Body-size evolution on islands: are adult size variations in tiger snakes a nonadaptive consequence of selection on birth size?

Mean adult size has been used as the traditional measure of body size to explain trends of insular gigantism and dwarfism in a wide array of taxa. However, patterns of variation in body size at birth have received surprisingly little attention, leaving open the possibility that adult body-size differences are nonadaptive consequences of selection acting on neonate body size. Here I used an empirical and correlative approach to test this hypothesis in a mosaic of 12 island and mainland snake populations in Australia. Data collected on 597 adult and 1,084 neonate tiger snakes showed that (1) both adult and neonate mean body sizes varied strongly across populations; (2) prey diversity and size convincingly explained birth-size variations: birth size-notably, gape size-correlated with prey size; (3) neonate snout-vent length was significantly correlated with neonate gape size; and (4) neonate snout-vent length was significantly correlated with adult snout-vent length. Postnatal growth rates recorded under common-garden conditions differed across populations and were correlated with mean prey size. These data collectively suggest that (1) prey size is the main driver for the evolution of body size at birth in gape-limited predators, (2) adult size variations may reflect selective forces acting on earlier life stages, and (3) adult size variations may also reflect resource availability during ontogeny (notably, prey diversity).     

Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes

It is a well-known phenomenon that islands can support populations of gigantic or dwarf forms of mainland conspecifics, but the variety of explanatory hypotheses for this phenomenon have been difficult to disentangle. The highly venomous Australian tiger snakes (genus Notechis) represent a well-known and extreme example of insular body size variation. They are of special interest because there are multiple populations of dwarfs and giants and the age of the islands and thus the age of the tiger snake populations are known from detailed sea level studies. Most are 5000-7000 years old and all are less than 10,000 years old. Here we discriminate between two competing hypotheses with a molecular phylogeography dataset comprising approximately 4800 bp of mtDNA and demonstrate that populations of island dwarfs and giants have evolved five times independently. In each case the closest relatives of the giant or dwarf populations are mainland tiger snakes, and in four of the five cases, the closest relatives are also the most geographically proximate mainland tiger snakes. Moreover, these body size shifts have evolved extremely rapidly and this is reflected in the genetic divergence between island body size variants and mainland snakes. Within south eastern Australia, where populations of island giants, populations of island dwarfs, and mainland tiger snakes all occur, the maximum genetic divergence is only 0.38%. Dwarf tiger snakes are restricted to prey items that are much smaller than the prey items of mainland tiger snakes and giant tiger snakes are restricted to seasonally available prey items that are up three times larger than the prey items of mainland tiger snakes. We support the hypotheses that these body size shifts are due to strong selection imposed by the size of available prey items, rather than shared evolutionary history, and our results are consistent with the notion that adaptive plasticity also has played an important role in body size shifts. We suggest that plasticity displayed early on in the occupation of these new islands provided the flexibility necessary as the island's available prey items became more depauperate, but once the size range of available prey items was reduced, strong natural selection followed by genetic assimilation worked to optimize snake body size. The rate of body size divergence in haldanes is similar for dwarfs (h(g) = 0.0010) and giants (h(g) = 0.0020-0.0025) and is in line with other studies of rapid evolution. Our data provide strong evidence for rapid and repeated morphological divergence in the wild due to similar selective pressures acting in different directions.     

Body Size Evolution in Insular Speckled Rattlesnakes (Viperidae: Crotalus mitchellii)

Background

Speckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) evolving in response to shifts in prey size.

Methodology/Principal Findings

Insular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Ángel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively.

Conclusions/Significance

Our data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism.     

Feeding preferences in 2 disjunct populations of tiger snakes, Notechis scutatus (Elapidae)

