Thomas Madsen

Restoration of an inbred adder population

The negative effects of inbreeding on population size are well documented in captive animals, but there is surprisingly little evidence that genetic factors cause a decline in wild populations, apart from a reported correlation of low levels of genetic variability with a high incidence of malformed or stillborn offspring. From the point of view of conservation strategies, it is not only the effect of genetic factors on population decline that needs to be considered, but also whether introducing novel genes can prevent or reverse such a decline. Here we show that the introduction of new genes into a severely inbred and isolated population of adders (Vipera berus) halted its precipitous decline towards extinction and expanded the population dramatically.

Is sperm really so cheap? Costs of reproduction in male adders, Vipera berus

Reproduction is energetically expensive for males as well as for females, but evolutionary biologists have typically regarded the energy costs of sperm production as trivial compared to the energy costs of overt reproductive behaviours, such as mate-searching, courtship, copulation and male–male combat. Adders (Vipera berus) offer an ideal model system in which to quantify the relative costs of spermatogenesis (and associated physiological preparations for reproductive activity) versus the costs of overt reproductive behaviours, because (i) these two activities occur sequentially rather than simultaneously, and are separated by a clear indicator (sloughing of the skin); and (ii) males do not feed during either of these phases. Hence, the rate of mass loss by males can be used as an index of energy costs. Surprisingly, the rate of mass loss is at least as high during the phase when males are immobile, building up sperm supplies, as when they move about and engage in reproductive behaviour. Rates of mass loss are not significantly correlated with male size or measures of the males participation in reproductive activities. Our data suggest that sperm production may be a major energy cost to reproducing male adders, and that this species offers a useful system in which to further investigate this possibility.

Major histocompatibility complex and mate choice in sand lizards

In mice and man, females prefer males with a major histocompatibility complex (MHC) genotype different to their own. We tested whether this phenomenon also occurs in the Swedish sand lizard (Lacerta agilis). Females in a laboratory experiment preferred to associate with odour samples obtained from more distantly related males at the MHC class 1 loci. Data on free-ranging lizards suggest that associations between males and females are nonrandom with respect to MHC genotype. However, male spatial distribution and mobility during the mating season suggest that the non-random pairing process in the wild may also be driven by corresponding genetic benefits to males pairing with less related females.

Inbreeding depression in an isolated population of adders Vipera berus

Abstract Although inbreeding depression is well-studied in captive animals, its role in natural populations remains controversial. We provide information on an isolated population of snakes (adders Vipera berus) that has been separated from neighbouring populations by the expansion of agricultural activities in southern Sweden. Total adult population size is

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection

Abstract Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male‐male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.

Is Thermoregulation Unimportant for Most Reptiles? An Example Using Water Pythons (Liasis fuscus) in Tropical Australia

Studies on reptilian thermoregulation have been dominated by research on small diurnal lizards living in the temperate zone, in environments where thermoregulatory challenges are severe and the animals (i) consequently devote considerable time and effort to the maintenance of high stable body temperatures and (ii) are constrained in their times and places of activity by thermal factors. However, most reptiles live in the Tropics, in more thermally benign regions where the animals ecology and behavior may be affected only trivially by thermoregulatory concerns. We present data on environmental temperatures and body temperatures of 26 radio-tracked water pythons (Liasis fuscus) in tropical Australia, to investigate the extent to which thermal considerations influence the day-to-day life of this species. Some effects are evident-for example, activity is reduced on cool nights, and gravid female pythons often bask in the few weeks prior to oviposition. Nonetheless, most pythons are able to maintain high and stable temperatures (approx. 30°C) throughout the year without overt thermoregulatory activities, because (i) ambient temperatures are generally high, (ii) microhabitats with distinctive thermal characteristics are easily accessible, and (iii) the large body sizes of adult pythons confer a high degree of thermal inertia. Overall, there are few features of the ecology of this species that appear to be substantially influenced by thermoregulation. Before we can place thermoregulatory biology in its proper perspective, we will need considerably more data on the thermal biology of tropical reptiles.

Seasonal Migration of Predators and Prey‐‐A Study of Pythons and Rats in Tropical Australia

Although seasonal migrations of large predatory mammals that follow mi- grating prey are well documented, no equivalent phenomenon has been described previously in terrestrial reptiles. We surveyed and radio-tracked water pythons (Liasisfuscus) in Fogg Dam and its adjacent floodplain in the wet-dry tropics of northern Australia to document patterns of movement, with particular emphasis on the ways in which the snakes exploit their major prey species, the dusky rat (Rattus colletti). The distribution and abundance of these rodents vary seasonally. During the dry season the rats live in soil crevices in the floodplain, but wet-season flooding forces them to higher ground, primarily to natural levee banks. Python and rat abundances on the floodplain adjacent to Fogg Dam were significantly correlated through time: both reached a maximum during the dry season, and fell dramat- ically during the wet season. Activity of pythons was centered around Fogg Dam during the dry season, but all of the radio-tracked snakes moved away from this area during the wet season. Most pythons migrated to the vicinity of levee banks on the floodplain up to 12 km away from their dry-season range. By migrating seasonally, water pythons can efficiently utilize a migratory prey species that would otherwise be unavailable for much of the year.

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...

Costs of reproduction in a population of European adders

Eleven years of data on a small population of adders (Vipera berus) in southern Sweden provide quantitative information on the nature and degree of costs faced by reproducing animals. Reproduction imposes both an energy cost (measured by loss in body mass) and a mortality cost on adders of both sexes. The extent of the energy cost is broadly independent of levels of reproductive activity in males, but mortality costs are highest for large males, perhaps because they are more obvious to predators. In females, energy costs include a high ‘fixed’ (fecundity-independent) component, such that a large litter may cost little more to produce than would a small litter. Energy costs and mortality costs are separate in males, but inter-related in females. Mortality of reproducing females is high (40% per year), primarily because post-parturient females are emaciated and must forage actively, hence increasing their vulnerability to predators. Females producing relatively large litters (high Relative Clutch Mass) lose more body mass, and are less likely to survive after reproducing. The observed low reproductive frequencies of female adders may result from the presence of high fecundity-independent costs of reproduction.

Female choice on male quantitative traits in lizards- why is it so rare?

Female choice on the basis of male traits has been described in an array of taxa but has rarely been demonstrated in reptiles. In the sand lizard (Lacerta agilis), and possibly in other non-territorial reptiles, a males contribution to a females fitness is restricted to his genes. In order to choose males of high genetic quality, females have to trade the fitness gain against the costs of active choice. In a Swedish population of sand lizards, long-lived males sired offspring with higher embryonic survival compared to offspring sired by short-lived males. In spite of this female sand lizards did not mate selectively with older and/or larger males. There appeared to be mo reliable cues to male longevity; age-specific male body size was highly variable. Furthermore, estimates of male nuptial coloration did not covary with ectoparasite load and, hence, females cannot use male coloration as a cue to heritable resistance to pathogenic parasite effects. When cues to male genetic quality are poor, or inaccurate, and males make no parental investment, we predict that female choice will be rare. Sand lizard females mating with many partners lay clutches with higher hatching success. Thus, females may obtain “good genes” for their young by multiple mating, thereby avoiding costs associated with mate choice.