Lin Schwarzkopf

Costs of reproduction in lizards: escape tactics and susceptibility to predation

SummaryIncreased basking and reduced agility in gravid female southern water skinks (Eulamprus tympanum) suggest that they will be more vulnerable to predators. However, gravid females shift their anti-predator tactics towards crypsis, by allowing potential predators (such as a human observer) to approach more closely than do males and non-gravid females. Gravid females were taken no more frequently than were non-gravid females or males when exposed to two types of natural predators, birds (kookaburra, Dacelo gigas) or snakes (common blacksnakes, Pseudechis porphyriacus) in field enclosures. Both these results suggest that the vulnerability of potential prey in this system is determined by the predators probability of detecting a potential prey item, not its probability of capturing the prey item after detection. Hence, laboratory-based measures of performance demonstrating reduced escape speed may sometimes have little relevance to actual fitness under field conditions, if the probability that an animal will be taken by a predator depends primarily on whether or not it is seen, rather than on how quickly it can escape.

Primate species richness in relation to habitat structure in Amazonian rainforest fragments

Abstract Factors affecting species richness in fragmented habitat are often of interest to conservation programs. Groups of red howler monkeys Alouatta seniculus , white-faced sakis Pithecia pithecia and golden-handed tamarins Saguinus midas occurred in five small (approx. 10 ha) fragments of Amazonian rainforest. However, not all of these species occurred in every fragment. The purpose of this study was to identify some of the factors affecting the number and distribution of primate species among these areas. To determine whether habitat structural diversity influenced primate species richness, height, diameter at breast height (DBH) and number of trees, DBH and number of lianas (woody vines) as well as height and number of palms in 15 20 × 20 m quadrats in each fragment were recorded. Also, height and number of secondary growth plants surrounding reserves were sampled. Structurally ‘complex’ reserves had high mean number of trees, high mean number of lianas, low mean percentage large trees (> 10 cm DBH) per quadrat, and streams. These characteristics were correlated with high species richness. P. pithecia occurred in the most structurally ‘complex’ fragments, S. midas in fragments surrounded by relatively tall secondary growth and A. seniculus in all five fragments. The relationships between the habitat requirements of the three species and their occurrence in these fragments are discussed.

Thermal biology of reproduction in viviparous skinks, Eulamprus tympanum: why do gravid females bask more?

SummaryIn some reptiles, gravid females bask more, and/or maintain higher body temperatures than do males or non-gravid females. This phenomenon is usually explained in terms of the female or her offspring benefitting from accelerated embryogenesis and early birth, but the effect of increased basking on gestation period has not been studied. In a laboratory experiment, gestation periods of gravid female skinks (Eulamprus tympanum) decreased with the duration of access to radiant heat. Embryonic development was more rapid in the laboratory than in the field, and there were no apparent adverse effects of this accelerated gestation on females or offspring. Number and mass of offspring, survival rates of embryos, relative clutch mass and female mass before and after parturition were not influenced by the decrease in gestation period caused by increased basking. Females selected similar temperatures in the laboratory and field (32° C), despite the availability of higher temperatures in the laboratory. Thus, gestation in the laboratory was accelerated by spending longer periods at usual basking temperatures, rather than by selecting higher temperatures. In the field, mean and modal body temperatures of active animals were similar in gravid females, males and non-gravid females, but gravid females appear to bask more of the time, even in cloudy weather when other members of the population do not bask. Hence, an apparent similarity in thermal regimes of gravid and non-gravid animals may mask significant underlying differences in thermoregulatory strategies.

Nest-site selection and offspring sex ratio in painted turtles, Chrysemys picta

We studied the influence of nest-site selection on nest temperature and clutch sex ratio in a northern population of painted turtles (Chrysemys picta). Comparison of characteristics of pairs of nests of individual females, within and between years, provided no evidence that these females followed a simple pattern in choosing nest sites and no indication that they influenced sex ratio by choosing sites with specific characteristics. Microhabitat features of nests were compared to the same characteristics of randomly chosen sites in the nesting area and in contrast to randomly chosen sites, nests were significantly warmer and more likely to occur on slopes. Sex ratios produced in nests were not correlated with any measured nest characteristic except temperature, and nest temperatures were more dependent on yearly variations in climate than on variations in site characteristics. Nest-temperature distributions were unimodal, suggesting that fe- males selected nest sites to maximize the probability that eggs would complete development, rather than to influence offspring sex ratio.

