Gregory P. Brown

Determining an individual’s sex early in life may clarify fundamental issues such as progeny sex ratios and sex-based divergences in ecology and behaviour prior to maturity and be useful in captive breeding of endangered species or in developing methods to control invasive pests by skewing offspring sex ratios. We measured morphological traits in captive-raised cane toads (Rhinella marina) at intervals from metamorphosis to more than 2 years of age, at which time we could reliably score each individual’s sex and hence, quantify the ontogenetic development of sexually dimorphic traits during juvenile life. Discriminant function analysis of 11 morphological traits (involving body size and dimensions of limbs, feet and parotoid glands, but not including sexually dimorphic traits expressed only in adults) correctly predicted sexes of around 70% of toads in a cohort ranging from 161 to 400 days of age, increasing to 95% in the oldest cohort. Despite the scarcity of research on this topic, published data on anurans reveal interspecific diversity: males and females diverge morphologically during juvenile life in some species but not others.

Geckos have high skin bacterial diversity, even though gecko skin has antimicrobial, self‐cleaning properties. To gain a better understanding of environment–animal–microbiome interactions in these reptiles, we investigated skin bacteria on seven northern Australian gecko species from five sites and two seasons ( n = 234) and found support for our hypotheses of divergent communities between species, sites and seasons. Despite that support, predictor variables had low explanatory power, which increased when focusing within a site or species, explaining up to 40% and 27% of the variation among samples at a site or on a species found in multiple sites, respectively. Weather explained even less variation, as temperature and rainfall did not account for site and season differences. Low explanatory power of these variables indicates that additional factors, or stochasticity, explain much of the bacterial assemblage on geckos. Next, research is needed to determine if these low‐biomass communities represent living symbionts. If so, assessing functional similarities, rather than taxonomic profiling, would clarify if bacterial communities have interactive roles with gecko hosts or represent short‐lived environmental hitch‐hikers and relic DNA.

The success of parthenogenic populations is facilitated by high fecundity, though studies show that parthenogenic hybrids can express traits of compromised fitness compared with their parent species, such as higher rates of evaporative water loss and greater susceptibility to ectoparasites. Recent works highlighting the importance of microbiomes on host health also show that both host lineage and the environment contribute to an animal's microbiome. In this study, we investigated how reproductive mode and the presence of mites affect evaporative water loss and if the skin bacterial microbiome differs between sympatric sexual and parthenogenic populations of the gecko Heteronotia binoei in central Australia. We collected sexual ( n = 17) and parthenogenic ( n = 66) H. binoei from six local sites (within 3 km of each other) to measure evaporative water loss, record mite infestations, and characterise skin bacterial communities from a subsample of individuals ( n = 17 per reproductive mode). Only parthenogenic individuals had mites, and mite infestations were not severe at our site. We found that neither reproductive mode nor the presence of mites affected evaporative water loss in our study populations, contrary to prior studies elsewhere for the species. In microbiome analyses, we found that reproductive mode significantly predicted community structure and composition; however, local site explained more of the observed variation than other variables. While these findings challenge previously observed differences in physiology between parthenogenic and sexual H. binoei , we found support for ectoparasite susceptibility in parthenogenic individuals. Our microbiome results reinforce that even in sympatry, host lineages harbour unique microbiomes, although the environment largely influences the skin bacterial communities in our study system.

Skin bacteria on amphibian hosts play an important role in host health, but those communities are also constantly shifting based on environmental and host-related feedback. On some hosts, stability of skin communities depends on relatively abundant taxa, with less abundant taxa more readily entering and exiting the system. Cane toads (Rhinella marina) have invaded widespread, diverse tropical ecosystems, with varying ecology, physiology, and behaviour in different environments. In this study, we described temporal patterns of skin bacterial communities on cane toads at a site in northern Australia through the wet and dry seasons over two years. Toads in the wild population were paired with a captive-held population, housed in a semi-natural environment, to detect effects of time and season on wild toads, explore bacterial transience and volatility in skin taxa, and determine the extent to which skin communities on captive toads represent those on the wild population. We found community differences by captivity status, sampling timepoint, and season, with increased richness in the wet season on wild toads. Bacterial communities also became more similar among individuals (lower dispersion) in the wet season. Captive toads harboured more stable communities over time, likely owing to the reduced bacterial reservoirs experienced while in captivity. We propose that cane toads, with varied movement patterns among their diverse invaded habitats, provide an interesting direction for future work understanding the influences of habitat and movement on skin microbes, and the flexibility of microbial symbiotic interactions in invasive hosts.

The physical burden of pregnancy may render females slower and less able to evade predation, favouring a reduction in feeding in order to avoid a reduction in survivorship. Life-history theory predicts that an organism’s optimal level of investment into reproduction depends upon whether or not the associated ‘costs’ (such as a decrease in rate of feeding) increase with higher fecundity. Anorexia during pregnancy is widespread among snakes, but there are few field data on fecundity-dependence of such costs. Over a 23-year period, we recorded reproductive condition and feeding status (based on palpation and production of faeces) for 3778 captures of free-ranging female natricine colubrid snakes (keelbacks, Tropidonophis mairii ) in tropical Australia. Pregnancy reduced feeding rates, and that decrease was greatest for females with higher reproductive investment (clutch mass relative to maternal mass). Our long-term data provide the first clear-cut evidence of fecundity-dependent costs of reproduction in free-ranging snakes.

