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

The genetic basis of dispersal in a vertebrate metapopulation

Dispersal affects evolutionary processes by changing population size and genetic composition, influencing the viability and persistence of populations. Investigating which mechanisms underlie variation in dispersal phenotypes and whether populations harbour adaptive potential for dispersal is crucial to understanding the eco-evolutionary dynamics of this important trait. Here, we investigate the genetic architecture of dispersal among successfully recruited individuals in an insular metapopulation of house sparrows. We use an extensive long-term individual-based ecological data set and high-density single-nucleotide polymorphism (SNP) genotypes for over 2500 individuals. We conducted a genome-wide association study (GWAS), and found a relationship between dispersal probability and a SNP located near genes known to regulate circadian rhythm, glycogenesis and exercise performance, among other functions. However, this SNP only explained 3.8% of variance, suggesting that dispersal is a polygenic trait. We then used an animal model to estimate heritable genetic variation (σA 2 ), which composes 10% of the total variation in dispersal probability. Finally, we investigated differences in σA 2 across populations occupying ecologically relevant habitat types (farm vs. non-farm) using a genetic groups animal model. We found different adaptive potentials across habitats, with higher mean breeding value, σA 2 , and heritability for the habitat presenting lower dispersal rates, suggesting also different roles of environmental variation. Our results suggest a complex genetic architecture of dispersal and demonstrate that adaptive potential may be environment dependent in key eco-evolutionary traits. The eco-evolutionary implications of such environment dependence and consequent spatial variation are likely to become ever more important with the increased fragmentation and loss of suitable habitats for many natural populations.

Climate match is key to predict range expansion of the world's worst invasive terrestrial vertebrates

Understanding the determinants of the range expansion of invasive alien species is crucial for developing effective prevention and control strategies. Nevertheless, we still lack a global picture of the potential factors influencing the invaded range expansion across taxonomic groups, especially for the world's worst invaders with high ecological and economic impacts. Here, by extensively collecting data on 363 distributional ranges of 19 of world's worst invasive terrestrial vertebrates across 135 invaded administrative jurisdictions, we observed remarkable variations in the range expansion across species and taxonomic groups. After controlling for taxonomic and geographic pseudoreplicates, model averaging analyses based on generalized additive mixed-effect models showed that species in invaded regions having climates more similar to those of their native ranges tended to undergo a larger range expansion. In addition, as proxies of propagule pressure and human-assisted transportation, the number of introduction events and the road network density were also important predictors facilitating the range expansion. Further variance partitioning analyses validated the predominant role of climate match in explaining the range expansion. Our study demonstrated that regions with similar climates to their native ranges could still be prioritized to prevent the spread of invasive species under the sustained global change.

Spatial sorting promotes rapid (mal)adaptation in the red-shouldered soapberry bug after hurricane-driven local extinctions

Predicting future evolutionary change is a critical challenge in the Anthropocene as geographic range shifts and local extinction emerge as hallmarks of planetary change. Hence, spatial sorting-a driver of rapid evolution in which dispersal-associated traits accumulate along expanding range edges and within recolonized habitats-might be of growing importance in ecology and conservation. We report on the results of a natural experiment that monitored recolonization of host plants by the seed-feeding, red-shouldered soapberry bug, Jadera haematoloma, after local extinctions from catastrophic flooding in an extreme hurricane. We tested the contribution of spatial sorting to generate rapid and persistent evolution in dispersal traits, as well as in feeding traits unrelated to dispersal. Long-winged dispersal forms accumulated in recolonized habitats and due to genetic correlation, mouthparts also became longer and this shift persisted across generations. Those longer mouthparts were probably adaptive on one host plant species but maladaptive on two others based on matching the optimum depth of seeds within their host fruits. Moreover, spatial sorting eroded recently evolved adaptive divergence in mouthpart length among all host-associated biotypes, an outcome pointing to profound practical consequences of the extreme weather event for local adaptation, population resilience and evolutionary futures.

