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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. [ABSTRACT FROM AUTHOR]

The structure of the skin may evolve rapidly during a biological invasion, for two reasons. First, novel abiotic challenges such as hydric conditions may modify selection of traits (such as skin thickness) that determine rates of evaporative water loss. Second, invaders might benefit from enhanced rates of dispersal, with locomotion possibly facilitated by thinner (and hence more flexible) skin. We quantified thickness of layers of the skin in cane toads (Rhinella marina) from the native range (Brazil), a stepping-stone population (Hawaii), and the invaded range in Australia. Overall, the skin is thinner in cane toads in Australia than in the native range, consistent with selection on mobility. However, layers that regulate water exchange (epidermal stratum corneum and dermal ground substance layer) are thicker in Australia, retarding water loss in hot dry conditions. Within Australia, epidermal thickness increased as the toads colonized more arid regions, but then decreased in the arid Kimberley region. That curvilinearity might reflect spatial sorting, whereby mobile (thin-skinned) individuals dominate the invasion front; or the toads' restriction to moist sites in this arid landscape may reduce the importance of water-conservation. Further work is needed to clarify the roles of adaptation versus phenotypic plasticity in generating the strong geographic variation in skin structure among populations of cane toads. [ABSTRACT FROM AUTHOR]

In many populations of terrestrial snakes, the phenotype of an individual (e.g. body size, sex, colour) affects its habitat use. One cause for that link is gape limitation, which can result in larger snakes eating prey that are found in different habitats. A second factor involves thermoregulatory opportunities, whereby individuals select habitats based upon thermal conditions. These ideas predict minimal intraspecific variation in habitat use in a species that eats small prey and lives in a thermally uniform habitat, such as the sea snake Emydocephalus annulatus , which feeds on tiny fish eggs and lives in inshore coral reefs. To test that prediction, we gathered data on water depths and substrate attributes for 1475 sightings of 128 free-ranging E. annulatus in a bay near Noumea, New Caledonia. Habitat selection varied among individuals, but with a preference for coral-dominated substrates. The body size and reproductive state of a snake affected its detectability in deep water, but overall habitat use was not linked to snake body size, colour morph, sex or pregnancy. A lack of ontogenetic shifts in habitat use allows extreme philopatry in E. annulatus , thereby reducing gene flow among populations and, potentially, delaying recolonization after local extirpation events. [ABSTRACT FROM AUTHOR]

An aquatic animal faces challenges not encountered by its terrestrial counterparts, promoting adaptive responses in multiple traits. For example, a thicker dermis might protect snakes when they are pushed against sharp objects by water currents, and might enable a snake to shed fouling organisms attached to its skin. We thus predicted that marine snakes should have thicker skin than terrestrial species, and that smaller sea snakes should have relatively thicker skin (because absolute, not relative, thickness determines vulnerability to fouling). Measurements of 192 snakes of 44 species supported those predictions. Many (but not all) sea snakes have skins 50% thicker than those of terrestrial and amphibious snake species, representing multiple independent evolutionary origins of thicker skin (in acrochordids, Laticauda sea kraits and both main clades of hydrophiine sea snakes). Marine snakes showed different allometries of skin thickness compared with their terrestrial counterparts; larger snakes had thicker skin within and among species of amphibious and terrestrial snakes, but larger aquatic snake species had thinner skin compared with smaller taxa. Interspecific variation in skin thickness was primarily due to increased collagen in the deep dermis, a physical barrier well suited to protecting against physical injury and to resisting penetration by epibionts. [ABSTRACT FROM AUTHOR]