Comparisons through time and space suggest rapid evolution of dispersal behaviour in an invasive species

During a biological invasion, we expect that the expanding front will increasingly become dominated by individuals with better dispersal abilities. Over many generations, selection at the invasion front thus will favour traits that increase dispersal rates. As a result of this process, cane toads (Bufo marinus) are now spreading through tropical Australia about 5-fold faster than in the early years of toad invasion; but how have toads changed to make this happen? Here we present data from radio-tracking of free-ranging cane toads from three populations (spanning a 15-year period of the toads’ Australian invasion, and across 1800 km). Our data reveal dramatic shifts in behavioural traits (proportion of nights when toads move from their existing retreat-site to a new one, and distance between those successive retreat-sites) associated with the rapid acceleration of toad invasion. Over a maximum period of 70 years (~50 generations), cane toads at the invasion front in Australia apparently have evolved such that populations include a higher proportion of individuals that make long, straight moves.

LIFE ON THE ROCKS: HABITAT USE DRIVES MORPHOLOGICAL AND PERFORMANCE EVOLUTION IN LIZARDS

As a group, lizards occupy a vast array of habitats worldwide, yet there remain relatively few cases where habitat use (ecology), morphology, and thus, performance, are clearly related. The best known examples include: increased limb length in response to increased arboreal perch diameter in anoles and increased limb length in response to increased habitat openness for some skinks. Rocky habitats impose strong natural selection on specific morphological characteristics, which differs from that imposed on terrestrial species, because moving about on inclined substrates of irregular sizes and shapes constrains locomotor performance in predictable ways. We quantified habitat use, morphology, and performance of 19 species of lizards (family Scincidae, subfamily Lygosominae) from 23 populations in tropical Australia. These species use habitats with considerable variation in rock availability. Comparative phylogenetic analyses revealed that occupation of rock-dominated habitats correlated with the evolution of increased limb length, compared to species from forest habitats that predominantly occupied leaf litter. Moreover, increased limb length directly affected performance, with species from rocky habitats having greater sprinting, climbing, and clinging ability than their relatives from less rocky habitats. Thus, we found that the degree of rock use is correlated with both morphological and performance evolution in this group of tropical lizards.

Locomotor performance in an invasive species: cane toads from the invasion front have greater endurance, but not speed, compared to conspecifics from a long-colonised area

Cane toads (Bufo marinus) are now moving about 5 times faster through tropical Australia than they did a half-century ago, during the early phases of toad invasion. Radio-tracking has revealed higher daily rates of displacement by toads at the invasion front compared to those from long-colonised areas: toads from frontal populations follow straighter paths, move more often, and move further per displacement than do toads from older (long-established) populations. Are these higher movement rates of invasion-front toads associated with modified locomotor performance (e.g. speed, endurance)? In an outdoor raceway, toads collected from the invasion front had similar speeds, but threefold greater endurance, compared to conspecifics collected from a long-established population. Thus, increased daily displacement in invasion-front toads does not appear to be driven by changes in locomotor speed. Instead, increased dispersal is associated with higher endurance, suggesting that invasion-front toads tend to spend more time moving than do their less dispersive conspecifics. Whether this increased endurance is a cause or consequence of behavioural shifts associated with rapid dispersal is unclear. Nonetheless, shifts in endurance between frontal and core populations of this invasive species point to the complex panoply of traits affected by selection for increased dispersal ability on expanding population fronts.

A gecko skin micro/nano structure - A low adhesion, superhydrophobic, anti-wetting, self-cleaning, biocompatible, antibacterial surface.

Geckos, and specifically their feet, have attracted significant attention in recent times with the focus centred around their remarkable adhesional properties. Little attention however has been dedicated to the other remaining regions of the lizard body. In this paper we present preliminary investigations into a number of notable interfacial properties of the gecko skin focusing on solid and aqueous interactions. We show that the skin of the box-patterned gecko (Lucasium sp.) consists of dome shaped scales arranged in a hexagonal patterning. The scales comprise of spinules (hairs), from several hundred nanometres to several microns in length, with a sub-micron spacing and a small radius of curvature typically from 10 to 20 nm. This micro and nano structure of the skin exhibited ultralow adhesion with contaminating particles. The topography also provides a superhydrophobic, anti-wetting barrier which can self clean by the action of low velocity rolling or impacting droplets of various size ranges from microns to several millimetres. Water droplets which are sufficiently small (10-100 μm) can easily access valleys between the scales for efficient self-cleaning and due to their dimensions can self-propel off the surface enhancing their mobility and cleaning effect. In addition, we demonstrate that the gecko skin has an antibacterial action where Gram-negative bacteria (Porphyromonas gingivalis) are killed when exposed to the surface however eukaryotic cell compatibility (with human stem cells) is demonstrated. The multifunctional features of the gecko skin provide a potential natural template for man-made applications where specific control of liquid, solid and biological contacts is required.