Evolution of personality and locomotory performance traits during a late Pleistocene island colonization in a tree frog

Recent empirical and theoretical studies suggest that personality and locomotory performance traits linked to dispersal abilities are crucial components of the dispersal syndromes, and that they can evolve during range expansions and colonization processes. Island colonization is one of the best characterized processes in dispersal biogeography, and its implication in the evolution of phenotypic traits has been investigated over a wide range of temporal scales. However, the effect of island colonization on personality and performance traits of natural populations, and how these traits could drive island colonization, has been little explored. Noteworthy, no studies have addressed these processes in the context of late Pleistocene range expansions. Here, we investigated the contribution of island colonization triggered by postglacial range expansions to intraspecific variation in personality and locomotory performance traits. We compared boldness, exploration, jumping performance, and stickiness abilities in populations from 3 equidistant areas of the Tyrrhenian tree frog Hyla sarda, 2 from the main island (Corsica Island), and 1 from the recently colonized island of Elba. Individuals from Elba were significantly bolder than individuals from Corsica, as they emerged sooner from a shelter (P = 0.028), while individuals from Corsica showed markedly higher jumping and stickiness performance (both P < 0.001), resulting as more performing than those of Elba. We discuss these results in the context of the major microevolutionary processes at play during range expansion, including selection, spatial sorting, founder effects, and their possible interaction with local adaptation processes.

Spatial ecology of the invasive Asian common toad in Madagascar and its implications for invasion dynamics

Invasion dynamics are determined, among other aspects, by the spatial behaviour of invasive populations. The invasive toad Duttaphrynus melanostictus is spreading inland from the eastern coast of Madagascar, causing considerable ecological impacts. Understanding the basic factors determining the spread dynamics can inform management strategies and provide insights into spatial evolutionary processes. We radio-tracked 91 adult toads in three localities along the invasion gradient to determine whether spatial sorting of dispersive phenotypes is occurring, and investigate intrinsic and extrinsic determinants of spatial behaviour. Overall, toads in our study appeared as habitat generalists, and their sheltering behaviour was tied to water proximity, with toads changing shelter more frequently closer to waterbodies. Toads showed low displacement rates (mean = 4.12 m/day) and quite a philopatric behaviour but were able to perform daily movements of over 50 m. We did not detect any spatial sorting of dispersal-relevant traits nor sex- or size-biased dispersal. Our results suggest that toads are more likely to expand their range during the wet season, and that the range expansion is probably dominated by short-distance dispersal at this stage of the invasion, although a future increase in invasion speed is expected, due to the capacity for long-distance movements of this species.

Travelling with a parasite: the evolution of resistance and dispersal syndromes during experimental range expansion

Rapid evolutionary change during range expansions can lead to diverging range core and front populations, with the emergence of dispersal syndromes (coupled responses in dispersal and life-history traits). Besides intraspecific effects, range expansions may be impacted by interspecific interactions such as parasitism. Yet, despite the potentially large impact of parasites imposing additional selective pressures on the host, their role on range expansions remains largely unexplored. Using microcosm populations of the ciliate Paramecium caudatum and its bacterial parasite Holospora undulata, we studied experimental range expansions under parasite presence or absence. We found that the interaction of range expansion and parasite treatments affected the evolution of host dispersal syndromes. Namely, front populations showed different associations of population growth parameters and swimming behaviours than core populations, indicating divergent evolution. Parasitism reshaped trait associations, with hosts evolved in the presence of the parasite exhibiting overall increased resistance and reduced dispersal. Nonetheless, when comparing infected range core and front populations, we found a positive association, suggesting joint evolution of resistance and dispersal at the front. We conclude that host-parasite interactions during range expansions can change evolutionary trajectories; this in turn may feedback on the ecological dynamics of the range expansion and parasite epidemics.

Sex-based differences in the use of post-fire habitats by invasive cane toads (Rhinella marina)

Wildfires can modify habitat attributes, and those changes may differentially affect males versus females within a species if there is pre-existing niche divergence between the sexes. We used radio-tracking and dissections to study invasive cane toads (Rhinella marina), and performed transect counts on native frogs and cane toads 12 months after extensive fires in forests of eastern Australia. Both toads and native frogs were encountered more frequently in burned sites than in unburned sites. Most microhabitat features were similar between burned versus unburned areas, but fire had differential impacts on the ecology of male versus female toads. In burned areas females were less numerous but were larger, in better body condition, and had consumed more prey (especially, coleopterans and myriapods). The impact of fire on attributes of retreat-sites (e.g., temperature, density of vegetation cover) also differed between the sexes. More generally, intraspecific divergence in ecological traits within a species (as a function of body size as well as sex) may translate into substantial divergences in the impacts of habitat change.

Long distance homing in the cane toad (Rhinella marina) in its native range

Many animals exhibit complex navigation over different scales and environments. Navigation studies in amphibians have largely focused on species with life histories that require accurate spatial movements, such as territorial poison frogs and migratory pond-breeding amphibians that show fidelity to mating sites. However, other amphibian species have remained relatively understudied, leaving open the possibility that well-developed navigational abilities are widespread. Here, we measured short-term space use in non-territorial, non-migratory cane toads (Rhinella marina) in their native range in French Guiana. After establishing site fidelity, we tested their ability to return home following translocations of 500 and 1000 m. Toads were able to travel in straight trajectories back to home areas, suggesting navigational abilities similar to those observed in amphibians with more complex spatial behavior. These observations break with the current paradigm of amphibian navigation and suggest that navigational abilities may be widely shared among amphibians.

