PubMed Health⌕ Search

Biomedical subjects

R Shine

Publications and source records attributed to R Shine.

At least 19 recordsLinked to original sources
PubMed2006

Do operational sex ratios influence sex allocation in viviparous lizards with temperature-dependent sex determination?

Under certain environmental situations, selection may favour the ability of females to adjust the sex ratio of their offspring. Two recent studies have suggested that viviparous scincid lizards can modify the sex ratio of the offspring they produce in response to the operational sex ratio (OSR). Both of the species in question belong to genera that have also recently been shown to exhibit temperature-dependent sex determination (TSD). Here we test whether pregnant montane water skinks (Eulamprus tympanum) utilise TSD to select offspring sex in response to population wide imbalances in the OSR, by means of active thermoregulation. We use a combination of laboratory and field-based experiments, and conduct the first field-based test of this hypothesis by maintaining females in outdoor enclosures of varying OSR treatments throughout pregnancy. Although maternal body temperature during pregnancy was influenced by OSR, the variation in temperature was not great enough to affect litter sex ratios or any other phenotypic traits of the offspring.

Animals↗
PubMed2006

Why do most tropical animals reproduce seasonally? Testing hypotheses on an Australian snake.

Most species reproduce seasonally, even in the tropics where activity occurs year-round. Squamate reptiles provide ideal model organisms to clarify the ultimate (adaptive) reasons for the restriction of reproduction to specific times of year. Females of almost all temperate-zone reptile species produce their eggs or offspring in the warmest time of the year, thereby synchronizing embryogenesis with high ambient temperatures. However, although tropical reptiles are freed from this thermal constraint, most do not reproduce year-round. Seasonal reproduction in tropical reptiles might be driven by biotic factors (e.g., peak periods of predation on eggs or hatchlings, or food for hatchlings) or abiotic factors (e.g., seasonal availability of suitably moist incubation conditions). Keelback snakes (Tropidonophis mairii, Colubridae) in tropical Australia reproduce from April to November, but with a major peak in May-June. Our field studies falsify hypotheses that invoke biotic factors as explanations for this pattern: the timing of nesting does not minimize predation on eggs, nor maximize food availability or survival rates for hatchlings. Instead, our data implicate abiotic factors: female keelbacks nest most intensely soon after the cessation of monsoonal rains when soils are moist enough to sustain optimal embryogenesis (wetter nests produce larger hatchlings, that are more likely to survive) but are unlikely to become waterlogged (which is lethal to eggs). Thus, abiotic factors may favor seasonal reproduction in tropical as well as temperate-zone animals.

Analysis of Variance↗
PubMed2005

The functional meaning of "prey size" in water snakes (Nerodia fasciata, Colubridae).

The evolutionary success of macrostomatan (enlarged-gape) snakes has been attributed to their ability to consume large prey, in turn made possible by their highly kinetic skulls. However, prey can be "large" in several ways, and we have little insight into which aspects of prey size and shape affect skull function during feeding. We used X-ray videos of broad-banded water snakes (Nerodia fasciata) feeding on both frogs and fish to quantify movements of the jaw elements during prey transport, and of the anterior vertebral column during post-cranial swallowing. In a sample of additional individuals feeding on both frogs and fish, we measured the time and the number of jaw protractions needed to transport prey through the buccal cavity. Prey type (fish vs. frog) did not influence transport kinematics, but did influence transport performance. Furthermore, wider and taller prey induced greater movements of most cranial elements, but wider prey were transported with significantly less anterior vertebral bending. In the performance trials, heavier, shorter, and wider prey took significantly more time and a greater number of jaw protractions to ingest. Thus, the functional challenges involved in prey transport depend not only upon prey mass, but also prey type (fish vs. frog) and prey shape (relative height, width and length), suggesting that from the perspective of a gape-limited predator, the difficulty of prey ingestion depends upon multiple aspects of prey size.

Animals↗
PubMed2005

Lower fecundity in parthenogenetic geckos than sexual relatives in the Australian arid zone.