Biological invasions often involve rapid modification of phenotypic traits. This is presumably in response to the novel pressures to which an invader is exposed, but the proximate basis for those changes remains unclear. Phenotypic changes may be generated by environmental factors (E), genetic factors (G) or the interaction between these two processes (G×E). To explore this issue, we obtained eight clutches of the cane toad (Rhinella marina) from three regions across its range of invasion within Australia, and raised the offspring under standard conditions (diet and/or exercise level manipulations) to clarify the proximate underpinnings of geographical divergence in phenotypic traits. Our results demonstrate that phenotypic variation among Australian cane toad populations is affected by genetics and environment, and an interaction between these two processes. Some traits (e.g. sprint distance) differed among populations, suggesting a heritable basis. Other traits (e.g. relative heart mass) were affected by experimental treatments but not by population. Intriguingly, other traits (e.g. body length) were affected by interactions between population of origin and experimental treatments. The relative importance of G, E and G × E differs among traits, but all three mechanisms have contributed to the rapid phenotypic divergence observed across the Australian range of invasive cane toads. [ABSTRACT FROM AUTHOR]

Measurements of > 3700 field-collected cane toads (Rhinella marina) show that head width relative to body length differs between the sexes and has become modified during the toad's translocation from French Guiana to Hawai'i and then Australia. Relative head width was highest in the native range, and declined progressively during the invasion. In long-colonized areas (French Guiana through to Queensland) male toads have wider heads than females, but this dimorphism decreases and eventually reverses at invasion fronts in both northern and southern Australia. To explore reasons for that variation, we conducted experiments on captive toads. A toad's head width affected its maximal ingestible prey size and prey-handling ability. Head width relative to body length was highly repeatable, consistent over ontogeny, and exhibited significant heritability (h2 = 0.20). Relative head widths differed between the sexes and populations-of-origin even if offspring were raised under standard conditions in captivity. Nonetheless, relative head width of a cane toad also is affected by prey availability. Captive toads raised on a diet of small prey items developed wider heads than did conspecifics raised on larger prey, partly compensating for the trophic limitations of smaller body size. Sexual and geographical divergences in relative head widths in cane toads are thus the combined result of rapid evolutionary divergence (in < 100 years) and an ability of individual toads to flexibly modify this important morphological feature depending upon local conditions. [ABSTRACT FROM AUTHOR]

Although primarily terrestrial, cane toads ( Rhinella marina) sometimes climb near-vertical surfaces (tree-trunks, cliffs, fences) during foraging or dispersal activities. We scored climbing ability (in laboratory trials) of 288 cane toads from four regions in Australia, plus two sites on the island of Hawai'i. We found strong divergence in climbing ability associated not only with sex and relative limb length, but also population of origin. Within each population, longer-limbed individuals (and hence, males rather than females) were better climbers, although the geographical divergence in climbing ability remained significant even when sex and limb length were included in multivariate regression models. The geographical difference in climbing ability (but not morphology) disappeared when the progeny were raised in captivity under identical conditions, without climbing opportunities. Although influenced by morphology, climbing ability in wild-caught cane toads appears to be driven primarily by local environmental conditions that facilitate and/or reward arboreal activity. [ABSTRACT FROM AUTHOR]

At an invasion front, energetic and physiological trade-offs may differ from those at the range-core as a result of selection for enhanced dispersal, combined with a low density of conspecifics (which reduces pathogen transmission and competition for food). We measured traits related to energy stores and immunity in wild cane toads ( Rhinella marina) across a 750-km transect from their invasion front in tropical Australia, back into sites colonized 21 years earlier. Several traits were found to vary with population age; some linearly and others in a curvilinear manner. The relative size of spleens and fat bodies was highest in the oldest and newest populations, where rates of lungworm infection were lowest. Toads from older populations produced more corticosterone in response to a standardized stressor, and had higher lymphocyte counts (but lower basophil counts). The amount of skin swelling elicited by phytohaemagglutinin injection did not vary geographically, although recruitment of leukocytes to the injected tissue was higher in toads from long-colonized areas. Because this was a field-based study, we cannot differentiate the effects of population age, toad density or pathogen pressure on our measures of stress and immune responses, nor can we distinguish whether the causation involves hard-wired adaptive processes or phenotypically plastic responses. Nonetheless, our data demonstrate substantial variation in immune systems among toads at varying distances from an invasion front, showing that a biological invasion imposes strong pressures on physiological systems of the invader. [ABSTRACT FROM AUTHOR]