Summary: Translocation-homing experiments reveal that non-territorial, non-migratory Rhinella marina can navigate to home areas following displacements exceeding regular, natural movements, suggesting a previously unconsidered prevalence of navigational abilities amongst amphibians.

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.

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.

How is adaptive potential distributed within species ranges?

Quantitative genetic variation (QGV) represents a major component of adaptive potential and, if reduced toward range-edge populations, could prevent a species' expansion or adaptive response to rapid ecological change. It has been hypothesized that QGV will be lower at the range edge due to small populations-often the result of poor habitat quality-and potentially decreased gene flow. However, whether central populations are higher in QGV is unknown. We used a meta-analytic approach to test for a general QGV-range position relationship, including geographic and climatic distance from range centers. We identified 35 studies meeting our criteria, yielding nearly 1000 estimates of QGV (including broad-sense heritability, narrow-sense heritability, and evolvability) from 34 species. The relationship between QGV and distance from the geographic range or climatic niche center depended on the focal trait and how QGV was estimated. We found some evidence that QGV declines from geographic centers but that it increases toward niche edges; niche and geographic distances were uncorrelated. Nevertheless, few studies have compared QGV in both central and marginal regions or environments within the same species. We call for more research in this area and discuss potential research avenues related to adaptive potential in the context of global change.

Spatial ecology of cane toads (Rhinella marina) in their native range: a radiotelemetric study from French Guiana

Like most invasive species, cane toads have attracted less research in their native range than in invaded areas. We radio-tracked 34 free-ranging toads in French Guiana, a source region for most invasive populations, across two coastal and two rainforest sites. Coastal toads generally sheltered in pools of fresh or brackish water but nocturnally foraged on beaches, whereas rainforest toads sheltered in forested habitats, moving into open areas at night. Over five days of monitoring, native toads frequently re-used shelters and moved little between days (means = 10-63 m/site) compared to invasion-front toads from Australia (~ 250 m). Larger toads moved less between days, but displaced in more consistent directions. At night, foraging toads travelled up to 200 m before returning to shelters. Foraging distance was related to body condition at coastal sites, with toads in poorer body condition travelling farther. Rain increased the probability of coastal toads sheltering in the dry habitats where they foraged. Dispersal and rainfall were lower at coastal sites, and the strategies utilized by coastal toads to minimize water loss resembled those of invasive toads in semi-desert habitats. This global invader already exhibits a broad environmental niche and substantial behavioural flexibility within its native range.

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?'

Environmental correlates of genetic variation in the invasive European starling in North America

Populations of invasive species that colonize and spread in novel environments may differentiate both through demographic processes and local selection. European starlings (Sturnus vulgaris) were introduced to New York in 1890 and subsequently spread throughout North America, becoming one of the most widespread and numerous bird species on the continent. Genome-wide comparisons across starling individuals and populations can identify demographic and/or selective factors that facilitated this rapid and successful expansion. We investigated patterns of genomic diversity and differentiation using reduced-representation genome sequencing of 17 winter-season sampling sites. Consistent with this species' high dispersal rate and rapid expansion history, we found low geographical differentiation and few F_ST outliers even at a continental scale. Despite starting from a founding population of ~180 individuals, North American starlings show only a moderate genetic bottleneck, and models suggest a dramatic increase in effective population size since introduction. In genotype-environment associations we found that ~200 single-nucleotide polymorphisms are correlated with temperature and/or precipitation against a background of negligible genome- and range-wide divergence. Given this evidence, we suggest that local adaptation in North American starlings may have evolved rapidly even in this wide-ranging and evolutionarily young system. This survey of genomic signatures of expansion in North American starlings is the most comprehensive to date and complements ongoing studies of world-wide local adaptation in these highly dispersive and invasive birds.