Theoretical models of the advantage of sexual reproduction typically assume that reproductive output is equal in sexual and parthenogenetic females. We tested this assumption by comparing fecundity between parthenogenetic and sexual races of gekkonid lizards in the Heteronotia binoei complex, collected across a 1200 km latitudinal gradient through the Australian arid zone. Under laboratory conditions, parthenogenetic geckos had approximately 30% lower fecundity when compared with their sexual progenitors, irrespective of body size. Reflecting clutch-size constraints in gekkonids, this fecundity difference was mainly because of fewer clutches over a shorter period. When parthenogens were compared more broadly with all coexisting sexual races across the latitudinal gradient, parthenogens had lower fecundity than sexuals only when corrected for body size. Differences in fecundity between parthenogens and coexisting sexual races depended on which sexual race was considered. There was no significant relationship between fecundity and parasite (mite) load, despite significantly higher mite loads in parthenogens than in sexual races.

Analysis of Variance↗
PubMed2004

Protein catabolism in pregnant snakes (Epicrates cenchria maurus Boidae) compromises musculature and performance after reproduction.

In many species the high energetic demands of reproduction induce a negative energy balance, and thus females must rely on tissue catabolism to complete the reproductive process. Previous works have shown that both fat and protein are energy resources during prolonged fasting in vertebrates. While many ecological studies on energy costs of reproduction have focused on variations in fat stores, the impact of protein investment on the female has not been thoroughly investigated. Notably, as there is no specialized storage form for proteins, intense catabolism is likely to entail structural (musculature) loss that may compromise maternal physical performance after reproduction. Measurements on captive rainbow boas ( Epicrates cenchria maurus) confirm that reproducing females undergo significant protein catabolism (as indicated by elevated plasma uric acid levels) and show considerable musculature loss during gestation (as detected by reduced width of the epaxial muscles). Protein mobilization entailed a significant functional loss that was illustrated by decrements in tests of strength and constriction after parturition. In wild situations, such effects are likely to decrease the snakes' ability to forage and apprehend prey. Hence, the time period needed to recover from reproduction can be extended not only because the female must compensate losses of both fat stores and functional muscle, but also because the ability to do so may be compromised. Performance alteration is likely to be of equal or greater importance than reduced energy stores in the physiological mediation of elevated post-reproduction mortality rates and infrequent reproductive bouts (e.g. biannual or triannual), two common ecological traits of female snakes.

Animals↗
PubMed2003

Competition between tadpoles and mosquito larvae.

Tadpoles and mosquito larvae often co-occur, and may compete for scarce resources. However, competition between such distantly related organisms has attracted less scientific attention than have interactions among closely related taxa. We examined ecological interactions in two tadpole-mosquito systems in southeastern Australia, one from freshwater ponds (Limnodynastes peronii and Culex quinquefasciatus) and one from brackish-water habitats (Crinia signifera and Ochlerotatus australis). Diets of these tadpoles and mosquito larvae overlap considerably, potentially leading to competition for food. Laboratory experiments show that, in both study systems, mosquitoes reduced the growth rates of tadpoles, and tadpoles reduced the growth rates and survival of mosquito larvae. These negative effects were seen even at high food levels. Thus, our study suggests that tadpoles and mosquito larvae affect each other strongly, and do so via pathways other than simple consumptive competition. Because mosquitoes are important vectors for human diseases, the global decline in amphibian populations may have more impact on human health than has generally been anticipated.

Analysis of Variance↗
PubMed2003

Effects of the insecticide endosulfan and presence of congeneric tadpoles on Australian treefrog (Litoria freycineti) tadpoles.

The organochlorine insecticide endosulfan is extensively used in Australia and can often contaminate aquatic environments. However, its effects on Australian frog species are unknown. We exposed tadpoles of the Australian treefrog Litoria frevcineti to endosulfan for 96 hours. Tadpoles exposed to 0.03 or 1.3 microg/L endosulfan grew more slowly than control tadpoles. Furthermore, feeding was inhibited in L. freycineti when the animals were observed halfway through their exposure to a concentration of 1.3 microg/L endosulfan for 96 h. This concentration (which periodically occurs in natural water bodies and rivers) also caused 17% tadpole mortality, and the survivors were more vulnerable to invertebrate (odonate) predation when tested 15 days after transfer to clean water. The presence of a sympatric species of tadpole, Litoria peronii, did not affect survivorship of L. freycineti, but reduced growth rates. Thus, short-term exposure to endosulfan in natural water bodies may influence tadpole viability either immediately or over an extended period.

Animals↗
PubMed2003

Lizards in 'nuclear families': a novel reptilian social system in Egernia saxatilis (Scincidae).

