Cutaneous shedding in amphibians causes shifts in bacterial microbiomes.

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.

Is developmental plasticity triggered by DNA methylation changes in the invasive cane toad (Rhinella marina)?

Many organisms can adjust their development according to environmental conditions, including the presence of conspecifics. Although this developmental plasticity is common in amphibians, its underlying molecular mechanisms remain largely unknown. Exposure during development to either 'cannibal cues' from older conspecifics, or 'alarm cues' from injured conspecifics, causes reduced growth and survival in cane toad (Rhinella marina) tadpoles. Epigenetic modifications, such as changes in DNA methylation patterns, are a plausible mechanism underlying these developmental plastic responses. Here we tested this hypothesis, and asked whether cannibal cues and alarm cues trigger the same DNA methylation changes in developing cane toads. We found that exposure to both cannibal cues and alarm cues was associated with local changes in DNA methylation patterns. These DNA methylation changes affected genes putatively involved in developmental processes, but in different genomic regions for different conspecific-derived cues. Genetic background explains most of the epigenetic variation among individuals. Overall, the molecular mechanisms triggered by exposure to cannibal cues seem to differ from those triggered by alarm cues. Studies linking epigenetic modifications to transcriptional activity are needed to clarify the proximate mechanisms that regulate developmental plasticity in cane toads.

A biological invasion modifies the dynamics of a host-parasite arms race.

By imposing novel selection pressures on both participants, biological invasions can modify evolutionary 'arms races' between hosts and parasites. A spatially replicated cross-infection experiment reveals strong spatial divergence in the ability of lungworms (Rhabdias pseudosphaerocephala) to infect invasive cane toads (Rhinella marina) in Australia. In areas colonized for longer than 20 years, toads are more resistant to infection by local strains of parasites than by allopatric strains. The situation reverses at the invasion front, where super-infective parasites have evolved. Invasion-induced shifts in genetic diversity and selective pressures may explain why hosts gain advantage over parasites in long-colonized areas, whereas parasites gain advantage at the invasion front.

The Well-Being of Older Offenders on Release in the Community.

In Canada and internationally the number of older offenders on parole and living in the community is increasing rapidly. Older offenders in the community are a vulnerable population at high risk for lack of well-being. Semi-structured interviews were conducted with N = 64 offenders aged 50 years and older on conditional release from custody in Canada, including long-term, recidivist, and first-time older offenders. Compared to their non-offender counterparts, older offenders in the community experience many of the same problems of aging and well-being, but are at greater risk for mental health problems, traumatic injuries, and for recidivists, substance abuse. Most long-term and first-time older offenders find themselves living at or below the poverty line. One third of older offenders experience social isolation from community and family due to their criminal history and incarceration. For those with Indigenous ancestry, Indigenous communities, and cultural organizations play a significant role in supporting older offenders.

The adaptive significance of large size at birth in marine snakes.

Evolutionary shifts from one habitat type to another can clarify selective forces that affect life-history attributes. Four lineages of snakes (acrochordids and three clades within the Elapidae) have invaded marine habitats, and all have larger offspring than do terrestrial snakes. Predation by fishes on small neonates offers a plausible selective mechanism for that shift, because ascending to breathe at the ocean surface exposes a marine snake to midwater predation whereas juvenile snakes in terrestrial habitats can remain hidden. Consistent with this hypothesis, snake-shaped models moving through a coral-reef habitat in New Caledonia attracted high rates of attack by predatory fishes, and small models (the size of neonatal terrestrial snakes) were attacked more frequently than were large models (the size of neonatal sea snakes). Vulnerability to predatory fishes may have imposed strong selection for increased offspring size in marine snakes.

Do changes in body mass alter white blood cell profiles and immune function in Australian cane toads (Rhinella marina)?

