Joseph R. Mendelson

Riding the wave: reconciling the roles of disease and climate change in amphibian declines.

We review the evidence for the role of climate change in triggering disease outbreaks of chytridiomycosis, an emerging infectious disease of amphibians. Both climatic anomalies and disease-related extirpations are recent phenomena, and effects of both are especially noticeable at high elevations in tropical areas, making it difficult to determine whether they are operating separately or synergistically. We compiled reports of amphibian declines from Lower Central America and Andean South America to create maps and statistical models to test our hypothesis of spatiotemporal spread of the pathogen Batrachochytrium dendrobatidis (Bd), and to update the elevational patterns of decline in frogs belonging to the genus Atelopus. We evaluated claims of climate change influencing the spread of Bd by including error into estimates of the relationship between air temperature and last year observed. Available data support the hypothesis of multiple introductions of this invasive pathogen into South America and subsequent spread along the primary Andean cordilleras. Additional analyses found no evidence to support the hypothesis that climate change has been driving outbreaks of amphibian chytridiomycosis, as has been posited in the climate-linked epidemic hypothesis. Future studies should increase retrospective surveys of museum specimens from throughout the Andes and should study the landscape genetics of Bd to map fine-scale patterns of geographic spread to identify transmission routes and processes.

Effects of bush encroachment on an assemblage of diurnal lizard species in central Namibia

Bush encroachment is a serious environmental and economic problem in Namibia, but little is known about impacts on native reptile diversity. Area-confined visual surveys were used to examine a diurnal lizard assemblage in central Namibian commercial ranchlands. Surveys were conducted in plots of open savanna habitat and proximal bush-encroached habitat. The following four species comprised 97.5% of all lizard observations: Pedioplanis undata, Mabuya varia, M. striata, and Lygodactylus bradfieldi. Pedioplanis undata was terrestrial, and the remaining three species were largely arboreal in our study plots. Mabuya varia was found in all savanna plots but was absent from all bush-encroached plots. Two species (P. undata and L. bradfieldi) were less abundant in bush-encroached plots. One species (M. striata) was more abundant in bush-encroached plots than in open savanna plots. Arboreal lizards demonstrated an avoidance to invasive woody plant species. Decreased diversity of habitat structure in bush-encroached habitats appears to influence native savanna lizard assemblages. Our results are consistent with accumulating evidence suggesting that bush encroachment and its associated ecological impacts are reorganizing savanna ecosystems throughout southern Africa. # 2002 Elsevier Science Ltd. All rights reserved.

The Challenge of Conserving Amphibian Megadiversity in Madagascar

Highly diverse and so far apparently untouched by emergent diseases, Malagasy frogs nevertheless are threatened by ongoing habitat destruction, making pro-active conservation actions especially important for preserving this unique, pre-decline, amphibian fauna.

Do rats terminate hypothalamic stimulation only in order to turn it on again

Rats will operate a shuttle box to both initiate and terminate electrical stimulation in the hypothalamus (ESH). Some believe that they terminate the ESH because it changes from rewarding to punishing if left on for more than a few seconds. Others believe that while the onset of ESH is rewarding, its reward strength rapidly diminishes and they terminate it only in order to turn it on again. If so, rats given only one opportunity each session to initiate and terminate the ESH should not terminate it, because they cannot turn it on again during the session. But in the present experiment it was found that they do terminate it, thus supporting the punishment hypothesis.

Facultative parthenogenesis discovered in wild vertebrates

Facultative parthenogenesis (FP)—asexual reproduction by bisexual species—has been documented in a variety of multi-cellular organisms but only recently in snakes, varanid lizards, birds and sharks. Unlike the approximately 80 taxa of unisexual reptiles, amphibians and fishes that exist in nature, FP has yet to be documented in the wild. Based on captive documentation, it appears that FP is widespread in squamate reptiles (snakes, lizards and amphisbaenians), and its occurrence in nature seems inevitable, yet the task of detecting FP in wild individuals has been deemed formidable. Here we show, using microsatellite DNA genotyping and litter characteristics, the first cases of FP in wild-collected pregnant females and their offspring of two closely related species of North American pitviper snakes—the copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus). Our findings support the view that non-hybrid origins of parthenogenesis, such as FP, are more common in squamates than previously thought. With this confirmation, FP can no longer be viewed as a rare curiosity outside the mainstream of vertebrate evolution. Future research on FP in squamate reptiles related to proximate control of induction, reproductive competence of parthenogens and population genetics modelling is warranted.

