Warnell School of Forestry and Natural Resources

During a recent study of red-backed salamanders (Plethodon cinereus), we discovered an intraerythrocytic organism typified by violet-staining, intracellular inclusions, consistent with descriptions of Cytamoeba or Aegyptianella (bacteria). Here we characterize its taxonomic status using molecular techniques and ask basic questions about its nature. Blood smears from 102 salamanders were examined from Pennsylvania, New York, and Virginia to determine prevalence, and whole blood from several infected animals was tested using a PCR which targets the 16S rRNA gene of bacteria. Phylogenetic analysis of partial 16S rRNA gene sequence (1201 bp) indicated this organism was in the order Rickettsiales and is likely a member of the family Anaplasmatacea. The organism differed from currently described taxa and was clearly differentiated from Aegyptianella pullorum of birds and “Candidatus Hemobacterium ranarum” (formally A. ranarum) of frogs. Of all salamanders, 17 (16.7%) were infected and these were significantly larger (snout–vent length) and had higher body condition scores than uninfected ones, and males were more likely to be infected than females. Erythrocytes affected by the pathogen were 5% larger than unaffected ones, but otherwise similar in morphology. Infected animals tended to have a greater number of circulating white blood cells, based on estimates from smears, indicating a nonspecific response to the pathogen by the innate immune system. Given its phylogenetic position, this pathogen is likely transmitted by an arthropod vector, and the male-biased prevalence strongly implicates trombiculid mites, which also live in leaf litter and affect male salamanders more so than females.

Although adult cane toads (Rhinella marina) are generally active only at night, a recent study reported that individuals of this species switched to diurnal activity in response to encountering a novel habitat type (deeply shaded gorges) in the course of their Australian invasion. our sampling over a broader geographic scale challenges the idea that this behaviour is novel; we documented diurnal behaviour both in the species' native range and in several sites within the invaded range, in multiple habitat types. Diurnal activity was most common in the tropics and in areas where toads attain high population densities and are in poor body condition, suggesting that the expansion of activity times may be induced by intraspecific competition for food. All animals are active for only a limited period of the diel cycle, and that timing is so consistent that we commonly define species as nocturnal (e.g., owls), diurnal (e.g., bearded dragons) or crepuscular (twilight hours; e.g., microbats). Organisms have evolved these life history traits to cope with the physiological demands of activity 1 ; to exploit different niches 2 ; and to temper various ecological interactions (such as predation 3). Nonetheless, facul-tative shifts in the diel timing of activity are common even within a species. For example, animals that are usually nocturnal may be driven to diurnal behaviour by factors such as nutritional stress, thermal extremes, predation risk and intraspecific competition 4. In many species of anuran amphibians (frogs and toads), adults are normally active only at night 5. Recently, Doody et al. 6 described a switch to diurnal behaviour in a species (the cane toad, Rhinella marina) that is usually nocturnal. Camera-trapping revealed that toads were nocturnally active in nearby sun-exposed gorges but were often active by day in shaded gorges. The authors attributed that shift as a response to novel conditions (deep shade) that do not occur elsewhere in the toads' broad distribution. In the course of our own fieldwork, we have sampled over a greater geographic range (but for shorter times at each place) and find that diurnal activity occurs in multiple populations of cane toads, in widely separated areas, and in several habitat types. Our radiotracking of cane toads in French Guiana revealed diurnal activity in native-range populations as well. Our data challenge Doody et al. 's 6 interpretation that diurnal activity of toads at their study sites is a novel response, elicited by a novel habitat type 6. Why, then, are toads sometimes active by day? Plausible explanations include diurnality as a response to: (1) availability of cool moist conditions by day, reducing the abiotic challenges of diurnal foraging; (2) compensation for restricted availability of sufficiently high temperatures at night, allowing foraging to take place over a longer period of time; (3) food scarcity, such that animals need to forage over a longer period in order to meet their energy requirements. The first two of these hypotheses predict that diurnal behaviour should be most common in cooler and moister parts of the toads' geographic range, whereas the third hypothesis predicts the opposite pattern (i.e., we expect more diurnality in hotter regions, because high temperatures increase metabolic rates and thus maintenance requirements) and also, predicts that diurnality will occur in areas where toads are abundant (increasing intraspecific competition for limited prey resources) and are in poor body condition. To test these competing predictions, we regressed the incidence of diurnal behaviour against other site-specific parameters.