The rapid evolution of increased dispersal rate during a population's range expansion provides a unique opportunity to detect trade-offs between dispersal and reproduction. If a high reproductive rate slows down an individual's dispersal, vanguard individuals should exhibit a lower reproductive output than conspecifics from long-colonized areas. In the present study, we demonstrate a reduction in reproductive rate in highly dispersive invasion-front populations of cane toads in tropical Australia. [ABSTRACT FROM AUTHOR]

Male body size can play an important role in the mating systems of anuran amphibians. We conducted laboratory- based trials with cane toads Rhinella (Bufo) marina from an invasive population in the wet-dry tropics of northern Australia, to clarify the effects of a male's body size on his reproductive success and behavior (mate choice). Males were stimulated with a synthetic hormone to induce reproductive readiness. Larger body size enhanced a male toad's ability to displace a smaller rival from amplexus, apparently because of physical strength: more force was required to dislodge a larger than a smaller amplectant male. A male's body size also affected his mate-choice criteria. Males of all body sizes were as likely to attempt amplexus with another male as with a female of the same size, and preferred larger rather than smaller sexual targets. However, this size preference was stronger in larger males and hence, amplexus was size-assortative. This pattern broke down when males were given access to already-amplectant male-female pairs: males of all body sizes readily attempted amplexus with the pair, with no size discrimination. An amplectant pair provides a larger visual stimulus, and prolonged amplexus provides a strong cue for sex identification (one of the individuals involved is almost certainly a female). Thus, a male cane toad's body size affects both his ability to defeat rivals in physical struggles over females, and the criteria he uses when selecting potential mates, but the impacts of that selectivity depend upon the context in which mating occurs. [ABSTRACT FROM AUTHOR]

Experimental evidence on the determinants of prey vulnerability is scarce, especially for vertebrates in the field. Invasive species offer robust opportunities to explore prey vulnerability, because the intensity of predation on or by such animals has not been eroded by coevolution. Around waterbodies in tropical Australia, native meat ants ( Iridomyrmex reburrus) consume many metamorph cane toads ( Bufo marinus, an invasive anuran). We document the determinants of toad vulnerability, especially the roles of toad body size and ant density. Larger metamorphs were attacked sooner (because they attracted more ants), but escaped more often. Overall, smaller toads were more likely to be killed. Ant densities influenced toad responses, as well as attack rate and success. Data on the immediate outcomes of attacks underestimate mortality: more than 73% of apparent ‘escapees’ died within 24 h. Because mortality during this period was independent of toad size, predation was less size selective than suggested by immediate outcomes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 738–751. [ABSTRACT FROM AUTHOR]

Previous research on developmentally plastic responses by reptile embryos has paid relatively little attention to tropical species, or to possible interactions between the effects of thermal and hydric regimes. In the present study, eggs of keelback snakes ( Tropidonophis mairii), from a tropical area with strong temporal and spatial variation in soil temperatures and moisture levels, were incubated. The phenotypic traits of hatchling snakes (body size, shape, muscular strength) were affected by moisture content of the incubation medium (vermiculite plus 100% vs. 50% water by mass), by mean incubation temperatures (25.7 vs. 27.9 °C) and by diel thermal variation (diel range 6.0 vs. 8.4 °C). Interactions between these factors were negligible. Cooler, more thermostable, moister conditions resulted in larger offspring, a trait under strong selection in this population. Thermal and hydric conditions covary in potential nest-sites (e.g. deeper nests are more thermostable as well as moister). This covariation may influence the evolution of reaction norms for embryogenesis. For example, if moister nests enhance offspring fitness and are cooler, then selection will favour the ability to develop in cool as well as moist conditions. Thus, the evolution of optimal incubation conditions with respect to one variable (e.g. temperature) may be driven by patterns of association with another variable (e.g. soil moisture) among natural nest-sites. Perhaps for this reason, the thermal optimum for incubation is surprisingly low in this tropical species. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 159–168. [ABSTRACT FROM AUTHOR]