Variation in food resources can result in dramatic fluctuations in the body condition of animals dependent on those resources. Decreases in body mass can disrupt patterns of energy allocation and impose stress, thereby altering immune function. In this study, we investigated links between changes in body mass of captive cane toads (Rhinella marina), their circulating white blood cell populations, and their performance in immune assays. Captive toads that lost weight over a three-month period had increased levels of monocytes and heterophils and reduced levels of eosinophils. Basophil and lymphocyte levels were unrelated to changes in mass. Because individuals that lost mass had higher heterophil levels but stable lymphocyte levels, the ratio of these cell types was also higher, partially consistent with a stress response. Phagocytic ability of whole blood was higher in toads that lost mass, owing to increased circulating levels of phagocytic cells. Other measures of immune performance were unrelated to mass change. These results highlight the challenges faced by invasive species as they expand their range into novel environments which may impose substantial seasonal changes in food availability that were not present in the native range. Individuals facing energy restrictions may shift their immune function towards more economical and general avenues of combating pathogens. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Geographic variation in bacterial assemblages on cane toad skin is influenced more by local environments than by evolved changes in host traits.

Bacterial assemblages on amphibian skin may play an important role in protecting hosts against infection. In hosts that occur over a range of environments, geographic variation in composition of bacterial assemblages might be due to direct effects of local factors and/or to evolved characteristics of the host. Invasive cane toads (Rhinella marina) are an ideal candidate to evaluate environmental and genetic mechanisms, because toads have evolved major shifts in physiology, morphology, and behavior during their brief history in Australia. We used samples from free-ranging toads to quantify site-level differences in bacterial assemblages and a common-garden experiment to see if those differences disappeared when toads were raised under standardised conditions at one site. The large differences in bacterial communities on toads from different regions were not seen in offspring raised in a common environment. Relaxing bacterial clustering to operational taxonomic units in place of amplicon sequence variants likewise revealed high similarity among bacterial assemblages on toads in the common-garden study, and with free-ranging toads captured nearby. Thus, the marked geographic divergence in bacterial assemblages on wild-caught cane toads across their Australian invasion appears to result primarily from local environmental effects rather than evolved shifts in the host.

Using the interRAI brief mental health screener to identify persons with mental disorders having repeat contact with police.

Repeat contacts between persons with serious mental disorder (PSMD) and police officers are costly to both police services and hospitals in terms of service utilization and can be viewed as a direct indicator of unmet needs and gaps in service provision. The intent of the current study was to examine the demographic and clinical characteristics of PSMD who had repeat contact with police officers in London, Ontario from 2016 to 2019 using data collected using the interRAI Brief Mental Health Screener (BMHS). Negative binomial regression was used on a sample of 4143 cases to develop a model predictive of repeat police contacts. The most parsimonious model predicting police contact based on items on the BMHS included age, command hallucinations, lack of insight, verbal abuse, known to possess weapons and family, friends and caregivers expressing concern over the possibility of self harm. Delusions were also independently significantly associated with repeat encounters. Unique to this study is the observation that possessing a weapon in the past 12 months was included in the predictive model. The results of the study add to the sparse research devoted to identifying the characteristics of PSMD who have repeat contact with police officers. Recommendations include integrating the findings into police training to ensure police officers flag those who have the potential for repeat encounters and refer them to appropriate community mental health service providers for proactive outreach services.

In an arms race between host and parasite, a lungworm's ability to infect a toad is determined by host susceptibility not parasite preference.

Evolutionary arms races can alter both parasite infectivity and host resistance, and it is difficult to separate the effects of these twin determinants of infection outcomes. We used a co-introduced, invasive host-parasite system (the lungworm Rhabdias pseudosphaerocephala and cane toads Rhinella marina), where rapid adaptation and dispersal have led to population differences in infection resistance. We quantified behavioural responses of parasite larvae to skin-chemical cues of toads from different invasive populations, and rates at which juvenile hosts became infected following standardized exposure to lungworms. Chemical cues from toad skin altered host-seeking behaviour by parasites, similarly among populations. The number of infection attempts (parasite larvae entering the host's body) also did not differ between populations, but rates of successful infection (establishment of adult worm in host lungs) were higher for range-edge toads than for range-core conspecifics. Thus, lower resistance to parasite infection in range-edge juvenile toads appears to be due to less effective immune defences of the host rather than differential behavioural responses of the parasite. In this ongoing host-parasite arms race, changing outcomes appear to be driven by shifts in host immunocompetence.

