How big is the dingo

Side effect: poison baits have made dingoes bigger

Ask your evolutionary biologist or ecologist for any unwanted side effects. As Michael Letnic from the University of New South Wales, who reports in the "Biological Journal of the Linnean Society" of an astonishing discovery: In the past decades the dingoes, the notorious Australian wild dogs, have grown larger in some regions of the country. And the reason for this should be precisely those poison baits that are laid out to decimate the dingo populations.

Between all stools

The relationship between humans and dingo is divided. On the one hand, the dog, which weighs around 20 kilograms, is one of Australia's iconic animals and is sometimes also kept as a pet. On the other hand, it has been massively fought since the 20th century for killing cattle. Attacks on people also occur; two cases in which children were abducted and killed by dingoes caused a particular stir.

But also from a purely biological point of view, the dingo plays an ambiguous role. As its Latin name Canis lupus dingo shows, it is a subspecies of the wolf - or more precisely one of the dog. Genetic analyzes indicate that dingoes are descended from domestic dogs. These are likely to have reached Australia around 4,000 years ago - long before the Europeans, but even longer after the first settlers in Australia, the ancestors of the Aborigines. Presumably it was seafarers from Southeast Asia who brought in dogs, which then went wild.

That is why the dingo now occupies a strange intermediate position: after all this time, should we still regard it as a feral domestic animal or as a real wild animal, as a harmful bio-invader like the foxes, cats and rabbits that the Europeans brought in, or as a natural component of the? Australian fauna? The native marsupials seem to have found their own answer to this: In 2016, researchers reported that small marsupials in the urban south of Australia avoid being near dogs - they are likely to have learned this lesson from living with dingoes for thousands of years. Wildcats, on the other hand, did not have them "to practice" during this period - and are therefore slaughtered by domestic cats year after year on an almost apocalyptic scale.

Combat ...

The dingo is likely to have one or the other species on its conscience, at least on mainland Australia. But it is fought mainly because of the damage it causes in livestock. Incredibly long border fences, shooting campaigns and, above all, poison baits dropped by helicopters are supposed to keep the dingo populations in check. The most common remedy is sodium fluoroacetate, a tasteless white powder that is used to prepare meat bait. In Australia it is mostly called "Doggone" after its catalog number "1080" or with the pragmatism typical of the country.

Michael Letnic has now pursued the question of how a few decades of the massive use of sodium fluoracetate have affected. To do this, he compared regions such as Kalgoorlie in southwest Australia or Pilbara in the northwest, where bait is laid out, with those where it is not, such as nature reserves or regions owned by Aborigines. Letnic's team concentrated on measuring the skull, since the size of the skull in dingoes is a good measure of the overall size of the animal. Around 600 dingo skulls from a period of 80 years were compared in this way.

... with unexpected consequences

The result was as clear as it was surprising: while the dingoes from regions without poison bait had changed nothing, their conspecifics from areas with poison use had grown larger on average. That sounds like the stuff that the eco-horror films of the 1970s were made of, but of course it moves in more modest dimensions. According to the researchers, the dingoes have gained six to nine percent in the affected areas. Incidentally, the higher of the two growth rates is that of the females. Their skulls have grown by 4.5 millimeters on average, those of the males by 3.6.

That might not seem like a lot, but extrapolated to total growth it corresponds to around one kilogram of body mass. For a vertebrate, such growth over the course of just a few decades is quite remarkable, the researchers point out. There is much research into the fact that pesticides have undesirable side effects on insects. However, your study would be among the first to show how such poisons can also become an evolutionary factor in large animals.

The presumed (secondary) causal relationship

But how can such a seemingly paradoxical side effect even come about? According to Letnic's colleague Mathew Crowther from the University of Sydney, several factors are likely to play a role here - including classic natural selection. A larger animal "needs" a greater amount of poison to die. Stately dingoes therefore have a higher chance of surviving a poison meal, producing offspring and passing on their genetic makeup, while smaller ones are taken out of the breeding race. In the medium to long term, the average size of the species must increase.

However, the nutritional situation also plays a role. Studies have shown how wildlife populations recover once the local dingoes have been decimated. Conversely, this also means that there is now more food available for the surviving dingoes. In the absence of internal competition, they do not suffer from hunger and can fully exploit their genetic potential.

According to the researchers, hybridization is unlikely to play a role. Dingoes often mate with domestic dogs, which could well bring genetic makeup for a larger body into the gene pool. In the selected study regions, however, the hybridization rate is very low, according to the researchers - and a possible influence of domestic dogs is therefore "highly unlikely". And when it comes to climate change, which is sometimes cited as an explanation for everything, the researchers also decline: If it gets hotter, mammals tend to be smaller than larger. In fact, the cause remains the poison.

Further growth?

So when you tried to kill the dingo you made it stronger. If you want to fight it further, this must be taken into account. And since kills and other methods are too inefficient, according to Letnic, only poison will remain an effective remedy in the future. And with him the choice between two strategies: Either take an ever higher dose of the same poison (which is already practiced anyway). Or take another poison. However, the researcher makes the same prognosis for both approaches: it will work for a while. And then the cycle starts all over again. (jdo, August 16, 2020)