New evidence suggests that thylacines were driven to extinction on the mainland by dingo attacks rather than competition for food, as previously thought.
Thylacines flourished widely until around 3500 years ago. Their disappearance from areas other than Tasmania coincided with the arrival of the dingo, which never reached Tasmania.
It is common for predators to kill smaller species that might represent competition. However, this was once dismissed as a possible explanation for the thylacine’s mainland disappearance since the Tasmanian tiger was larger than dingos.
Dr Mike Letnic of the University of NSW says this view ignores Bergmann’s Rule – the observation that animals are larger in colder climates – so Letnic and colleagues at the University of Sydney compared the size of thylacine and dingo bones from similar areas. They found a substantial overlap in size, but some adult thylacines were much smaller than any dingoes.
Evidence from Tasmania shows that thylacines were highly sexually dimorphic, with the females much smaller than the males. Although it is not possible to identify the sex of most of the fossils, it seems likely that male thylacines were of similar size to dingoes while females were not much larger than foxes.
“Recent studies have shown that foxes are suppressed in areas that have many dingoes, and it appears that the dingoes kill the foxes. Hence we believe that the same mechanism occurred 3500–5000 years ago with dingoes killing thylacines,” says Dr Mathew Crowther of Sydney University’s School of Biological Sciences.
Male thylacines may have been large enough to fight off dingos, but the females would probably have succumbed, leaving the species unable to breed.
In PloS One the researchers argued that direct attacks by dingoes may have wiped out the thylacines. “Recent analysis has found the evidence for competition as a driver of extinction is weak,” Letnic says. “It can have a big effect on abundance, but usually a species can find a location in which it can survive. Direct killing is harder to avoid, particularly with an introduced species, which the smaller animal may lack adaptations to avoid.”
Letnic acknowledges that the theory lacks direct evidence, such as dingo bite marks on thylacine fossils. However, he says finding this would be “like looking for a needle in a haystack”.
The paper also raises the possibility that changes in Aboriginal culture and economy, coinciding with the dingo’s arrival, may have contributed, as might the dingo’s greater tendency to hunt in packs.
Dingoes were similarly sized to male thylacines but were considerably larger than female thylacines on mainland Australia during the Holocene. Small size may have made female thylacines particularly susceptible to direct killing by dingoes and such killing could have driven thylacines to extinction. Due to their lower metabolic rate and convergent morphology, thylacines would have also been susceptible to resource competition with dingoes, but competition is generally thought to be a weaker extinction threat than predation. Our results provide support for the hypothesis that direct killing by larger dingoes contributed to the extinction of the thylacine on mainland Australia. However, attributing the extinction of the thylacine to just one cause is problematic because the arrival of dingoes coincided with another potential extinction driver, the intensification of the human economy.