Current Reptile Research
For more than 150 years, biologists have believed that a single process – natural selection – is responsible for cumulative evolutionary changes in organisms. One of the most exciting results from our cane toad studies is to reveal an additional process, that we call “spatial sorting”. Unlike natural selection, it doesn’t require traits (such as more rapid dispersal) to benefit the individual animals that exhibit those traits. Instead, characteristics evolve because genes for those traits accumulate in a small part of the species range – the invasion front. Any gene that makes a cane toad quicker is likely to end up at the rapidly-moving invasion front, whereas any gene that slows a toad down gets left behind. Over many generations, this accumulation of “genes for speed” ends up producing the remarkably athletic invasion-front toads we see today at the invasion front.
For more than 20 years we have studied populations of water pythons (Liasis fuscus) and filesnakes (Acrochordus arafurae) in the wet dry tropics of northern Australia. The work has documented strong links between weather patterns (stochastic annual variation in rainfall) and the abundance of prey species (rats and fish). In turn, these fluctuations in resource availability influence the survival, growth and reproductive output of the snakes in these ecosystems. The studies have also generated a massive data base - from radiotelemetry, as well as mark-recapture programs - to clarify aspects of these poorly known species. The study has been a collaborative one between Thomas Madsen and Rick Shine. More recently, Bea Ujvari has joined the team.
Greg Brown has been studying the ecology and reproductive biology of colubrid snakes in the same area of the Australian tropics. The work explores seasonality of reproduction and habitat use in slatey-grey snakes (Stegonotus cucullatus), Macleay's water snake (Enhydris polylepis, pictured) and keelbacks (Tropidonophis mairii) on the Adelaide River floodplain. Greg also is examining the ecology and impact of introduced cane toads (Bufo marinus), that arrived in our study area in the 2004-05 wet season.
Although most reptiles reproduce by laying eggs, more than 100 separate lineages have evolved live birth (viviparity). This transition has usually occurred in cold climates. We have worked extensively on the origin of this trait, and especially on the selective forces involved. Current research by Rick Shine and Melanie Elphick focuses on the upper elevational boundary to egg-laying lizards in the Brindabella Mountains near Canberra. We have monitored natural nests of skinks (Bassiana duperreyi) for several years, and relocated eggs among nests to deconfound genetic and environmental influences. In current work we are examining the fate of eggs artificially translocated to nest-sites above the elevational (and thermal) limits for egg-layers.
Using the same study system as for the viviparity studies, Rick Shine and Melanie Elphick have examined the ways in which the incubation conditions experienced by an egg modify the size, shape, behaviour and locomotor speed of the hatchling that emerges from that egg. Strong effects are apparent, suggesting that this sensitivity to incubation temperature may have played an important role in topics such as the evolution of temperature-dependent sex determination.
Comparative analyses show that the mating system affects patterns of sexual dimorphism in snakes, as in other kinds of animals. Males tend to grow larger than females where males engage in physical contact with each other during mating season. However, males are the smaller sex where male-male combat does not occur. In recent work, Rick Shine and collaborators (notably Dr. Robert Mason of Oregon State University) have experimentally tested hypotheses on determinants of mating success in male garter snakes (Thamnophis sirtalis) in Manitoba, Canada. Together with Sohan Shetty, Rick has conducted similar studies on sea snakes (Laticauda colubrina, pictured) in Fiji. The sea snake studies have recently expanded to Vanuatu and New Caledonia.
Niche divergence between the sexes can amplify or constrain the degree of sexual dimorphism generated by aspects of reproductive biology. Studies on carpet pythons by David Pearson provide striking examples of massive shifts in dimorphism among areas with different spectra of available prey-species.
Field studies on garter snakes (Thamnophis sirtalis parietalis, pictured) and sea snakes (see above), incorporating experimental arenas to control extraneous variables, have clarified the ways in which male snakes obtain mating opportunities. A remarkable subtlety and complexity is emerging. For example, some male garter snakes produce skin chemicals that resemble those of females, thus stimulating intense courtship from other males.
A wide variety of projects are designed to provide information relevant to management and conservation. One study recently carried out in Sumatra involved examination of thousands of reticulated pythons (Python reticulatus), blood pythons (P. curtis) and varanid lizards (Varanus salvator) that had been captured for the commercial leather industry. We generated data on the sizes, sexes, food habits and reproductive biology of these animals to assess the sustainability of current harvest levels.
Dr. Jonathan Webb has been examining the population ecology of an endangered snake species from south-eastern Australia (the broad-headed snake Hoplocephalus bungaroides). In collaboration with Rick, Jonathan is clarifying the processes responsible for this species decline and possible techniques to aid its recovery. This work will involve major field experiments on the roles of vegetation shading and shelter-site availability as determinants of reptile abundance.
Two groups of venomous snakes have made the transition from terrestrial to aquatic life. We are currently studying representatives of both these lineages. Work on laticaudid sea snakes has taken Rick from Fiji to Vanuatu and to New Caledonia, where he and Xavier Bonnet have established a major project. Studies have also commenced on a hydrophiid sea snake (Emydocephalus annulatus, pictured).
Introduced to Australia more than 70 years ago in a futile attempt to control agricultural pests, feral cane toads (Bufo marinus) have now spread through much of tropical Australia. Native predators attempting to eat these toads are killed by their powerful toxins. Rick Shine, Jonno Webb and Ben Phillips (with help from Michael Crossland, Matt Greenlees and John Llewelyn) are examining the ecological impacts of toads as they arrive at our long-term study site at Fogg Dam near Darwin, and also longer-term effects of toads in long-colonised areas further to the east.
Photographs © R. Shine, P. Harlow, D. O'Connor, G. Barrott-Brown, S. Shetty, M. Elphick, P. Laboute, D. Warner, J. Webb and C. Masters