Recent research has revealed unsuspected complexity in social organization among squamate reptiles. In particular, large Australian scincid lizards of the genus Egernia have been reported to occur in large aggregations of closely related individuals. However, the 'nuclear family' structure found in many other 'social' organisms (especially birds) has not been reported from reptiles. Our field studies on black rock skinks (Egernia saxatilis) in southeastern Australia document exactly this pattern. We quantified group composition using behavioural observations at regular intervals over three field seasons, and took tissue samples for parentage analysis. On the focal rock outcrop 72% of lizards were typically found as part of a stable social grouping, with individuals physically associated with other group members in a third of observations. Eighty-five per cent of juveniles lived in social groups, 65% in family groups with at least one of their parents (including 39% with both parents as revealed by parentage analysis of five microsatellite loci). Broader sampling in surrounding areas revealed similar patterns of group size, composition and relatedness. Overall, of the groups that contained more than one adult, 83% contained a single adult pair. Long-term monogamy and group stability were evident from our genetic data, with up to three annual cohorts of full-sib offspring living with their biological parents. Our data expand the range of social systems known for reptiles, and reveal strong convergence towards 'nuclear family' systems in distantly related vertebrates.

Animals↗
PubMed2003

When to be born? Prolonged pregnancy or incubation enhances locomotor performance in neonatal lizards (Scincidae).

The degree of offspring development at hatching (or birth) varies among species within most major vertebrate lineages; altricial vs. precocial birds offer the clearest example of a trade-off between early hatching and the degree of locomotor development of the hatchling. No such diversity has been reported for reptiles, but we suggest that natural selection may fine-tune the time of hatching (in oviparous species) or birth (in viviparous species) to optimize offspring phenotypes and hence, maximize fitness. This hypothesis predicts enhanced neonatal performance after more prolonged incubation or gestation, within as well as among populations. Both published and original data on Australian scincid lizards support this prediction. In a field study, viviparous alpine skinks (Niveoscincus microlepidotus) that gave birth later in the season had faster-running offspring, that had a higher probability of surviving through the first year of life. The enhanced performance and survival were not secondary results of larger offspring size. After controlling for effects of mean incubation temperature, prolonged development also correlated with enhanced locomotor performance in hatchlings from eggs of an oviparous skink (Bassiana duperreyi) incubated at warm temperatures (> 20 degrees C) but not at cooler temperatures (< 20 degrees C). We suggest that embryonic reptiles control their date of hatching or birth and thus, their stage of development at this critical life-history transition.

Adaptation, Physiological↗
PubMed2002

Influences on venom yield in Australian tigersnakes (Notechis scutatus) and brownsnakes (Pseudonaja textilis: Elapidae, Serpentes).

The rates at which venomous animals produce venoms are of obvious biological and medical importance, but factors influencing those rates remain poorly understood. We gathered data on venom yield (wet mass of venom) and percentage solids (dry mass of the venom divided by wet mass) for 53 eastern brownsnakes (Pseudonaja textilis) and 36 mainland tigersnakes (Notechis scutatus) over a 4-year period at Venom Supplies Pty. Ltd, a commercial venom production facility in South Australia. Tigersnakes yielded about threefold more venom (by wet mass) than brownsnakes, but with slightly lower percentage solids. Both species showed significant geographic variation in percentage solids. Venom yields varied as a function of the snake's sex and geographic origin, but these effects were secondary consequences of geographic and sex-based differences in body size. Relative head size affected venom yield in brownsnakes but not tigersnakes. Overall, the amount of venom that a snake produced during milking was affected by its species, its geographic origin, its body size and relative head size, and by the time of year that it was milked, as well as by interactions among these factors. Body size was the most important effect on venom yield, with yields increasing more rapidly with size in brownsnakes than in tigersnakes. Research at the intersection of snake ecology and venom characteristics has great potential, but will require a genuinely interdisciplinary approach.

Animals↗
PubMed2001

Changes in plasma progesterone in relation to vitellogenesis and gestation in the viviparous snake Vipera aspis.