Variations at both the genetic and phenotypic levels play animportant role in responses to food and food-related stimuli.Knowledge of such variations is crucial to understanding howpopulations adapt to changing environments. We investigatedthe dietary preferences of 2 tiger snake populations and comparedthe responses of diet-naive animals (laboratory-born neonates),diet-controlled animals (laboratory-reared juveniles), and naturaldiet–experienced animals (wild-caught adults) to visualand chemical cues from 6 prey types (mouse, skink, silver gull,chicken, shearwater, and frog). The mainland population inhabitsa swamp, feeds mostly on frogs, and suffers heavy predation.The second population inhabits a small nearby offshore islandwith no standing water (no frogs); feeds mostly on skinks, mice,and, as adults, silver gull chicks; and suffers no known predation.Although different prey are eaten in the 2 populations, adultwild-caught snakes from both populations showed a significantpreference for 3 types of prey (frog, mouse, and chick), irrespectiveof their natural diet. Neonates responded to all prey cues morethan they did to control stimuli in both populations. However,the island neonates showed significantly higher interest insilver gull chick stimuli (the main prey of the island adultsnakes) than did their mainland conspecifics. Laboratory-bredjuveniles displayed behavioral plasticity by significantly increasingtheir response to mice after being fed baby mice for 7 months.We conclude that genetic-based differences in food-related cuesare important in tiger snakes but that they are also capableof behavioral plasticity. Island adult and neonate snakes exhibitedresponses to prey types no longer consumed naturally (frog),suggesting that behavioral characters may have been retainedfor long periods under relaxed selection. Island neonates showeda strong interest in a novel prey item (silver gull). This resultcomplements previous work describing how island snakes havedeveloped the ability to swallow larger prey than usual, aswell as seemingly developing a taste for them.     

Rejection of non-adaptive hypotheses for intraspecific variation in trophic morphology in gape-limited predators

Recent studies have interpreted intraspecific divergence in relative head sizes in snakes as evidence for adaptation of the trophic apparatus in gape-limited predators to local prey size. However, such variation might also arise from non-adaptive processes (such as allometry, correlated response, genetic drift, or non-adaptive phenotypic plasticity). We test predictions from these alternative hypotheses using data on the allometric relationship between head size and body size in two wide-ranging snake species: eight populations of adders ( Vipera berus ) and 30 populations of common gartersnakes ( Thamnophis sirtalis ). Our data enable strong rejection of the alternative (non-adaptive) hypotheses, because the relationship between head and body size differed significantly among populations, the geographic distance separating pairs of populations explained less than 1.5% of their divergence in allometric coefficients, and the within-population allometric coefficients were higher than the among-population coefficients in each species. In addition, the geographical variability of allometric coefficients in females did not parallel that in males, suggesting that allometric coefficients have evolved independently in the two sexes. Phenotypic plasticity also cannot explain the data, because laboratory studies show that the allometric relationship between head size and body size is relatively insensitive to differing growth rates. We conclude that the intraspecific head size divergence in these snakes is better explained by spatially heterogeneous selection to optimize prey handling ability, than by non-adaptive processes.     

‘On being the right size’ – Do aliens follow the rules?

Aim

To assess whether mammalian species introduced onto islands across the globe have evolved to exhibit body size patterns consistent with the ‘island rule,’, and to test an ecological explanation for body size evolution of insular mammals.

Location

Islands worldwide.

Methods

We assembled data on body mass, geographical characteristics (latitude, maximum elevation) and ecological communities (number of mammalian competitors, predators and prey) for 385 introduced populations across 285 islands, comprising 56 species of extant, non‐volant mammals. We used linear regression, ANCOVA and regression tree analyses to test whether introduced populations of mammals exhibit the island rule pattern, whether the degree of body size change increased with time in isolation and whether residual variation about the general trend can be attributed to the geographical and ecological characteristics of the islands.

Results

Introduced populations follow the predicted island rule trend, with body size shifts more pronounced for populations with greater residence times on the islands. Small mammals evolved to larger body sizes in lower latitudes and on islands with limited topographic relief. Consistent with our hypothesis on the ecology of evolution, body size of insular introduced populations was influenced by co‐occurring species of mammalian competitors, predators and prey.

Conclusion

The island rule is a pervasive pattern, exhibited across a broad span of geographical regions, taxa, time periods and, as evidenced here, for introduced as well as native mammals. Time in isolation impacts body size evolution profoundly. Body size shift of introduced mammals was much more pronounced with increasing residence times, yet far less than that exhibited by native, palaeo‐insular mammals (residence times > 10,000 years). Given the antiquity of many species introductions, it appears that much of what we view as the natural character and ecological dynamics of recent insular communities may have been rendered artefacts of ancient colonizations by humans and commensals.     

