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.

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.

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.

Disease Exposure and Antifungal Bacteria on Skin of Invasive Cane Toads, Australia.

Cane toads, an invasive species in Australia, are resistant to fungal pathogens affecting frogs worldwide (Batrachochytrium dendrobatidis). From toad skin swabs, we detected higher proportions of bacteria with antifungal properties in Queensland, where toad and pathogen distributions overlap, than in other sites. This finding suggests that site-specific pathogen pressures help shape skin microbial communities.

The cost of chemical defence: the impact of toxin depletion on growth and behaviour of cane toads ( Rhinella marina).

Many animals capable of deploying chemical defences are reluctant to use them, suggesting that synthesis of toxins imposes a substantial cost. Typically, such costs have been quantified by measuring the elevation in metabolic rate induced by toxin depletion (i.e. during replenishment of toxin stores). More generally, we might expect that toxin depletion will induce shifts in a broad suite of fitness-relevant traits. In cane toads ( Rhinella marina), toxic compounds that protect against predators and pathogens are stored in large parotoid (shoulder) glands. We used correlational and experimental approaches in field and laboratory settings to investigate impacts of toxin depletion on growth rate and behaviour in cane toads. In free-ranging toads, larger toxin stores were associated with smaller gonads and livers, suggesting energetic trade-offs between toxin production and both reproduction and energy metabolism. Experimental removal of toxin (by manually squeezing parotoid glands) reduced rates of growth in body mass in both captive and free-ranging toads. Radio tracking demonstrated that de-toxined toads dispersed more slowly than did control toads. Given that toxin stores in cane toads take several months to fully replenish, deploying toxin to repel a predator may impose a substantial cost, explaining why toads use toxin only as a final line of defence.

Pathology Associated With an Outbreak of Entamoebiasis in Wild Cane Toads (Rhinella marina) in Tropical Australia.

Infection due to Entamoeba spp. is known to cause serious disease in primates (Entamoeba histolytica) and snakes (Entamoeba invadens), but there are no detailed descriptions of the pathology associated with Entamoeba spp. infection in amphibians. In 2014, an outbreak of entamoebiasis associated with a novel species of Entamoeba induced clinical illness and poor body condition in free-ranging cane toads in Australia's Northern Territory. Here, we describe the gross pathology, histology, and clinical pathology linked to the outbreak. The study compared 25 toads with invasive entamoebiasis, defined as histologically visible amoebas within tissue, and 12 toads without invasive entamoebiasis. Grossly, affected toads had mild to marked congestion of colonic serosal vasculature, with variable thickening of the intestinal wall and serosanguineous to hemorrhagic colonic content. Histologically, invasive entamoebiasis manifested primarily as moderate to severe, variably hyperplastic to ulcerative colitis. The small intestine was affected in 10 of 25 toads, and 5 of 25 toads also had gastric lesions. Amoebas consistent in morphology with Entamoeba sp. were commonly intermingled with mucosal epithelium, frequently along the basement membrane, with deeper invasion into the superficial lamina propria in only 5 toads. Toads with invasive entamoebiasis had neutrophilia, monocytosis, and lymphopenia, and thus elevated neutrophil to lymphocyte ratios, suggestive of an inflammatory and/or stress leukogram.

Invasive Colonic Entamoebiasis in Wild Cane Toads, Australia.

We detected a disease syndrome in free-ranging Australian cane toads involving atypical behavior and emaciation that is associated with a previously undescribed Entamoeba sp. that infiltrates the colonic lining, causing it to slough. The organism may become seasonally pathogenic when toads are under hydric and nutritional stress.

Invasion history alters the behavioural consequences of immune system activation in cane toads.

Acute activation of the immune system often initiates a suite of behavioural changes. These "sickness behaviours"-involving lethargy and decreased activity-may be particularly costly on invasion fronts, where evolutionary pressures on dispersal favour individuals that move large distances. We used a combination of field and laboratory studies to compare sickness behaviours of cane toads from populations differing in invasion history. To do this we stimulated immune system activation by injecting lipopolysaccharide (LPS) to mimic bacterial infection. We predicted that LPS would result in less severe sickness behaviour in toads from range-edge populations because they had undergone selection for rapid and sustained dispersal (activities in conflict with lethargy and decreased activity). Contrary to our prediction, LPS injection caused a greater reduction in dispersal-relevant traits in invasion-front individuals than in conspecifics from the range-core. Our data suggest that the rapid invasion of cane toads through tropical Australia has seen an evolutionary shift in the magnitude of sickness behaviour elicited by pathogen infection. The increased sickness behaviour among range-edge toads suggests a shift away from pathogen tolerance (seen in range-core populations) towards resistance to pathogen attack. But as a consequence, when pathogens do become successfully established, toads from invasion-front populations may have less capacity to tolerate their ill-effects.