Rapid divergence of parasite infectivity and host resistance during a biological invasion

By perturbing co-evolved interactions, biological invasions provide an opportunity to study the evolution of interactions between hosts and their parasites on ecological timescales. We studied the interaction between the cane toad (Rhinella marina) and its direct-lifecycle lungworm (Rhabdias pseudosphaerocephala) that was brought from South America to Australia with the toads in 1935. Compared with infective parasite larvae from long-established (range-core) toad populations, parasite larvae from toads near the invasion front were larger, lived longer and were better able to resist exposure to toxin from the parotoid glands of toads. Experimentally, we infected the common-garden-reared progeny of toads from range-core and invasion-front populations within Australia with lungworms from both populations. Infective larvae from invasion-front (vs. range-core) populations of the parasite were more successful at entering toads (by skin penetration) and establishing infections in the lungs. Toads from invasion-front populations were less prone to infection by either type of larvae. Thus, within 84 years, parasites at an invasion front have increased infectivity, whereas hosts have increased resistance to parasite infection compared with range-core populations. Rapid evolution of traits might affect host–parasite interactions during biological invasions, generating unpredictable effects both on the invaders and on native ecosystems.

The accelerating anuran: evolution of locomotor performance in cane toads (Rhinella marina, Bufonidae) at an invasion front

As is common in biological invasions, the rate at which cane toads (Rhinella marina) have spread across tropical Australia has accelerated through time. Individuals at the invasion front travel further than range-core conspecifics and exhibit distinctive morphologies that may facilitate rapid dispersal. However, the links between these morphological changes and locomotor performance have not been clearly documented. We used raceway trials and high-speed videography to document locomotor traits (e.g. hop distances, heights, velocities, and angles of take-off and landing) of toads from range-core and invasion-front populations. Locomotor performance varied geographically, and this variation in performance was linked to morphological features that have evolved during the toads' Australian invasion. Geographical variation in morphology and locomotor ability was evident not only in wild-caught animals, but also in individuals that had been raised under standardized conditions in captivity. Our data thus support the hypothesis that the cane toad's invasion across Australia has generated rapid evolutionary shifts in dispersal-relevant performance traits, and that these differences in performance are linked to concurrent shifts in morphological traits.

How biological invasions affect animal behaviour: A global, cross-taxonomic analysis

In the Anthropocene, species are faced with drastic challenges due to rapid, human-induced changes, such as habitat destruction, pollution and biological invasions. In the case of invasions, native species may change their behaviour to minimize the impacts they sustain from invasive species, and invaders may also adapt to the conditions in their new environment in order to survive and establish self-sustaining populations. We aimed at giving an overview of which changes in behaviour are studied in invasions, and what is known about the types of behaviour that change, the underlying mechanisms and the speed of behavioural changes. Based on a review of the literature, we identified 191 studies and 360 records (some studies reported multiple records) documenting behavioural changes caused by biological invasions in native (236 records from 148 species) or invasive (124 records from 50 species) animal species. This global dataset, which we make openly available, is not restricted to particular taxonomic groups. We found a mild taxonomic bias in the literature towards mammals, birds and insects. In line with the enemy release hypothesis, native species changed their anti-predator behaviour more frequently than invasive species. Rates of behavioural change were evenly distributed across taxa, but not across the types of behaviour. Our findings may help to better understand the role of behaviour in biological invasions as well as temporal changes in both population densities and traits of invasive species, and of native species affected by them.

Eco-evolutionary dynamics of range expansion

Understanding the movement of species' ranges is a classic ecological problem that takes on urgency in this era of global change. Historically treated as a purely ecological process, range expansion is now understood to involve eco-evolutionary feedbacks due to spatial genetic structure that emerges as populations spread. We synthesize empirical and theoretical work on the eco-evolutionary dynamics of range expansion, with emphasis on bridging directional, deterministic processes that favor evolved increases in dispersal and demographic traits with stochastic processes that lead to the random fixation of alleles and traits. We develop a framework for understanding the joint influence of these processes in changing the mean and variance of expansion speed and its underlying traits. Our synthesis of recent laboratory experiments supports the consistent role of evolution in accelerating expansion speed on average, and highlights unexpected diversity in how evolution can influence variability in speed: results not well predicted by current theory. We discuss and evaluate support for three classes of modifiers of eco-evolutionary range dynamics (landscape context, trait genetics, and biotic interactions), identify emerging themes, and suggest new directions for future work in a field that stands to increase in relevance as populations move in response to global change.

Colonization history affects heating rates of invasive cane toads

Amphibians in hot climates may be able to avoid high temperatures by controlling their rates of heating. In northern Australia, invasive cane toads (Rhinella marina) experience hot dry conditions in newly-colonized (western) sites but milder conditions in longer-occupied (eastern) sites. Under standardized conditions, toads from western sites heated less rapidly than did conspecifics from an eastern site. The availability of free water slowed heating rates of eastern but not western toads. Thus, the colonization of climatically extreme sites has been accompanied by a rapid shift in the toads’ ability to remain cool under hot conditions, even when free water is not available.