Plasticity matches phenotype to local conditions despite genetic homogeneity across 13 snake populations

In a widespread species, a matching of phenotypic traits to local environmental optima is generally attributed to site-specific adaptation. However, the same matching can occur via adaptive plasticity, without requiring genetic differences among populations. Adult sea kraits (Laticauda saintgironsi) are highly philopatric to small islands, but the entire population within the Neo-Caledonian Lagoon is genetically homogeneous because females migrate to the mainland to lay their eggs at communal sites; recruits disperse before settling, mixing up alleles. Consequently, any matching between local environments (e.g. prey sizes) and snake phenotypes (e.g. body sizes and relative jaw sizes (RJSs)) must be achieved via phenotypic plasticity rather than spatial heterogeneity in gene frequencies. We sampled 13 snake colonies spread along an approximately 200 km northwest–southeast gradient (n > 4500 individuals) to measure two morphological features that affect maximum ingestible prey size in gape-limited predators: body size and RJS. As proxies of habitat quality (HQ), we used protection status, fishing pressure and lagoon characteristics (lagoon width and distance of islands to the barrier reef). In both sexes, spatial variation in body sizes and RJSs was linked to HQ; albeit in different ways, consistent with sex-based divergences in foraging ecology. Strong spatial divergence in morphology among snake colonies, despite genetic homogeneity, supports the idea that phenotypic plasticity can facilitate speciation by creating multiple phenotypically distinct subpopulations shaped by their environment.     

The evolution of prey body size reaction norms in diverse communities

1. Heterogeneous predation risks can select for predator-specific plastic defences in prey populations. However, diverse predation threats can generate diffuse selection, which, in turn, can lead to the evolution of more generalized reaction norms. Unreliable predator cues also can select for more generalized plasticity in prey. 2. Here, I evaluated the extent to which variation in risk from a focal predator vs. variation in risk from predator diversity and composition were associated with variation in body mass reaction norms in 18 prey populations. Toward this end, I assayed the body mass reaction norms in a common garden experiment for spotted salamander larvae Ambystoma maculatum in response to marbled salamander predators Ambystoma opacum, local predator richness and the densities of two auxiliary predator species. 3. When raised under controlled conditions, prey larvae generally were smaller when exposed to A. opacum kairomones. Among populations, the mean and slope of body mass variation was unrelated to A. opacum's local density. 4. Predator richness and several key environmental factors were not associated with reaction norm variation. Instead, the density of an auxiliary newt predator species was correlated with reduced mass reaction norm slopes. Results suggest that diffuse selection from auxiliary predators can modify the evolution of life-history plasticity.     

Aerobic metabolism during predation by a boid snake

We quantified the oxygen uptake rates (VO(2)) and time spent, during the constriction, inspection, and ingestion of prey of different relative sizes, by the prey-constricting boid snake Boa constrictor amarali. Time spent in prey constriction varied from 7.6 to 16.3 min, and VO(2) during prey constriction increased 6.8-fold above resting values. This was the most energy expensive predation phase but neither time spent nor metabolic rate during this phase were correlated with prey size. Similarly, prey size did not affect the VO(2) or duration of prey inspection. Prey ingestion time, on the other hand, increased linearly with prey size although VO(2) during this phase, which increased 4.9-fold above resting levels, was not affected by prey size. The increase in mechanical difficulty of ingesting larger prey, therefore, was associated with longer ingestion times rather than proportional increases in the level of metabolic effort. The data indicate that prey constriction and ingestion are largely sustained by glycolysis and the intervening phase of prey inspection may allow recovery between these two predatory phases with high metabolic demands. The total amount of energy spent by B. c. amarali to constrict, inspect, and ingest prey of sizes varying from 5 to 40% of snake body mass varied inversely from 0.21 to 0.11% of the energy assimilated from the prey, respectively. Thus, prey size was not limited by the energetic cost of predation. On the contrary, snakes feeding on larger prey were rewarded with larger energetic returns, in accordance with explanations of the evolution of snake feeding specializations.     

