Professor Rick Shine's team from the University of Sydney have selected a species of large lungworms (Rhabdias spp). for this research. These worms occur in high densities in Australian toads and have a direct lifecycle, entering the toad through its skin or digestive tract and migrating to the lungs where they grow and mature. Eggs and larvae are passed out in the toads faeces into the water or land, where they go through a free-living sexual stage before producing infective larvae that enter other toads. Experimental infection of metamorphling European toads (Bufo bufo) with Rhabdias bufonis significantly reduced locomotor performance, feeding, activity and growth (Goater and Ward 1991, Goater et al. 1993).

The Sydney university team suspected that because adult toads visit a water body only briefly to reproduce, by chance we might expect some clutches to be lungworm-free. The teams radio-tracking studies at the invasion front have shown that the toads move remarkably long distances, up to a kilometre in a single night - and during the tropical wet-season they keep heading in the same direction, almost never returning to sites they have previously occupied. If parasites slow down the toads they infect, then parasite-free toads will disperse more rapidly than parasitised animals - and if so, we would expect toads at the invasion front (because they are the progeny of toads that have themselves been at the invasion front the previous generation) to be those that have escaped parasite infection. We should therefore see lower parasite loads in toads close to the invasion front than in older populations.

To answer this question, the scientists involved in this project collected and dissected toads over most of their tropical range in Queensland and the Northern Territory. Importantly, they were able to obtain a data set which covered a long timeframe,which has allowed confounding factors (such as local weather conditions) to be excluded from durations of toad residency in specific localities.

To determine whether or not infection with Rhabdias affected toad behaviour (e.g. activity level, food intake) or locomotor performance (speed and/or endurance), adult toads were collected from an invasion-front population in the Adelaide River floodplain of the Northern Territory. The parasite did not occur in this area and the study animals had no prior experience of Rhabdias. These animals were transported to Townsville (an area with high Rhabdias densities) where they were experimentally infected (random control and treatment groups).

Various behavioural and biological criteria were scored and toads tested for speed and endurance over a three week period (May-June 2006) under controlled conditions. Half the toad groups were then infected with Rhabdias, and the remainder treated with a control.

After a period to allow for parasite growth, the toads were all retested for locomotor performance.