Approved and Off-Label Use of Prescribed Psychotropic Medications among Federal Canadian Inmates.

OBJECTIVE: To examine psychotropic medication prescription practices in federal Canadian penitentiaries. METHOD: 468 files were drawn from a purposive sample of thirteen Canadian federal institutions representing the five regions, different security levels, and male and female designated facilities. Information on the names of all psychotropic medications prescribed, indications for use, dosage, frequency, and route of administration was retrieved. Designation of approved or off-label use of medications was determined by consulting: (1) the Health Canada (2016) Drug Product Database, (2) the Canadian Compendium of Pharmaceuticals and Specialties 2016, and (3) the American Hospital Formulary Service Drug Information 2016. Prescription rates were examined by gender, Indigenous ancestry, drug class, institutional infractions, and current offence. RESULTS: 36.2% of prescriptions for psychotropic medication were coded as 'off-label'. Anxiolytic/hypnotics drugs were the psychotropic drugs most commonly used for off-label purposes. There were no differences in the prevalence of approved versus off-label prescriptions based on Indigenous ancestry or gender, and no pattern of elevated off-label prescription practices for offenders involved in institutional misconducts or those sentenced for the most serious crimes. CONCLUSIONS: The rates of prescribing 'off-label' psychotropic medication are not elevated relative to other correctional settings or to rates cited in Canadian surveys conducted in the community.

Biotic interactions mediate the influence of bird colonies on vegetation and soil chemistry at aggregation sites.

Colonial-nesting organisms can strongly alter the chemical and biotic conditions around their aggregation sites, with cascading impacts on other components of the ecosystem. In tropical Australia, Metallic Starlings (Aplonis metallica) nest in large colonies far above the forest canopy, in emergent trees. The ground beneath those trees is open, in stark contrast to the dense foliage all around. We surveyed the areas beneath 27 colony trees (and nearby randomly chosen trees lacking bird colonies) to quantify the birds' impacts on soil and vegetation characteristics, and to test alternative hypotheses about the proximate mechanisms responsible for the lack of live vegetation beneath colony trees. Nutrient levels were greatly elevated beneath colony trees (especially, those with larger colonies), potentially reaching levels toxic to older trees. However, seedlings thrived in the soil from beneath colony trees. The primary mechanism generating open areas beneath colony trees is disturbance by scavengers (feral pigs and native Turkeys) that are attracted in vast numbers to these nutrient hotspots. Seedlings flourished within exclosures inaccessible to vertebrate herbivores, but were rapidly consumed if unprotected. Our results contrast with previous studies of colonies of seabirds on remote islands, where a lack of large terrestrial herbivores results in bird colonies encouraging rather than eliminating vegetation in areas close to the nesting site. In our continental study system, scavengers may rapidly dilute the spatial heterogeneity generated by the massive nutrient subsidy from bird colonies.

Rapid shifts in dispersal behavior on an expanding range edge.

Dispersal biology at an invasion front differs from that of populations within the range core, because novel evolutionary and ecological processes come into play in the nonequilibrium conditions at expanding range edges. In a world where species' range limits are changing rapidly, we need to understand how individuals disperse at an invasion front. We analyzed an extensive dataset from radio-tracking invasive cane toads (Rhinella marina) over the first 8 y since they arrived at a site in tropical Australia. Movement patterns of toads in the invasion vanguard differed from those of individuals in the same area postcolonization. Our model discriminated encamped versus dispersive phases within each toad's movements and demonstrated that pioneer toads spent longer periods in dispersive mode and displayed longer, more directed movements while they were in dispersive mode. These analyses predict that overall displacement per year is more than twice as far for toads at the invasion front compared with those tracked a few years later at the same site. Studies on established populations (or even those a few years postestablishment) thus may massively underestimate dispersal rates at the leading edge of an expanding population. This, in turn, will cause us to underpredict the rates at which invasive organisms move into new territory and at which native taxa can expand into newly available habitat under climate change.

Invader immunology: invasion history alters immune system function in cane toads (Rhinella marina) in tropical Australia.

Because an individual's investment into the immune system may modify its dispersal rate, immune function may evolve rapidly in an invader. We collected cane toads (Rhinella marina) from sites spanning their 75-year invasion history in Australia, bred them, and raised their progeny in standard conditions. Evolved shifts in immune function should manifest as differences in immune responses among the progeny of parents collected in different locations. Parental location did not affect the offspring's cell-mediated immune response or stress response, but blood from the offspring of invasion-front toads had more neutrophils, and was more effective at phagocytosis and killing bacteria. These latter measures of immune function are negatively correlated with rate of dispersal in free-ranging toads. Our results suggest that the invasion of tropical Australia by cane toads has resulted in rapid genetically based compensatory shifts in the aspects of immune responses that are most compromised by the rigours of long-distance dispersal.