Rapid prey‐induced shift in body size in an isolated snake population (Notechis scutatus,Elapidae)

Abstract Geographic divergence in phenotypic traits between long‐isolated populations likely has a genetic basis, but can phenotypic plasticity generate such divergence rapidly in the initial stages of isolation? Australian tiger snakes (Notechis scutatus, Elapidae) provide a classic model system for the evolution of body size: mean adult sizes are relatively invariant in mainland populations, but many offshore islands have dwarf or giant populations. Previous work has shown a genetic basis to this divergence in long‐isolated islands (>10 000 years), but what of the initial stages of this process? Human translocation of mainland snakes to Carnac Island 90 years ago gives us a unique opportunity to assess the proximate reasons for the giant size of Carnac Island animals compared with mainland conspecifics. Our data suggest a major role for phenotypic plasticity. Feeding trials on captive snakes from both island and mainland populations showed a strong link between food intake and growth rates, similar in the two populations. Snakes given abundant food grew much larger than we have ever recorded in the wild, demonstrating that observed mean body sizes are driven by food availability rather than genetic limits to growth. In combination with earlier work showing genetic divergence in growth rates in snakes from long‐isolated islands, our data suggest that geographical divergence in mean adult body sizes in this system initially is driven by a rapid shift due to phenotypic plasticity, with the divergence later canalized by a gradual accumulation of genetic differentiation.     

Testing mechanisms of Bergmann's rule: phenotypic decline but no genetic change in body size in three passerine bird populations

Bergmann's rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether "Bergmann size clines" are due to a genetic response or are a consequence of phenotypic plasticity. Here, we use data on body size (mass and tarsus length) from three long-term (1979-2008) study populations of great tits (Parus major) that experienced a temperature increase to examine mechanisms behind Bergmann's rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than a decrease, in body mass. There was no support for microevolutionary change, and thus the observed declines in body mass were likely a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as adaptive evolutionary responses to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.     

Allometry and selection in a novel predator–prey system: Australian snakes and the invading cane toad

Because many organismal traits vary with body size, interactions between species can be affected by the respective body sizes of the participants. We focus on a novel predator–prey system involving an introduced, highly toxic anuran (the cane toad, Bufo marinus ) and native Australian snakes. The chance of a snake dying after ingesting a toad depends on the size of the snake and the size of the toad, and ultimately reflects the effect of four allometries: (1) physiological tolerance (the rate that physiological tolerance to toad toxin changes with snake size); (2) swallowing ability (the rate that maximal ingestible toad size (i.e. snake head size) increases with snake body size); (3) prey size (the rate that prey size taken by snakes increases with snake head size) and (4) toad toxicity (the rate that toxicity increases with toad size). We measured these allometries, and combined them to estimate the rate at which a snake's resistance changes with toad toxicity. The parotoid glands (and thus, toxicity) of toads increased disproportionately with toad size (i.e. relative to body size, larger toads were more toxic) but simultaneously, head size relative to body size (and thus, maximal ingestible prey size relative to predator size) declined with increasing body size in snakes. Thus, these two allometries tended to cancel each other out. Physiological tolerance to toxins did not vary with snake body size. The end result was that across snake species, mean adult body size did not affect vulnerability. Within species, however, smaller predators were more vulnerable, because the intraspecific rate of decrease in relative head size of snakes was steeper than the rate of increase in toxicity of toads. Thus, toad invasion may cause disproportionate mortality of juvenile snakes, and adults of the sex with smaller mean adult body sizes.     

Environmental and genetic variation in the haematocrit of fledgling pied flycatchers Ficedula hypoleuca

We exposed females of a highly placentotrophic viviparous scincid lizard (Pseudemoia pagenstecheri) to various environmental factors during pregnancy, and quantified the effects of these treatments on their offspring. The clear result was that the phenotypes of neonatal lizards can be substantially modified by the environment that their mother experiences during gestation. Restricting prey availability to the females reduced the size of their offspring. Limiting the females' basking opportunities delayed their seasonal timing of parturition, and modified body proportions (tail length relative to snout-vent length) of the neonates. More surprisingly, female lizards that were regularly exposed to the scent of sympatric lizard-eating snakes gave birth to offspring that were heavier, had unusually long tails relative to body length, and were highly sensitive to the odour of those snakes (as measured by tongue-flick responses). The neonates' antipredator responses were also modified by the experimental treatment to which their mother was exposed. The modifications in body mass, tail length and response to snake scent plausibly reduce the offspring's vulnerability to predatory snakes, and hence may constitute adaptive maternal manipulations of the neonatal phenotype. Received: 6 July 1998 / Accepted: 5 December 1998     

Interactions between locomotion,feeding, and bodily elongation during the evolution of snakes