Phylogeography of the flat‐tailed horned lizard (Phrynosoma mcallii) and systematics of the P. mcallii–platyrhinos mtDNA complex

Two species of horned lizards are sympatric along the periphery of the Salton Trough. Phrynosoma mcallii, endemic to the trough, is of conservation concern because its limited habitat has been fragmented by human activities. A more common and widespread species, Phrynosoma platyrhinos, occurs around the periphery of the trough and much further to the North. The two species are syntopic at a few localities, where morphological intermediates have also been found. Here, we used nested clade phylogeographical analysis (NCPA) and analysis of molecular variance (amova), to examine 781 bp of mitochondrial DNA (nad4 and two tRNAs) from 82 individuals of P. mcallii. We tested whether populations of this species were recently connected, or if they were historically isolated prior to human modification of the region. Our NCPA results indicated significant population structure associated with the Colorado River, suggesting limited gene flow and potential isolation across this barrier. Populations west of the Colorado River, currently isolated from one another by human development, show less genetic differentiation. We also collected homologous sequence data from 34 individuals of P. platyrhinos and seven specimens morphologically intermediate between P. mcallii and P. platyrhinos, as a preliminary investigation of hybridization between these two species. From phylogenetic results of these data, we identified a species (Phrynosoma goodei) previously recognized as a subspecies of P. platyrhinos. Six of the morphologically intermediate specimens shared mtDNA haplotypes with P. goodei, while one was nested among P. mcallii haplotypes.

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free-swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.

Shifts in the Relative Abundance of Snakes in a Desert Grassland

Distribution, diversity and relative abundance of snake species on roads through desert grasslands in Arizona and New Mexico were compared to data in a previously published survey conducted about 30 years ago. We found a significant shift in the relative abundance of snake species: Thamnophis marcianus and Crotalus atrox have increased in relative abundance and C. scutulatus has decreased. These changes are correlated with succession of local Semidesert Grasslands to Chihuahuan Desertscrub. Analysis of distributions of Crotalus revealed that C. atrox was more common than C. scutulatus in scrub habitat while C. scutulatus was more common in the remaining grasslands.

Schedule-Induced Polydipsia: The Role of Orolingual Factors and a New Hypothesis

When a food-deprived animal is intermittently fed small amounts of food, it rapidly develops a tendency to ingest water immediately following the ingestion of each morsel (Falk, 1961, 1969). An appropriate choice of various parameters can lead these animals to ingest greatly excessive quantities of water. In one experiment, female rats with a mean free-feeding weight of 264 g were deprived of food until their body weights dropped to 70–80% of normal. When they were given the opportunity to bar-press for 45-mg dry food pellets on a variable-interval 1-min schedule, they ingested a mean of 92.5 ml of water per 3.17-hr session (Falk, 1961). Such quantities of intake are greatly excessive whether compared to the normal 24-hr intake or to the amount of water that would be ingested if the rats were allowed to eat the same amount of food ad libitum. This phenomenon, which is referred to as schedule-induced polydipsia or by the acronym SIP (Falk, 1964), can be explained neither in terms of traditional behavioral phenomena, such as adventitious reinforcement or timing behavior (Falk, 1969), nor in terms of water-regulatory variables, such as impaired renal concentrating ability or de facto water deprivation (Falk, 1969; Stricker and Adair, 1966).

Effects of water temperature on the reward value and satiating capacity of water in water-deprived rats

Since orolingual cooling is a primary reward for water-deprived rats, cold water should be more rewarding than warm water. Hooded rats were allowed to bar press on a VI 1-min schedule for water at approximately 12° or 36°C. Testing started when their body weights had dropped to 80% of their predeprivation levels and was continued as their weights were allowed to drift slowly upward. Mean number of bar presses for cold and body-temperature water were compared under the various body-weight conditions, and in almost all cases the rats bar pressed at a higher rate for cold water. When these same animals were again deprived to 80% body weight, they showed an overwhelming preference for cold water over warm water in 10-min choice tests. A third experiment with these animals replicated Kapatos and Golds (1972) finding that rats offered either only warm water or only cold water in different test sessions drink more warm water than cold. The results of these experiments are consistent with the hypothesis that cold water is both more rewarding and more satiating than is body-temperature water, but it could be that the intake of cold water is terminated prematurely by the drop in body temperature which it produces.