Frequency-dependent Batesian mimicry maintains colour polymorphism in a sea snake population.

Evolutionary theory suggests that polymorphic traits can be maintained within a single population only under specific conditions, such as negative frequency-dependent selection or heterozygote advantage. Non-venomous turtle-headed sea snakes (Emydocephalus annulatus) living in shallow bays near Noumea in New Caledonia exhibit three colour morphs: black, black-and-white banded, and an intermediate (grey-banded) morph that darkens with age. We recorded morph frequencies during 18 consecutive years of surveys, and found that the numbers of recruits (neonates plus immigrants) belonging to each morph increased in years when that morph was unusually rare in the population, and decreased when that morph was unusually common. Thus, morph frequencies are maintained by negative frequency-dependent selection. We interpret the situation as Batesian mimicry of highly venomous sea snakes (Aipysurus, Hydrophis, Laticauda) that occur in the same bays, and range in colour from black-and-white banded to grey-banded. Consistent with the idea that mimicry may protect snakes from attack by large fish and sea eagles, behavioural studies have shown that smaller fish species in these bays flee from banded snakes but attack black individuals. As predicted by theory, mimetic (banded) morphs are less common than the cryptically-coloured melanic morph.

Divergence in life-history traits among three adjoining populations of the sea snake Emydocephalus annulatus (Hydrophiinae, Elapidae).

Life-history traits such as rates of growth, survival and reproduction can vary though time within a single population, or through space among populations, due to abiotically-driven changes in resource availability. In terrestrial reptiles, parameters such as temperature and rainfall generate variation in life-histories-but other parameters likely are more important in marine systems. We studied three populations of sea snakes (Emydocephalus annulatus) in adjacent bays in the IndoPacific archipelago of New Caledonia. The extreme philopatry of individual snakes allows us to unambiguously allocate each animal to one of the three populations. Although water temperatures and rainfall do not differ over this small scale, one site experiences more intense winds, restricting opportunities for foraging. Our 18-year mark-recapture dataset (> 1,200 snakes, > 2,400 captures) reveals significant divergence among populations in life-history traits. Survival rates and population densities were similar among sites, but snakes at the most wind-exposed site (Anse Vata) exhibited lower body condition, slower growth, less frequent production of litters, and smaller litters. Weather-driven variation in feeding rates thus may affect life-history traits of marine snakes as well as their terrestrial counterparts, but driven by different parameters (e.g., wind exposure rather than variation in temperatures or rainfall).

Increased rates of dispersal of free-ranging cane toads (Rhinella marina) during their global invasion.

Invasions often accelerate through time, as dispersal-enhancing traits accumulate at the expanding range edge. How does the dispersal behaviour of individual organisms shift to increase rates of population spread? We collate data from 44 radio-tracking studies (in total, of 650 animals) of cane toads (Rhinella marina) to quantify distances moved per day, and the frequency of displacement in their native range (French Guiana) and two invaded areas (Hawai'i and Australia). We show that toads in their native-range, Hawai'i and eastern Australia are relatively sedentary, while toads dispersing across tropical Australia increased their daily distances travelled from 20 to 200 m per day. That increase reflects an increasing propensity to change diurnal retreat sites every day, as well as to move further during each nocturnal displacement. Daily changes in retreat site evolved earlier than did changes in distances moved per night, indicating a breakdown in philopatry before other movement behaviours were optimised to maximise dispersal.