Tradeoffs between dispersal and reproduction at an invasion front of cane toads in tropical Australia

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal. Our dissections of >1,200 field-collected cane toads (Rhinella marina) at a site in tropical Australia reveal rapid changes in morphological and reproductive traits over a three-year period after the invaders first arrived. As predicted, individuals with dispersal-enhancing traits (longer legs, narrower heads) had reduced reproductive investment (lower gonad mass). Post-invasion, the population was increasingly dominated by individuals with less dispersive phenotypes and a higher investment into reproduction (including, increased expression of sexually dimorphic traits in males). These rapid shifts in morphology and reproductive biology emphasise the impacts of the invasion process on multiple, interlinked aspects of organismal biology.

Flight capacity increases then declines from the core to the margins of an invasive species' range

Individuals that disperse farther than other individuals are more likely to be on the frontlines of spreading populations and may be more likely to mate with one another as a consequence of their spatial proximity. Over generations, this process-known as spatial sorting-can produce patterns of increasing dispersal ability from a population's core towards the spreading front. By contrast, when the spread of a population is limited by the availability of suitable habitat, theory predicts that range boundaries can select against more dispersive phenotypes and produce patterns of decreasing dispersal capacity towards population margins. In a common garden study of invasive kudzu bugs (Megacopta cribraria)-which are limited by the availability of hostplants in their southern and western margins-I show that midrange individuals fly 49% farther than individuals in the core and 37% farther than individuals at margins. This result highlights that other processes, such as maternal effects or selection at range boundaries, may create more complicated patterns of dispersal ability across landscapes than predicted by models of spatial sorting alone.

The Snail's Charm

In 2017, The American Naturalist celebrated its 150th anniversary. It was founded as a journal of natural history, yet it developed into an important vehicle of the evolutionary synthesis. During the early years of the journal and through much of the twentieth century, evolutionary theory was developed to explain the history of nature before humankind existed to alter it-when time was expansive and uncommon events, though rare, were frequent enough to effect evolutionary change. Today, with the influence of human activity, dispersal patterns are fundamentally altered, genetic variation is locally limiting in small and fragmented populations, and environments are changing so rapidly that time itself seems limited. How can we use this theory, which was built to explain the past and which depends on an excess of chances and time, to address the challenges of the present and the future when chances are fewer and time seems so short? And does the habit of naturalists to observe, describe, and cultivate a fascination with nature have a place in contemporary science?

Behavioural divergence during biological invasions: a study of cane toads (Rhinella marina) from contrasting environments in Hawai'i

Invasive species must deal with novel challenges, both from the alien environment and from pressures arising from range expansion per se (e.g. spatial sorting). Those conditions can create geographical variation in behaviour across the invaded range, as has been documented across regions of Australia invaded by cane toads; range-edge toads are more exploratory and willing to take risks than are conspecifics from the range-core. That behavioural divergence might be a response to range expansion and invasion per se, or to the different environments encountered. Climate differs across the cane toads' invasion range from the wet tropics of Queensland to the seasonally dry climates of northwestern Western Australia. The different thermal and hydric regimes may affect behavioural traits via phenotypic plasticity or through natural selection. We cannot tease apart the effects of range expansion versus climate in an expanding population but can do so in a site where the colonizing species was simultaneously released in all suitable areas, thus removing any subsequent phase of range expansion. Cane toads were introduced to Hawai'i in 1932; and thence to Australia in 1935. Toads were released in all major sugarcane-growing areas in Hawai'i within a 12-month period. Hence, Hawai'ian cane toads provide an opportunity to examine geographical divergence in behavioural traits in a climatically diverse region (each island has both wet and dry sides) in the absence of range expansion subsequent to release. We conducted laboratory-based behavioural trials testing exploration, risk-taking and response to novelty using field-caught toads from the wet and dry sides of two Hawai'ian islands (Oahu and Hawai'i). Toads from the dry side of Oahu had a higher propensity to take risks than did toads from the dry side of Hawai'i. Toads from Oahu were also more exploratory than were conspecifics from the island of Hawai'i. However, toads from wet versus dry climates were similar in all behaviours that we scored, suggesting that founder effects, genetic drift, or developmentally plastic responses to ecological factors other than climate may have driven behavioural divergence between islands.