Mathematical modelling of spatial sorting and evolution in a host-parasite system.

There have been numerous empirical and agent-based modelling studies on the spatial self-structuring of traits, particularly in regard to dispersal ability (termed spatial sorting) of cane toads in northern Australia, but few mathematical modelling studies. In this study, we formulate a reaction-diffusion based partial-integro-differential equation model based on an earlier model by Bouin et al. (2012) to examine this spatial self-structuring of traits in both a cane toad population and lungworm parasite population, which evolves with the cane toad population. In particular, the traits we focus on are dispersal ability for the cane toad population and both prepatent period and larval size for the lungworm parasite population. Apart from the spatial self-structuring of these traits, our results confirm a number of observations made in empirical and agent-based studies; particularly, that there is a noticeable lag between the host and parasite population which is critically dependent on the parasite functional response to host densities, that older populations regress back to lower dispersal speeds and that spatial sorting can still occur with a disadvantage in reproductivity and/or survival in more motile individuals. Moreover, we find that such a disadvantage in reproductivity and/or survival is unlikely to be large if spatial sorting is to have a noticeable effect on the rate of range expansion, as it has been observed to have over the last 60 years in northern Australia.

The straight and narrow path: the evolution of straight-line dispersal at a cane toad invasion front.

At the edge of a biological invasion, evolutionary processes (spatial sorting, natural selection) often drive increases in dispersal. Although numerous traits influence an individual's displacement (e.g. speed, stamina), one of the most important is path straightness. A straight (i.e. highly correlated) path strongly enhances overall dispersal rate relative to time and energetic cost. Thus, we predict that, if path straightness has a genetic basis, organisms in the invasion vanguard will exhibit straighter paths than those following behind. Our studies on invasive cane toads (Rhinella marina) in tropical Australia clearly support this prediction. Radio-tracking of field-collected toads at a single site showed that path straightness steadily decreased over the first 10 years post-invasion. Consistent with an evolved (genetic) basis to that behavioural shift, path straightness of toads reared under common garden conditions varied according to the location of their parents' origin. Offspring produced by toads from the invasion vanguard followed straighter paths than did those produced by parents from long-established populations. At the individual level, offspring exhibited similar path straightness to their parents. The dramatic acceleration of the cane toad invasion through tropical Australia has been driven, in part, by the evolution of a behavioural tendency towards dispersing in a straight line.

An evolutionary process that assembles phenotypes through space rather than through time.

In classical evolutionary theory, traits evolve because they facilitate organismal survival and/or reproduction. We discuss a different type of evolutionary mechanism that relies upon differential dispersal. Traits that enhance rates of dispersal inevitably accumulate at expanding range edges, and assortative mating between fast-dispersing individuals at the invasion front results in an evolutionary increase in dispersal rates in successive generations. This cumulative process (which we dub "spatial sorting") generates novel phenotypes that are adept at rapid dispersal, irrespective of how the underlying genes affect an organism's survival or its reproductive success. Although the concept is not original with us, its revolutionary implications for evolutionary theory have been overlooked. A range of biological phenomena (e.g., acceleration of invasion fronts, insular flightlessness, preadaptation) may have evolved via spatial sorting as well as (or rather than) by natural selection, and this evolutionary mechanism warrants further study.

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.

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.

The early toad gets the worm: cane toads at an invasion front benefit from higher prey availability.

In biological invasions, rates of range expansion tend to accelerate through time. What kind of benefits to more rapidly dispersing organisms might impose natural selection for faster rates of dispersal, and hence the evolution of range-edge acceleration? We can answer that question by comparing fitness-relevant ecological traits of individuals at the invasion front compared with conspecifics in the same area a few years post-invasion. In tropical Australia, the rate of invasion by cane toads (Rhinella marina) has increased substantially over recent decades, due to shifts in heritable traits. Our data on field-collected cane toads at a recently invaded site in the Australian wet-dry tropics span a 5-year period beginning with toad arrival. Compared with conspecifics that we monitored in the same sites post-invasion, toads in the invasion vanguard exhibited higher feeding rates, larger energy stores, better body condition and faster growth. Three processes may have contributed to this pattern: (i) higher prey availability at the front (perhaps due to reduced competition from conspecifics); (ii) the lack of viability-reducing parasites and pathogens in invasion-front toads; and (iii) distinctive (active, fast-growing) phenotypes of the invasion-front toads. Nutritional benefits to individuals in the invasion vanguard (whether because of higher prey availability, or lower pathogen levels) thus may have conferred a selective advantage to accelerated dispersal in this system.