Culling dingoes with poison may be making them bigger

Australia’s dingoes are getting bigger, and it may be because of humans. New research suggests the change is happening only in places where the wild canine’s populations are controlled with poison.

The findings could illustrate for the first time that, when targeted with pesticides, changes to the physical traits of “pest” species can occur in bigger animals, not just insects and rodents.   

Scientists had noticed an increase in the size of some dingoes, but that there hasn’t been much understanding of what was causing it, says Michael Letnic, an ecologist at the University of New South Wales in Sydney. He wondered if it was the consequence of decades of the dingoes’ status as a livestock pest.  

Dingoes (Canis lupus dingo) have long had an uneasy relationship with farmers and ranchers in rural Australia. The predators can attack livestock, usually sheep. Shooting and fencing have been used to control dingo populations and protect livestock. But in the 1960s and 1970s, a new tool was also employed in western and southern Australia: a poison called sodium monofluoroacetate, or 1080. Odorless and tasteless, the powder could be mixed into bits of meat and scattered across the landscape as deadly bait for dingoes to snatch up. 

A dose’s effectiveness is dependent on a dingo’s mass, which led Letnic to test the idea that 1080 use might be related to dingoes’ size change. He and Mathew Crowther, an ecologist at the University of Sydney, delved into museum collections of dingo skulls, collected from across three areas that have been exposed to 1080 for about 50 to 60 years, and one region where baiting is banned. The skulls date from 1930 to the present day, so by measuring their length (a proxy for a dingo’s body size), the researchers could compare the sizes of the animals before and after poisoning began. 

After examining more than 500 skulls, the team found that in baited regions, female dingoes’ skulls have grown 4.5 millimeters longer, on average, in the era after 1080 was introduced. Male skulls are 3.6 millimeters longer than they were. These changes equate to a roughly 6 and 9 percent jump in body mass in males and females, respectively, or about a one kilogram increase on average, the team reports July 31 in the Biological Journal of the Linnean Society. In contrast, skulls from dingoes in the unbaited region did not significantly change in length over the same time period.     

Dingoes are top predators whose appetites send ripple effects through the food web (SN: 1/13/14). Kangaroo numbers increase when dingo populations are controlled, so the combination of extra prey availability and reduced competition may make it easier for dingoes that aren’t killed by the poison to find food and grow. “By reducing the dingo population, [1080 is] changing the environment that dingoes are growing up in,” Letnic says. Bigger dingoes may then, in turn, be more tolerant of the poison’s effects, their body size outpacing a relatively constant dosage over the years.

“We’ve known for a long time that if we spray our fields with pesticides, then the insects that we’re trying to kill change and develop resistance” to the pesticides, Letnic says. “This work suggests that when we use pesticides on big animals, we can produce comparable changes.”

Still, the study is based on correlations rather than experimental manipulation of dingo populations, so pinning down precisely what’s causing the change is tricky. But the team’s search for possible alternative explanations for the size increase came up short. Climate change can cause size shifts, but animals tend to get smaller as temperatures rise, not bigger. Interbreeding with domesticated dogs might make the dingoes bigger, but the skulls all came from areas of Australia with negligible rates of dog-dingo hybrids. 

Kiyoko Gotanda, an evolutionary biologist at the University of Cambridge not involved with the research, says that while the effects of hunting on animal traits are often investigated, she’s “unaware of studies looking at how using poisoning as a control method for vertebrates might also induce [physical or behavioral] change… I would also be interested to learn if changes to body size occur once you stop using poison control on the predators,” she says.

If dingoes are growing in size in response to 1080 exposure, there could be ecological implications down the line. Bigger dingoes can hunt bigger prey, notes Letnic, which could have unknown impacts on Australian ecosystems. And dingoes aren’t the only worry. The poison is also used to control other “pests,” including invasive red foxes, which devour many threatened animals. If the foxes become tolerant of 1080, the conservation consequences could be harsh, Letnic says.