Melanie J. Elphick

Longterm effects of incubation temperatures on the morphology and locomotor performance of hatchling lizards (Bassiana duperreyi, Scincidae)

The phenotypes of hatchling reptiles are known to be affected by the thermal environments they experience during incubation, but the evolutionary and ecological significance of this phenotypic plasticity remains unclear. Crucial issues include: (i) the magnitude of effects elicited by thermal regimes in natural nests (as opposed to constant-temperature incubation); (ii) the persistence of these effects during ontogeny; and (iii) the consistency of these effects across different test conditions (does the thermal regime during embryogenesis simply shift the hatchling’s thermal optimum for performance, or actually modify overall performance ability regardless of temperature?). We examined these questions by incubating eggs of scincid lizards (Bassiana duperreyi) from montane southeastern Australia, under two fluctuating-temperature regimes that simulated ‘cold’ and ‘hot’ natural nests. These thermal regimes substantially modified hatchling morphology (mass, body length, tail length, and the relationship between these variables), locomotor performance (running speeds over distances of 25cm and 1m), antipredator ‘tactics’ and survival rates. The differences in locomotor performance persisted throughout the 20 weeks of our experiment. Lizards that emerged after ‘hot’ incubation were faster runners than their ‘cold’-incubated siblings under all thermal conditions that we tested. Thus, incubation temperatures modified overall locomotor ability, with only a minor effect on the set-point for optimum performance. The magnitude, persistence and consistency of these incubation-induced phenotypic modifications suggest that they may play an important role in evolutionary and ecological processes within lizard populations.

THE INFLUENCE OF NEST TEMPERATURES AND MATERNAL BROODING ON HATCHLING PHENOTYPES IN WATER PYTHONS

Previous work on phenotypic plasticity in hatchling reptiles has dealt almost exclusively with lizards and turtles from temperate zone habitats, in taxa where the only maternal control over incubation regimes is exerted via nest site selection. In contrast, water pythons (Liasis fuscus) in northern Australia are tropical snakes that show facultative maternal brooding, with shivering thermogenesis to warm the clutch. Thus, incubation temperatures of this species are influenced both by nest site selection and by maternal care. We experimentally simulated three thermal regimes typical of different types of natural nests in our study population. These were (1) hot, stable temperatures typical of nests laid in the burrows of varanid lizards (constant 32°C); (2) lower and more variable temperatures typical of nests laid inside tree root boles, either with maternal attendance (diel range 27.1°–32.9°C); or (3) in root boles but without maternal attendance (24.3°–32.9°C). We incubated 187 eggs from 15 clutches obt...

Nesting lizards (Bassiana duperreyi) compensate partly, but not completely, for climate change

Species in which ambient temperatures directly determine offspring sex may be at particular risk as global climates change. Whether or not climate change affects sex ratio depends upon the effectiveness of buffering mechanisms that link ambient regimes to actual nest temperatures. For example, females may simply lay nests earlier in the season, or in more shaded areas, such that incubation thermal regimes are unchanged despite massive ambient fluctuation. Based on eight years of monitoring nests over a 10-year period in the field at an alpine site in southeastern Australia, we show that, even though lizards (Bassiana duperreyi, Scincidae) have adjusted both nest depth and seasonal timing of oviposition in response to rising ambient temperatures, they have been unable to compensate entirely for climate change. That inability stems from the fact that the seasonal progression of soil temperatures, and thus, the degree to which thermal regimes at the time of laying predict subsequent conditions during incubation, also has shifted with climate change. As a result, mean incubation temperatures in natural nests now have crossed the thermal threshold at which incubation temperature directly affects offspring sex in this population.

The effect of short-term weather fluctuations on temperatures inside lizard nests, and on the phenotypic traits of hatchling lizards

Can short-term stochastic variation in local weather conditions modify the thermal conditions inside lizard nests, and thus (potentially) the developmental rates, hatching success, and phenotypic traits of hatchlings from these nests? This hypothesis requires that (i) natural nests are poorly buffered thermally, such that ambient regimes affect temperatures inside the nest, and (ii) short-term thermal variations modify attributes of the offspring. Field data on natural nests of the sub-alpine skink Bassiana duperreyi confirm the existence of this first effect, and laboratory experiments substantiate the latter. Exposure to warmer-than-usual temperatures for 2 weeks during the 9to 16-week incubation period doubled hatching success, and significantly modified hatchling phenotypes (hatching dates, offspring size and locomotor performance). The proportion of development completed prior to this exposure influenced the degree of response. Exposure to a brief ‘window’ of higher-than-usual temperatures soon after oviposition had more effect on hatching time, egg survival and hatchling phenotypes than if the exposure occurred later in development. Thus, minor variations in weather conditions during incubation may have substantial effects on reptile populations.  2001 The Linnean Society of London