Viviparity has arisen from oviparity on more than 100 independent occasions in squamate reptiles, providing an unique opportunity to investigate the ways in which endocrine control of gestation length has been modified by natural selection during this major transition in reproductive modes. Intuitively, the evolution of viviparity might be expected to involve an increasingly important role for the steroid hormone progesterone, rather than estradiol. Unfortunately, published data on this topic in snakes are scarce and often contradictory. Females of the viviparous snake Vipera aspis reproduce with a lower than annual frequency, providing the opportunity to examine steroid profiles simultaneously in vitellogenic, pregnant, and postparturient versus nonreproductive females. From 1990 to 1994, more than 500 blood samples were collected from more than 100 females. Progesterone, estradiol-17beta, and several plasma metabolites were assayed by radioimmunoassay and spectrophotometry. In contrast to earlier studies, we found significant differences between plasma progesterone levels in reproducing (10.5 +/- 9.1 ng ml(-1), N = 168) and nonreproducing (5.1 +/- 4.2 ng ml(-1), N = 121) females. Estradiol reached high levels during vitellogenesis (4.8 +/- 4.0 ng ml(-1), N = 16), and progesterone levels increased during gestation (from 3.7 ng ml(-1) before gestation to 18.7 ng ml(-1) at midgestation). However, experimental elevation of plasma progesterone levels with implants (up to 44.4 ng ml(-1)) did not modify plasma metabolite levels or delay parturition.

Animals↗
PubMed2001

Does total reproductive effort evolve independently of offspring size?

In all species, patterns of reproductive allocation have important fitness consequences and therefore important implications for life-history evolution. Nearly universally, theory in this field has modeled as independent the evolution of total allocation to offspring and the subsequent division of this allocation into many small versus few large offspring. Yet, some theory and a very small amount of experimental evidence suggest that these life-history traits may be evolutionarily linked. Using comparative analyses of copepod life histories, we illustrate that rather than being evolutionarily independent these traits can be linked, in this case, across a very large clade of invertebrates. Our results indicate that a more complete understanding of the evolution of these traits will require greater consideration of simultaneous allocation decisions, rather than sequential ones, and other genetic and selective mechanisms.

Animals↗
PubMed2001

Bumpus in the snake den: effects of sex, size, and body condition on mortality of red-sided garter snakes.

Huge breeding aggregations of red-sided garter snakes (Thamnophis sirtalis parietalis) at overwintering dens in Manitoba provide a unique opportunity to identify sources of mortality and to clarify factors that influence a snake's vulnerability to these factors. Comparisons of sexes, body sizes, and body condition of more than 1000 dead snakes versus live animals sampled at the same time reveal significant biases. Three primary sources of mortality were identified. Predation by crows, Corvus brachyrhynchos (590 snakes killed), was focussed mostly on small snakes of both sexes. Crows generally removed the snake's liver and left the carcass, but very small snakes were sometimes brought back to the nest. Suffocation beneath massive piles of other snakes within the den (301 dead animals) involved mostly small males and (to a lesser extent) large females; snakes in poor body condition were particularly vulnerable. Many emaciated snakes (n = 142, mostly females) also died without overt injuries, probably due to depleted energy reserves. These biases in vulnerability are readily interpretable from information on behavioral ecology of the snakes. For example, sex biases in mortality reflect differences in postemergence behavior and locomotor capacity, the greater attractiveness of larger females to males, and the high energy costs of reproduction for females.

Animals↗
PubMed2000

The transvestite serpent: why do male garter snakes court (some) other males?

In large mating aggregations of red-sided garter snakes, Thamnophis sirtalis parietalis, in Manitoba, male courtship is directed not only to females, but also to other males with female-like skin lipids ('she-males'). We show that 'she-maleness' is an intrinsic property of a male rather than an artefact of lipid transfer from females, and that male-male courtship is very common in the field. She-males were distinctive in terms of appearance (they were heavier than other males and more often covered with mud), behaviour (they were inactive and rarely courted females) and performance (they were slow crawlers, ineffective courters and easily outcompeted by other males in mating trials). 'She-maleness' was not a characteristic of a particular subset of males, as envisaged in previous work; instead, it was a transitory phase that most (perhaps all) male snakes passed through soon after they first emerged from the winter den. Recently emerged males spent their first day or two relatively inactive, while restoring physiological functions (including locomotor performance and courtship ability). Experimental application of female skin lipids on to males dramatically decreased courtship levels of the recipient snakes. Thus, recently emerged males may derive two kinds of benefit from mimicking female skin lipids. First, female mimicry 'switches off' the male's own (energetically expensive) courtship at a time when that courtship would be unproductive. Second, it may disadvantage his rivals by distracting them from females, and increasing their energy expenditure. Copyright 2000 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

Why is sex determined by nest temperature in many reptiles?

Temperature-dependent sex determination (TSD) is widespread in reptiles but its adaptive significance remains controversial. The most plausible theoretical models for the evolution of TSD rely upon differential fitness of male and female offspring incubated under different thermal regimes. Several mechanisms might generate these fitness differentials, and recent work provides empirical support for some of them.

Journal Article↗