Population dynamics of the sea snake Emydocephalus annulatus (Elapidae, Hydrophiinae).

For sea snakes as for many types of animals, long-term studies on population biology are rare and hence, we do not understand the degree to which annual variation in population sizes is driven by density-dependent regulation versus by stochastic abiotic factors. We monitored three populations of turtle-headed sea snakes (Emydocephalus annulatus) in New Caledonia over an 18-year period. Annual recruitment (% change in numbers) showed negative density-dependence: that is, recruitment increased when population densities were low, and decreased when densities were high. Windy weather during winter increased survival of neonates, perhaps by shielding them from predation; but those same weather conditions reduced body condition and the reproductive output of adult snakes. The role for density-dependence in annual dynamics of these populations is consistent with the slow, K-selected life-history attributes of the species; and the influence of weather conditions on reproductive output suggests that females adjust their allocation to reproduction based on food availability during vitellogenesis.

Host defense or parasite cue: Skin secretions mediate interactions between amphibians and their parasites.

Amphibian skin secretions (substances produced by the amphibian plus microbiota) plausibly act as a first line of defense against parasite/pathogen attack, but may also provide chemical cues for pathogens. To clarify the role of skin secretions in host-parasite interactions, we conducted experiments using cane toads (Rhinella marina) and their lungworms (Rhabdias pseudosphaerocephala) from the range-core and invasion-front of the introduced anurans' range in Australia. Depending on the geographical area, toad skin secretions can reduce the longevity and infection success of parasite larvae, or attract lungworm larvae and enhance their infection success. These striking differences between the two regions were due both to differential responses of the larvae, and differential effects of the skin secretions. Our data suggest that skin secretions play an important role in host-parasite interactions in anurans, and that the arms race between a host and parasite can rapidly generate spatial variation in critical features of that interaction.

Untangling the influence of biotic and abiotic factors on habitat selection by a tropical rodent.

Understanding how animal populations respond to environmental factors is critical because large-scale environmental processes (e.g., habitat fragmentation, climate change) are impacting ecosystems at unprecedented rates. On an overgrazed floodplain in north-western Australia, a native rodent (Pale Field Rat, Rattus tunneyi) constructs its burrows primarily beneath an invasive tree (Chinee Apple, Ziziphus mauritiana) rather than native trees. The dense thorny foliage of the Chinee Apple may allow high rat densities either because of abiotic effects (shade, in a very hot environment) or biotic processes (protection from trampling and soil compaction by feral horses, and/or predation). To distinguish between these hypotheses, we manipulated Chinee Apple foliage to modify biotic factors (access to horses and predators) but not shade levels. We surveyed the rat population with Elliott traps under treatment and control trees and in the open woodland, in two seasons (the breeding season-January, and the nesting season-May). In the breeding season, we ran giving-up density experiments (GUD) with food trays, to assess the perceived risk of predation by rats across our three treatments. Selective trimming of foliage did not affect thermal regimes underneath the trees but did allow ingress of horses and we observed two collapsed burrows as a consequence (although long term impacts of horses were not measured). The perceived predation risk also increased (GUD values at food trays increased) and was highest in the open woodland. Our manipulation resulted in a shift in rat sex ratios (indicating female preference for breeding under control but not foliage-trimmed trees) and influenced rat behaviour (giving-up densities increased; large dominant males inhabited the control but not treatment trees). Our data suggest that the primary benefit of the Chinee Apple tree to native rodents lies in physical protection from predators and (potentially) feral horses, rather than in providing cooler microhabitat.

Intergenerational effects of manipulating DNA methylation in the early life of an iconic invader.