Information from lizard lineages that have evolved a highly elongate (snake‐like) body form may clarify the selective forces important in the early evolution of snakes. Lizards have evolved bodily elongation via two distinct routes: as an adaptation to burrowing underground or to rapid locomotion above ground. These two routes involve diametrically opposite modifications to the body plan. Burrowing lizards have elongate trunks, small heads, short tails, and relatively constant body widths, whereas surface‐active taxa typically have shorter trunks, wider heads, longer tails, and more variable body widths. Snakes resemble burrowing rather than surface‐active (or aquatic) lizards in these respects, suggesting that snakes evolved from burrowing lizards. The trunk elongation of burrowing lizards increases the volume of the alimentary tract, so that an ability to ingest large meals (albeit consisting of small individual prey items) was present in the earliest snakes. Subsequent shifts to ingestion of wide‐bodied prey came later, after selection dismantled other gape‐constraining morphological attributes, some of which may also have arisen as adaptations to burrowing through hard soil (e.g. relatively small heads, rigid skulls). Adaptations of snake skulls to facilitate ingestion of large prey have evolved to compensate for the reduction of relative head size accompanying bodily elongation; relative to predator body mass, maximum sizes of prey taken by snakes may not be much larger than those of many lizards. This adaptive scenario suggests novel functional links between traits, and a series of testable predictions about the relationships between squamate morphology, habitat, and trophic ecology. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 293–304.     

An experimental study of body size and food size relations in crucian carp,Carassius carassius

Synopsis In dense, single-species assemblages of crucian carp, competition is intense and results in populations of stunted fish. To explore mechanisms underlying this competition, we measured handling times, return rates, and prey choice for five sizes of crucian carp feeding on six sizes of a standardized food. Handling times increased with prey size and decreased with fish size. Return rates (dry mass ingested per unit handling time) increased dramatically with fish size, and generally decreased with increasing prey size, especially for small fish. Patterns of return rates among size-classes suggested that one or more size-related shifts in feeding efficiency exist for crucian carp; combined with physiological stresses related to winter anoxia, the inability of fish to make these shifts may contribute to size structures observed in high-density populations. Comparisons of relations among fish size, prey size, and return rates for crucian carp and bluegill, Lepomis macrochirus, suggest that similar intraspecific competitive relations exist between generalist species with size-structured populations. Despite differences in return rates among prey sizes, the extent of food selectivity based on prey size exhibited by crucian carp in two types of choice trials was less than predicted. Crucian carp commonly take in several items before mechanically processing food with their pharyngeal apparatus; this multiple prey processing may contribute to disparities between observed choice patterns and those predicted based on return rates for single prey.     

Experimental studies of adaptive differentiation in Bahamian Anolis lizards

Populations of the lizards Anolis carolinensis and A. sagrei were experimentally introduced onto small islands in the Bahamas. Less than 15 years after introduction, we investigated whether the populations had diverged and, if so, whether differentiation was related to island vegetational characteristics or propagule size. No effect of founding population size was evident, but differentiation of A. sagrei appears to have been adaptive, a direct relationship existed between how vegetationally different an experimental island was from the source island and how much the experimental population on that island had diverged morphologically. Populations of A. carolinensis had also diverged, but were too few for quantitative comparisons. A parallel exists between the divergence of experimental populations of A. sagrei and the adaptive radiation of Anolis lizards in the Greater Antilles; in both cases, relative hindlimb length and perch diameter are strongly correlated. This differentiation could have resulted from genetic change or environmentally-driven phenotypic plasticity. Laboratory studies on A. sagrei from a population in Florida indicate that hindlimb length exhibits adaptive phenotypic plasticity. Further studies are required to determine if the observed differences among the experimental populations are the result of such plasticity. Regardless of whether the differences result from plasticity, genetic change, or both, the observation that anole populations differentiate rapidly and adaptively when exposed to novel environmental conditions has important implications for understanding the adaptive radiation of Caribbean anoles.     

Insular shifts in body size of rice frogs in the Zhoushan Archipelago, China

1. Differences in body size between mainland and island populations have been reported for reptiles, birds and mammals. Despite widespread recognition of insular shifts in body size in these taxa, there have been no reports of such body size shifts in amphibians. 2. We provide the first evidence of an insular shift in body size for an amphibian species, the rice frog Rana limnocharis. We found significant increases in body size of rice frogs on most sampled islands in the Zhoushan archipelago when compared with neighbouring mainland China. 3. Large body size in rice frogs on islands was significantly related to increased population density, in both breeding and non-breeding seasons. Increases in rice frog density were significantly related to higher resource availability on islands. Increased resource availability on islands has led to higher carrying capacities, which has subsequently facilitated higher densities and individual growth rates, resulting in larger body size in rice frogs. We also suggest that large body size has evolved on islands, as larger individuals are competitively superior under conditions of harsh intraspecific competition at high densities. 4. Increases in body size in rice frogs were not related to several factors that have been implicated previously in insular shifts in body size in other taxa. We found no significant relationships between body size of rice frogs and prey size, number of larger or smaller frog species, island area or distance of islands from the mainland. 5. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in body size across a range of terrestrial vertebrate taxa, and provide support for recent theoretical work concerning the importance of resource availability for insular shifts in body size.     