Physiological plasticity in a successful invader: rapid acclimation to cold occurs only in cool-climate populations of cane toads (Rhinella marina)

Physiological plasticity may facilitate invasion of novel habitats; but is such plasticity present in all populations of the invader or is it elicited only by specific climatic challenges? In cold-climate areas of Australia, invasive cane toads (Rhinella marina) can rapidly acclimate to cool conditions. To investigate whether this physiological plasticity is found in all invasive cane toads or is only seen in cool climates, we measured the acclimation ability of toads from across Australia and the island of Hawai'i. We collected toads from the field and placed them at either 12 or 24°C for 12 h before measuring their righting response as a proxy for critical thermal minimum (CTmin). Toads from the coolest Australian region (New South Wales) demonstrated plasticity (as previously reported), with exposure to 12°C (vs. 24°C) decreasing CTmin by 2°C. In toads from other Australian populations, CTmins were unaffected by our thermal treatments. Hawai'ian toads from a cool, wet site also rapidly acclimated to cool conditions, whereas those from warmer and drier Hawai'ian sites did not. Thermal plasticity has diverged among populations of invasive cane toads, with rapid acclimation manifested only in two cool-climate populations from widely separated sites. Predictions about the potential range of invasive species thus must consider the possibility of geographic (intraspecific) heterogeneity in thermal plasticity; data from other parts of the species' range may fail to predict levels of plasticity elicited by thermal challenges.

Methods for invasive species control are transferable across invaded areas

Cane Toads (Rhinella marina) are invasive pests in many parts of the world, including the Japanese island of Ishigaki. Extensive research in Australia has identified promising new methods for control, but also has shown that toads exhibit geographic variation in many traits (suggesting that methods developed in one location may not work in another). Can the approaches developed in Australia play a useful role for controlling this invasive species in Japan? Our experimental trials on Ishigaki Island suggest that these new methods can be successfully applied to Japan. First, Cane Toad embryos exposed to chemical cues of conspecific tadpoles exhibited a reduction in viability (subsequent growth and development). This response appears to be species-specific, with native frog embryos not being affected by exposure to cues from toad tadpoles, and Cane Toad embryos not being affected by exposure to cues from native frog tadpoles. Second, Cane Toad tadpoles were attracted to traps containing water from conspecific eggs, and toxin from adult conspecifics. Third, adult Cane Toads were attracted to acoustic cues of calling males, with sex differences in rates of attraction to specific versions of a synthetic call (males were attracted to choruses whereas females were attracted to low-frequency calls). Our results suggest that the methods developed by Australian researchers are applicable to controlling invasive Cane Toads in Japan.

Invasive cane toads are unique in shape but overlap in ecological niche compared to Australian native frogs

Invasive species are an important issue worldwide but predicting invasiveness, and the underlying mechanisms that cause it, is difficult. There are several primary hypotheses to explain invasion success. Two main hypothesis based on niche spaces stand out as alternative, although not exclusive. The empty niche hypothesis states that invaders occupy a vacant niche space in the recipient community, and the niche competition hypothesis states that invaders overlap with native species in niche space. Studies on trait similarity/dissimilarity between the invader and native species can provide information on their niche overlap. Here, we use the highly invasive and well‐studied cane toad (Rhinella marina) to test these two hypotheses in Australia, and assess its degree of overlap with native species in several niche dimensions. We compare extensive morphological and environmental data of this successful invader to 235 species (97%) of native Australian frogs. Our study is the first to document the significant morphological differences between the invasive cane toad and a continent‐wide frog radiation: despite significant environmental overlap, cane toads were distinct in body size and shape from most Australian frog species, suggesting that in addition to their previously documented phenotypic plasticity and wide environmental and trophic niche breadth, their unique shape also may have contributed to their success as an invasive species in Australia. Thus, the invasive success of cane toads in Australia may be explained through them successfully colonizing an empty niche among Australian anurans. Our results support that the cane toad's distinct morphology may have played a unique role in the invasiveness of this species in Australia, which coupled with a broad environmental niche breadth, would have boosted their ability to expand their distribution across Australia. We also propose RLLR (Relative limb length ratio) as a potentially useful measure of identifying morphological niche uniqueness and a potential measure of invasiveness potential in anuran amphibians.