In response to novel environments, invasive populations often evolve rapidly. Standing genetic variation is an important predictor of evolutionary response but epigenetic variation may also play a role. Here, we use an iconic invader, the cane toad (Rhinella marina), to investigate how manipulating epigenetic status affects phenotypic traits. We collected wild toads from across Australia, bred them, and experimentally manipulated DNA methylation of the subsequent two generations (G1, G2) through exposure to the DNA methylation inhibitor zebularine and/or conspecific tadpole alarm cues. Direct exposure to alarm cues (an indicator of predation risk) increased the potency of G2 tadpole chemical cues, but this was accompanied by reductions in survival. Exposure to alarm cues during G1 also increased the potency of G2 tadpole cues, indicating intergenerational plasticity in this inducible defence. In addition, the negative effects of alarm cues on tadpole viability (i.e. the costs of producing the inducible defence) were minimized in the second generation. Exposure to zebularine during G1 induced similar intergenerational effects, suggesting a role for alteration in DNA methylation. Accordingly, we identified intergenerational shifts in DNA methylation at some loci in response to alarm cue exposure. Substantial demethylation occurred within the sodium channel epithelial 1 subunit gamma gene (SCNN1G) in alarm cue exposed individuals and their offspring. This gene is a key to the regulation of sodium in epithelial cells and may help to maintain the protective epidermal barrier. These data suggest that early life experiences of tadpoles induce intergenerational effects through epigenetic mechanisms, which enhance larval fitness. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

First line of defence: Skin microbiota may protect anurans from infective larval lungworms.

Functional roles of the rich microbiota of the skin are not fully understood, but include protection against microbial diseases and other environmental challenges. In experimental studies, we show that reducing the microbiota from cane toad (Rhinella marina) skin by gently wiping with absorptive gauze resulted in threefold higher rates of infection by lungworms (Rhabdias pseudosphaerocephala) following standardised exposure to infective skin-penetrating larvae. Higher concentrations of microbial DNA were associated with lower rates of lungworm entry. Our data suggest that microbial activity on the anuran skin comprises an important line of defence against attack by macroparasites as well as by fungi and other microbes.

Anthropogenically modified habitats favor bigger and bolder lizards.

Anthropogenic activities often create distinctive but discontinuously distributed habitat patches with abundant food but high risk of predation. Such sites can be most effectively utilized by individuals with specific behaviors and morphologies. Thus, a widespread species that contains a diversity of sizes and behavioral types may be pre-adapted to exploiting such hotspots. In eastern Australia, the giant (to >2 m) lizard Varanus varius (lace monitor) utilizes both disturbed (campground) and undisturbed (bushland) habitats. Our surveys of 27 sites show that lizards found in campgrounds tended to be larger and bolder than those in adjacent bushland. This divergence became even more marked after the arrival of a toxic invasive species (the cane toad, Rhinella marina) caused high mortality in larger and bolder lizards. Some of the behavioral divergences between campground and bushland lizards may be secondary consequences of differences in body size, but other habitat-associated divergences in behavior are due to habituation and/or nonrandom mortality.

Variation in size and shape of toxin glands among cane toads from native-range and invasive populations.

If optimal investment in anti-predator defences depends on predation risk, invading new regions (and thus, encountering different predators) may favour shifts in that investment. Cane toads offer an ideal system to test this prediction: expensive anti-predator toxins are stored mainly in parotoid glands whose dimensions are easy to measure, and toad invasions have changed the suites of predators they encounter. Although plasticity may influence parotoid morphology, comparisons between parents and progeny revealed that gland dimensions were highly heritable. That heritability supports the plausibility of an evolved basis to variation in gland dimensions. Measurements of 3779 adult toads show that females have larger glands than males, invasive populations have larger glands than in the native-range, and that parotoid sexual size dimorphism varies strongly among invaded areas. Geographic variation in parotoid morphology may be driven by predation risk to both adult toads and offspring (provisioned with toxins by their mother), with toxins allocated to eggs exacerbating the risk of cannibalism but reducing the risk of interspecific predation. Investment into chemical defences has evolved rapidly during the cane toad's international diaspora, consistent with the hypothesis that organisms flexibly adjust resource allocation to anti-predator tactics in response to novel challenges.