Island/mainland body size differences in Australian varanid lizards

Island varanids seem to be an exception to the rule that territorial vertebrate taxa often become gigantic relative to mainland relatives when on islands, whereas non-territorial species become dwarfed (Case 1978). However, no systematic island/mainland studies have examined the empirical size trends in this group of carnivorous lizards. We perform such an analysis for the Australian region and critically evaluate various selective agents that might be responsible for size changes in several island populations. Insular gigantism occurs at least four times among the island populations examined. The magnitude of size change is positively correlated to prey abundance on the islands (as indirectly measured through a condition index of the lizards, essentially a measure of how fat they arc) and the size of prey: islands with large prey have large varanids and vice versa. Since the island population with the largest size change, the Reevesby Varanus rosenbergi, was introduced less than 100 years ago, these size changes can be quite rapid. This might indicate that selective coefficients are strong; however, we can not exclude the possibility that these size differences have no genetic component and simply reflect environmental differences in growth rate and shifts in age structure between island and mainland locations.     

Patterns of tail breakage in the ladder snake (Rhinechis scalaris) reflect differential predation pressure according to body size

Predator-prey interactions are key factors in the evolution of defensive tactics. In snakes, shy organisms from which direct evidence of predator-prey interactions is difficult to obtain, injuries are potential indicators of both the nature and frequency of interactions. We studied the incidence of tail breakage and body scarring in the ladder snake, Rhinechis scalaris, an actively foraging Mediterranean snake, and tested several hypotheses that link body injuries and snake life-history traits, mainly under sexual and ontogenetic aspects. Evidence is presented supporting an ontogenetic shift in the frequency of tail breakage, with the incidence of tail loss increasing as a logistic function of snake size. We relate this finding to the adaptive significance of ontogenetic shifts in dorsal pattern and the reaction of snakes to approaching predators; small individuals are more likely to remain immobile than are medium to large individuals, the former aided by a concealing dorsal pattern with transverse lines, and the latter by a striped pattern. This species exhibited sex differences in body scarring but not in tail breakage, nor did we encounter evidence to suggest that snakes experience multiple tail breaks over time, thus failing to support the sexual difference and multiple tail breakage hypotheses. Moreover, we failed to find a lower frequency of body scarring than tail breakage; hence, our results do not afford evidence that frequency of tail breakage represents an inefficiency of predators in catching or dispatching ophidian prey.     

Predator behaviour and morphology mediates the impact of an invasive species: cane toads and death adders in Australia

The arrival of an invasive species can have severe impacts on native species. The extent of the impact, as well as the speed at which native species may mount an adaptive response, depend upon the correlation between impact and the individual phenotypes of the native species. Strong correlation between phenotype and impact within the native species raises the possibility of rapid adaptive response to the invader. Here, we examine the impact of a dangerous newly arrived prey species (the highly toxic cane toad Bufo marinus ) on naïve predators (death adders Acanthophis praelongus ) in northern Australia. During laboratory trials and field radiotracking, toads killed 48% of the adders we studied. Long-term monitoring of the population also suggests a massive decline (>89%) in recent years concurrent with the arrival of toads. Variation in snake physiology (resistance to toad toxin) had little bearing on snake survival in the field. Snake behaviour (tendency to attack toads) and morphology (body size and head size), however, were strong predictors of snake survival. Smaller snakes with relatively small heads, and snakes that were unwilling to attack toads in the laboratory, had much higher survival rates in the field. These results show that toads have a massive impact on death adder populations, but that snake phenotypes strongly mediate this impact. Thus natural selection is operating on these adder populations and an adaptive response is a possibility. If these adders can rapidly shift toad-relevant morphological and behavioural traits (either through plastic or evolved means), they will ultimately face a lowered impact from this toxic invader.