Locomotor performance of cane toads differs between native-range and invasive populations

Invasive species provide a robust opportunity to evaluate how animals deal with novel environmental challenges. Shifts in locomotor performance-and thus the ability to disperse-(and especially, the degree to which it is constrained by thermal and hydric extremes) are of special importance, because they might affect the rate that an invader can spread. We studied cane toads (Rhinella marina) across a broad geographical range: two populations within the species' native range in Brazil, two invasive populations on the island of Hawai'i and eight invasive populations encompassing the eastern, western and southern limits of the toad invasion in Australia. A toad's locomotor performance on a circular raceway was strongly affected by both its temperature and its hydration state, but the nature and magnitude of those constraints differed across populations. In their native range, cane toads exhibited relatively low performance (even under optimal test conditions) and a rapid decrease in performance at lower temperatures and hydration levels. At the other extreme, performance was high in toads from southern Australia, and virtually unaffected by desiccation. Hawai'ian toads broadly resembled their Brazilian conspecifics, plausibly reflecting similar climatic conditions. The invasion of Australia has been accompanied by a dramatic enhancement in the toads' locomotor abilities, and (in some populations) by an ability to maintain locomotor performance even when the animal is cold and/or dehydrated. The geographical divergences in performance among cane toad populations graphically attest to the adaptability of invasive species in the face of novel abiotic challenges.

Variation in Major Histocompatibility Complex diversity in invasive cane toad populations

Context The cane toad (Rhinella marina), a native species of central and southern America, was introduced to Australia in 1935 as a biocontrol agent after a complex history of prior introductions. The population rapidly expanded and has since spread through much of the Australian landmass, with severe impacts on the endemic wildlife, primarily via toxicity to predators. The invasion process has taken its toll on the cane toad, with changes in the immunological capacity across the Australian invasive population. Aims To investigate the immunogenetic underpinnings of these changes, we studied the diversity of the Major Histocompatiblity Complex (MHC) genes in introduced cane toad populations. Methods We studied the diversity of two MHC genes (the classical class I UA locus and a class II DAB locus) and compared these with neutral microsatellite markers in toads from the Australian site of introduction and the Australian invasion front. We also included toads from Hawai’i, the original source of the Australian toads, to infer founder effect. Key results Diversity across all markers was low across Australian and Hawai’ian samples, consistent with a reduction in genetic diversity through multiple founder effects during the course of the successive translocations. In Australia, allelic diversity at the microsatellite markers and the UA locus was reduced at the invasion front, whereas all three alleles at the DAB locus were maintained in the invasion-front toads. Conclusions Loss of allelic diversity observed at the microsatellite markers and the UA locus could be the result of drift and bottlenecking along the invasion process, however, the persistence of DAB diversity warrants further investigation to disentangle the evolutionary forces influencing this locus. Implications Through the use of different molecular markers, we provide a preliminary description of the adaptive genetic processes occurring in this invasive population. The extremely limited MHC diversity may represent low immunogenetic competence across the Australian population, which could be exploited for invasive species management.

It is lonely at the front: contrasting evolutionary trajectories in male and female invaders

Invasive species often exhibit rapid evolutionary changes, and can provide powerful insights into the selective forces shaping phenotypic traits that influence dispersal rates and/or sexual interactions. Invasions also may modify sexual dimorphism. We measured relative lengths of forelimbs and hindlimbs of more than 3000 field-caught adult cane toads (Rhinella marina) from 67 sites in Hawai'i and Australia (1-80 years post-colonization), along with 489 captive-bred individuals from multiple Australian sites raised in a 'common garden' (to examine heritability and reduce environmental influences on morphology). As cane toads spread from east to west across Australia, the ancestral condition (long limbs, especially in males) was modified. Limb length relative to body size was first reduced (perhaps owing to natural selection on locomotor ability), but then increased again (perhaps owing to spatial sorting) in the invasion vanguard. In contrast, the sex disparity in relative limb length has progressively decreased during the toads' Australian invasion. Offspring reared in a common environment exhibited similar geographical divergences in morphology as did wild-caught animals, suggesting a genetic basis to the changes. Limb dimensions showed significant heritability (2-17%), consistent with the possibility of an evolved response. Cane toad populations thus have undergone a major shift in sexual dimorphism in relative limb lengths during their brief (81 years) spread through tropical Australia.

Constructing an Invasion Machine: The Rapid Evolution of a Dispersal-Enhancing Phenotype During the Cane Toad Invasion of Australia

Biological invasions can induce rapid evolutionary change. As cane toads (Rhinella marina) have spread across tropical Australia over an 80-year period, their rate of invasion has increased from around 15 to 60 km per annum. Toads at the invasion front disperse much faster and further than conspecifics from range-core areas, and their offspring inherit that rapid dispersal rate. We investigated morphological changes that have accompanied this dramatic acceleration, by conducting three-dimensional morphometric analyses of toads from both range-core and invasion-front populations. Morphology of heads, limbs, pectoral girdles and pelvic girdles differed significantly between toads from the two areas, ranging from 0.5% to 16.5% difference in mean bone dimensions between populations, with invasion-front toads exhibiting wider forelimbs, narrower hindlimbs and more compact skulls. Those changes plausibly reflect an increased reliance on bounding (multiple short hops in quick succession) rather than separate large leaps. Within an 80-year period, invasive cane toads have converted the basic anuran body plan – which evolved for occasional large leaps to evade predators – into a morphotype better-suited to sustained long-distance travel.

