate of such studies. We present fitness estimates of laboratory‐raised hybrids between the chorus frogs Pseudacris feriarum and Pseudacris nigrita—two species that have undergone reproductive character displacement where they come into secondary contact. By studying viability, mating success, and fertility across the life cycle, we find strong support for reinforcement as the force driving displacement in this system. Specifically, we find hybrid fitness is reduced by 44%. This reduction results from both sexual selection against hybrid males and natural selection on male fertility, but not viability selection. Sexual selection against hybrid males is four times stronger than natural selection. Hybrid female fitness is not reduced, however, suggesting that Haldane's rule may be operating in this system if males are heterogametic. We also found higher variation in hybrid male fertilization success relative to P. feriarum males, suggesting that the hybrid incompatibility genes are polymorphic within one or both of the parent species.     

Geological and climatic forces driving speciation in the continentally distributed trilling chorus frogs (Pseudacris)

Tertiary geological events and Quaternary climatic fluctuations have been proposed as important factors of speciation in the North American flora and fauna. Few studies, however, have rigorously tested hypotheses regarding the specific factors driving divergence of taxa. Here, we test explicit speciation hypotheses by correlating geologic events with divergence times among species in the continentally distributed trilling chorus frogs (Pseudacris). In particular, we ask whether marine inundation of the Mississippi Embayment, uplift of the Appalachian Mountains, or modification of the ancient Teays-Mahomet River system contributed to speciation. To examine the plausibility of ancient rivers causing divergence, we tested whether modern river systems inhibit gene flow. Additionally, we compared the effects of Quaternary climatic factors (glaciation and aridification) on levels of genetic variation. Divergence time estimates using penalized likelihood and coalescent approaches indicate that the major lineages of chorus frogs diversified during the Tertiary, and also exclude Quaternary climate change as a factor in speciation of chorus frogs. We show the first evidence that inundation of the Mississippi Embayment contributed to speciation. We reject the hypotheses that Cenozoic uplift of the Appalachians and that diversion of the Teays-Mahomet River contributed to speciation in this clade. We find that by reducing gene flow, rivers have the potential to cause divergence of lineages. Finally, we demonstrate that populations in areas affected by Quaternary glaciation and aridification have reduced levels of genetic variation compared to those from more equable regions, suggesting recent colonization.     

The effects of five organophosphorus insecticides on thermal stress in tadpoles of the Pacific tree frog, Hyla regilla

Three week old tadpoles of Hyla regilla were exposed to the insecticides temephos, fenthion, methyl parathion, chlorpyrifos, and malathion for 24 h at concentrations less than and at field applied rates as recommended for mosquito control in California. The thermal tolerance was significantly lowered by all chemicals and concentrations tested. Chlorpyrifos and methyl parathion were the most active compounds tested whereas malathion was the least toxic to H regilla tadpoles. Activity was depressed, in comparison to control tadpoles, by the chemicals methyl parathion (100 ppb) and malathion (500 ppb).