Sex differences in optimal incubation temperatures in a scincid lizard species

Abstract Most theoretical models for the evolution of temperature-dependent sex determination (TSD) rely upon differential fitness of male and female offspring incubated under different thermal regimes. However, there are few convincing data on this topic. We studied incubation effects in a lizard species (Bassiana duperreyi, Scincidae) with genotypic sex determination, so that we could separate effects due to incubation temperatures from those due to offspring gender. We incubated eggs under two different fluctuating-temperature regimes that simulated hot and cold natural nest-sites. The effects of our incubation treatments on phenotypes of the hatchling lizards (morphology and locomotor performance) differed between the sexes. Females emerging from eggs exposed to the “hot nest” treatment (diel cycling, 23–31°C) were larger, and ran faster, than did their sisters from the “cold nest” treatment (16–24°C). Males showed a smaller and less consistent phenotypic response than females. These incubation-induced responses were relatively stable during the first few weeks of life post-hatching, at least in captive lizards maintained under laboratory conditions. These kinds of sex differences in the phenotypic responses of hatchling reptiles to incubation conditions provide a plausible basis for the evolution of temperature-dependent sex determination in reptiles.

Field experiments on foraging in free-ranging water snakes Enhydris polylepis (Homalopsinae)

Laboratory-based experimental studies of snake foraging have revealed complex and flexible reliance upon multiple cues for prey location and capture: thus, major features of foraging biology depend upon the detailed context of predator–prey encounters and prey antipredator tactics. Accordingly, we need field-based studies to evaluate and extend conclusions from laboratory studies, by (1) recording locations and behaviours of both predators and prey, and (2) experimentally manipulating prey stimuli to identify causal determinants of snake feeding responses. In the Australian wet–dry tropics, the homalopsine snake Enhydris polylepis forages at night in shallow water for sleeping fish. Such fish allow close approach, but flip upwards (and thus at least briefly, out of the water) when touched. Experiments showed that these snakes locate their prey primarily by visual cues, especially movement, rather than by scent or waterborne vibrations. Strikes are elicited by tactile cues, especially the splash of water as a fish leaps upwards. Demonstrating the value of field-based study, data on the location of prey (very shallow water) and their antipredator behaviour (flipping rather than swimming) thus enables us to identify the cue (splashing) most important for eliciting the snakes strike. A conventional laboratory study would not have yielded this insight.

The behavioural consequences of sex reversal in dragons

Sex differences in morphology, physiology, and behaviour are caused by sex-linked genes, as well as by circulating sex-steroid levels. Thus, a shift from genotypic to environmental sex determination may create an organism that exhibits a mixture of male-like and female-like traits. We studied a lizard species (Central Bearded Dragon, Pogona vitticeps), in which the high-temperature incubation of eggs transforms genetically male individuals into functional females. Although they are reproductively female, sex-reversed dragons (individuals with ZZ genotype reversed to female phenotype) resemble genetic males rather than females in morphology (relative tail length), general behaviour (boldness and activity level), and thermoregulatory tactics. Indeed, sex-reversed ‘females’ are more male-like in some behavioural traits than are genetic males. This novel phenotype may impose strong selection on the frequency of sex reversal within natural populations, facilitating rapid shifts in sex-determining systems. A single period of high incubation temperatures (generating thermally induced sex reversal) can produce functionally female individuals with male-like (or novel) traits that enhance individual fitness, allowing the new temperature-dependent sex-determining system to rapidly replace the previous genetically based one.

Potential targets for selection during the evolution of viviparity in cold-climate reptiles

Viviparity (live-bearing) has evolved from oviparity (egg-laying) in more than 100 lineages of squamate reptiles (lizards and snakes). This transition generally has occurred in cool climates, where thermal differentials between eggs in the (cool) nest versus the (warm) maternal oviduct influence embryonic development, in ways that may enhance offspring fitness. To identify specific traits potentially under selection, we incubated eggs of a montane scincid lizard at conditions simulating natural nests, maternal body temperatures, and an intermediate stage (2-week uterine retention of eggs prior to laying). Incubation at maternal temperatures throughout incubation affected the hatchling lizard’s activity level and boldness, as well as its developmental rate, morphology, and locomotor ability. A treatment that mimicked the initial stages of the transition toward viviparity had a major effect on some hatchling traits (locomotor speeds), a minor effect on others (tail length, total incubation period) and no effect on yet others (offspring behaviors). More generally, different aspects of the phenotype are sensitive to incubation conditions at different stages of development; thus, the evolution of reptilian viviparity may have been driven by a succession of advantages that accrued at different stages of embryogenesis.