Is the enhanced dispersal rate seen at invasion fronts a behaviourally plastic response to encountering novel ecological conditions?

As a population expands into novel areas (as occurs in biological invasions), the range edge becomes dominated by rapidly dispersing individuals-thereby accelerating the rate of population spread. That acceleration has been attributed to evolutionary processes (natural selection and spatial sorting), to which we add a third complementary process: behavioural plasticity. Encountering environmental novelty may directly elicit an increased rate of dispersal. When we reciprocally translocated cane toads (Rhinella marina) among study sites in southern Australia, the transported animals massively increased dispersal rates relative to residents (to an extent similar to the evolved increase between range-core versus invasion-front toad populations in Australia). The responses of these translocated toads show that even range-core toads are capable of the long-distance dispersal rates of invasion-front conspecifics and suggest that rapid dispersal (rather than evolving de novo) has simply been expanded from facultative to constitutive expression.

Effects of parenting quality on adolescents' personality resemblance to their parents. The TRAILS study

This study considers the development of resemblance between 741 adolescents and their biological parents, across six NEO-PI-R personality traits known to be important in psychological problems: anger-hostility, impulsiveness, vulnerability, assertiveness, excitement-seeking, and self-discipline. We modelled the association between perceived parental warmth and rejection at age eleven and personality resemblance to parents at about age sixteen. Parenting experienced during early adolescence was related to the degree and direction in which adolescents resembled their parents five years later in life. Rejection, especially from fathers, significantly predicted a smaller resemblance to both the parents. Girls were more strongly affected by parental quality than boys, and there was some indication that adolescents responded in opposite ways to parenting from mothers and fathers. This study is a first step in uncovering the complex interplay between parenting, gender, and the current generation's ability to develop personality traits independent from the previous generation.

Toads on Lava: Spatial Ecology and Habitat Use of Invasive Cane Toads (Rhinella marina) in Hawai'i

Most ecological research on cane toads (Rhinella marina) has focused on invasive populations in Australia, ignoring other areas where toads have been introduced. We radio-tracked and spool-tracked 40 toads, from four populations on the island of Hawai'i. Toads moved extensively at night (mean 116 m, from spool-tracking) but returned to the same or a nearby retreat-site each day (from radio-tracking, mean distance between successive retreat sites 11 m; 0 m for 70% of records). Males followed straighter paths during nocturnal movements than did females. Because moist sites are scarce on the highly porous lava substrate, Hawai'ian toads depend on anthropogenic disturbance for shelter (e.g. beneath buildings), foraging (e.g. suburban lawns, golf courses) and breeding (artificial ponds). Foraging sites are further concentrated by a scarcity of flying insects (negating artificial lights as prey-attractors). Habitat use of toads shifted with time (at night, toads selected areas with less bare ground, canopy, understory and leaf-litter), and differed between sexes (females foraged in areas of bare ground with dense understory vegetation). Cane toads in Hawai'i thrive in scattered moist patches within a severely arid matrix, despite a scarcity of flying insects, testifying to the species' ability to exploit anthropogenic disturbance.

The genetic backburn: using rapid evolution to halt invasions

The impact of an invasive species depends upon the extent of area across which it ultimately spreads. A powerful strategy for limiting impact, then, is to limit spread, and this can most easily be achieved by managing or reinforcing natural barriers to spread. Using a simulation model, we show that rapid evolutionary increases in dispersal can render permeable an otherwise effective barrier. On the other hand, we also show that, once the barrier is reached, and if it holds, resultant evolutionary decreases in dispersal rapidly make the barrier more effective. Finally, we sketch a strategy--the genetic backburn--in which low-dispersal individuals from the range core are translocated to the nearside of the barrier ahead of the oncoming invasion. We find that the genetic backburn--by preventing invasion front genotypes reaching the barrier, and hastening the evolutionary decrease in dispersal--can make barriers substantially more effective. In our simulations, the genetic backburn never reduced barrier strength, however, the improvement to barrier strength was negligible when there was substantial long-distance dispersal, or when there was no genetic variation for dispersal distance. The improvement in barrier strength also depended on the trade-off between dispersal and competitive ability, with a stronger trade-off conferring greater power to the genetic backburn.