Males and females within a population may differ in dietary composition either as a non-adaptive consequence of sexual dimorphism, or because specific food types enhance fitness more in one sex than the other. To test between those two explanations, we can ask whether the consumption of a food type (a) is constrained by sexually dimorphic traits such as body size, or (b) differentially benefits the sex that consumes that food more frequently. A 23-year field study of Slatey-Grey Snakes (Stegonotus australis) in tropical Australia provided data on 663 meals, of which 130 were reptile eggs (primarily from Keelback Snakes (Tropidonophis mairii)). Over the same range of snake SVLs, eggs were consumed more often by female than by male Slatey-Grey Snakes (25.8 versus 15.2% of records), but consumption of reptile eggs was independent of snake body size. Female Slatey-Grey Snakes were not more common or more active than males during Keelback nesting periods, but they were more likely to be captured in the vicinity of Keelback oviposition sites than were males. In years with higher availability of Keelback eggs, female Slatey-Grey Snakes had higher clutch sizes and clutch masses (plausibly reflecting the nutritional benefits of eating reptile eggs to provision reptile eggs). In combination, our results suggest an adaptive basis to the sex-based divergence in dietary composition in Slatey-Grey Snakes.

Low population densities at the expanding edge of a biological invasion are predicted to reduce intraspecific competition such that invading organisms evolve to prioritize adaptations for rapid rates of population growth and dispersal over those that facilitate success in competitive interactions. The larvae (tadpoles) of anuran species that breed in isolated waterbodies provide excellent models with which to test this prediction, because they compete for limited food resources and are unable to disperse until after metamorphosis. Consistent with our predictions, experiments with captive‐raised larvae of cane toads ( Rhinella marina ) show that tadpoles from range core populations were more effective competitors (based on rates of growth and development), especially against tadpoles from invasion front populations, and especially when food was limited (intensifying competition for resources). By contrast, response to predators—a selection pressure presumed constant regardless of invasion history—was largely unchanged between core and edge populations. Limiting food availability reduced survival rate and size at metamorphosis by similar amounts in both core and edge populations. The spread of cane toads across tropical Australia thus has been accompanied by a reduction in the competitive ability of tadpoles, as predicted under the hypothesis that competitive ability will decrease at an expanding range edge. Other traits, associated with response to predation and food limitation, appeared unchanged.

Biological invasions can disrupt the close and longstanding coevolved relationships between host and parasites. At the same time, the shifting selective forces acting on demography during invasion can result in rapid evolution of traits in both host and parasite. Hosts at the invasion front may reduce investment into costly immune defences and redistribute those resources to other fitness-enhancing traits. Parasites at the invasion front may have reduced pathogenicity because traits that negatively impact host dispersal are left behind in the expanding range. The host’s immune system is its primary arsenal in the coevolutionary ‘arms race’ with parasites. To assess the effects of invasion history on immune responses to parasite infection, we conducted a cross-infection experiment which paired common-garden reared cane toads and lungworm parasites originating from various sites in their invaded range across northern Australia. Infected toads had larger spleens and higher concentrations of eosinophils than did uninfected toads. Infected toads also exhibited lower bacteria-killing ability, perhaps reflecting a trade-off of resources towards defences that are more specifically anthelminthic. The impact of infection intensity on multiple immune measures differed among toads and parasites from different parts of the invasion trajectory, supporting the hypothesis that invasion has disrupted patterns of local adaptation.

The invasion of cane toads ( Rhinella marina ) across tropical Australia has resulted in the rapid evolution of traits that enable higher rates of dispersal, and that adapt toads to hot dry climates. In anurans, a larger heart facilitates both locomotor activity and desiccation tolerance. Heart size is also often affected, either directly or indirectly, by parasite infections. To test the effects of invasion history and parasite exposure on heart size, we studied common garden‐reared toads whose parents were sourced from diverse locations, and experimentally exposed them to larvae of a nematode lungworm ( Rhabdias pseudosphaerocephala) . Offspring of invasion‐front parents had larger hearts than did conspecifics from long‐established populations. Exposure to infective lungworm larvae decreased heart mass in toads from all populations. Our study suggests that cardiovascular function, like other traits, has evolved rapidly during the toad invasion; and that lungworm parasites can modify the cardiovascular function and hence aerobic capacity of their host.

Considerable research has focused on microbes on amphibian skin, as they act as the first line of defense against invading pathogens. This effort has generated substantial data on patterns across species, space, time, and ontogeny, alongside a growing list of beneficial antifungal symbionts. Though there is evidence of stability in amphibian skin microbial communities, there is also an indication that regular skin shedding reduces cultivable bacteria, with regrowth and recolonization in the period between sheds. This suggests that skin communities are in constant flux, and we lack an understanding of how the membership and structure of those communities are affected by shedding events. In this study, we conducted experiments on cane toads (Rhinella marina) to investigate the influence of shedding on skin microbiomes. We first used quantitative PCR to verify a positive correlation between bacterial loads and time in the days after shedding. We then resampled individuals over time to describe changes in community composition in the 38 h after shedding using amplicon sequencing. Similar to trends of bacterial loads, we found increases in alpha diversity over time after shedding, suggesting that shedding reduces bacterial diversity as it knocks down bacterial loads. During the 38‐h period, community structure became similar to pre‐shed communities in some individuals, but there was no consistent pattern in structural changes among individuals. In light of the amphibian chytridiomycosis pandemic, understanding how physiological events such as skin shedding affect beneficial bacteria and communities on amphibians would provide important insight into amphibian ecology.