Costs of Reproduction in Water Skinks

Costs of reproduction measured under laboratory conditions may have little bearing on actual costs observed in the field, because both environmental conditions and behavior can modify the manifestation of these costs. I studied survival, growth, and reproduction in a wild population of southern water skinks (Eulamprus tympanum) over 3 yr to measure costs of reproduction in females, to examine how the environment influences these costs, and to assess the extent to which females are able to reduce costs. The proportion of mature females that reproduced each year varied from 30-60%. Frequent reproduction appeared costly in terms of both survival rates and growth. Growth rates and survival rates were negatively correlated with reproduction during the 3-yr study. In the coolest and wettest year of the study, when food intake by females was relatively low, gravid females and females that had reproduced in the previous year survived less well than nonreproducing females. There was no evidence of a survival cost of reproduction in the other two years of the study. Reproduction also imposed a fecundity cost, as overall, mean growth rates of reproducing females were reduced relative to those of nonreproducing females of the same body size. Reduced growth rate translates to reduced litter size in these skinks, because fecundity increases with body size. These results suggest that reproduction can be costly in terms of both survival and future reproduction in these skinks, but that survival costs are not consistent from year to year and are mediated by environmental conditions. Females appear to adjust their frequency of reproduction to minimize these costs, as those females that skipped opportunities for reproduction were those expected to gain the greatest growth and fecundity benefit by skipping.

Environmental and social factors influence chorusing behaviour in a tropical frog: examining various temporal and spatial scales

Abstract Many animals use conspicuous display to attract mates, and there should be selection for displays to occur at times and places that maximise the probability of mating, while minimising energetic costs and predator attraction. To select the best times for display, individuals may use environmental cues, the presence of other individuals, or both, but few studies have examined these sources of variation in display activity. In this study, we examined physical environmental and social factors triggering displays in a tropical, terrestrially breeding frog, Cophixalus ornatus. To measure the influence of physical environmental conditions on calling activity, we recorded temperature, rainfall, moon illumination/visibility, humidity, barometric pressure and intensity of calling activity throughout a breeding season at six locations along a 560-m transect. The intensity of calling varied daily, seasonally, and at a small spatial scale. Variation in calling activity from day to day was large. There was also a strong seasonal trend in calling activity: few males called at the start of the season, activity peaked shortly after the beginning of the season, and then declined linearly from the peak to the end of the season. There was also consistent variation among sites along the transect, which may have been due to variations in frog density at each site, or to consistent microscale variations in physical conditions, or both. After statistically removing consistent local variation among sites, a principal components analysis suggested that a maximum of 35.8% of the variation in calling activity among days was due to factors common to all sites, such as weather, moon illumination, or large-scale social facilitation (e.g. of choruses by other choruses). The remainder of the variation among sites (64.2%) was due to site-specific factors, such as small-scale social facilitation or unmeasured, apparently stochastic effects, such as microenvironmental physical factors that do not vary consistently over sites. Regressions of environmental variables on residual calling activity (after removing consistent effects of site and season), alone or in combination, accounted for very little of the variation in the number of calling males (maximum 10%). Thus, our data, showing strong seasonal effects and consistent variation among sites combined with large amounts of variation in the number of calling males at small spatial scales, suggest that environmental conditions, such as temperature, rainfall, moon illumination and barometric pressure, which act over large spatial scales, may determine the overall environmental envelope within which calling can occur but do not account for most of the variation in the number of calling males on a day-to-day or site-to-site basis. Similarly, variations in the number of calling males at small spatial scales suggest that social facilitation is a relatively unimportant trigger for displays on a large scale in these frogs. On the other hand, our data suggest that social facilitation may have important effects on variation in the number of calling males on a day-to-day and site-to-site basis. We used playback experiments to assess whether the sound of calling could initiate displays. We played either a taped chorus or white noise in areas where few (zero to two) males were calling. The number of calling males increased both during and after the chorus stimulus, whereas there was no increase in calling in response to white noise. These data suggest that examining variation in calling activity at small spatial scales can reveal the sources of variation for the number of calling males, and indicate that, in these frogs, males tend to use the calling of other individuals as a cue to determine when to display.