Abstract
Initial research on the spread of cane toads (Rhinella marina) through tropical Australia reported a high incidence of spinal arthritis (spondylosis) in toads at the invasion front (where toads disperse rapidly), but not in areas colonized earlier (where toads are more sedentary). The idea that spondylosis was a cost of rapid dispersal was challenged by wider spatial sampling which linked rates of spondylosis to hot (tropical) climates rather than to dispersal rates. Here, the authors of these competing interpretations collaborate to reinterpret the data. Our reanalysis supports both previous hypotheses; rates of spondylosis are higher in populations established by fast-dispersing toads, and are higher in tropical than in temperate environments; they are also higher in larger toads. The functional reason for climatic effects is unclear, but might involve effects on the soil-living bacteria involved in the induction of spondylosis; and/or may reflect higher movement (as opposed to dispersal) or more pronounced dry-season aggregation rates of toads in tropical conditions.
Highlights
Following their release in northeastern Australia in 1935, cane toads dispersed at increasing speeds to the west[1,2,3,4]
Queensland, midway through the current invasion range, that had a prevalence of spinal arthritis more than double that of any other population, and five times higher than the mean prevalence across all populations
Bower et al.[3] used “date since population establishment” as a proxy for invasion speed, the hypothesis that spondylosis is induced by rapid dispersal invokes invasion speed rather than “time since toad arrival” as the critical factor
Summary
Queensland, midway through the current invasion range, that had a prevalence of spinal arthritis more than double that of any other population, and five times higher than the mean prevalence across all populations. Excluding this sample did not qualitatively modify any of our conclusions. The correlation between arrival times and invasion speeds is non-linear in tropical Australia (because the rate of toad dispersal has accelerated dramatically4) and non-existent at the southern invasion front (where toads of all populations disperse slowly[4]). To test the putative link between dispersal rate and spondylosis, we should examine the relationship between the prevalence of spondylosis and rates of dispersal at the time that toads arrived at each site
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