How are tigers and bears alike?

evolution : Ligers and cappuccino bears

Ligers are an attraction in the few zoos where such animals live, if only because of their ancestry. Her father is a lion, her mother a tigress. Such mixtures of two species do not normally reproduce with one another. They are accordingly rare and sensational.

Unless humans intervene like a mule. These descendants of a donkey stallion and a horse mare were responsible for the transport of loads and were bred.

Mother polar bear, father grizzly bear: hybrids are more common than expected

Such mixtures of two species, known as hybrids, hardly play a role in nature, assumed evolutionary biologists. Until cappuccino bears appeared in the Canadian Arctic with a polar bear mother and a grizzly bear father. And until researchers found evidence in the genome of different species in groups as diverse as bears, big cats and mice that such hybrids are not only relatively common, but can also play a role in evolution.

Axel Janke from the University of Frankfurt am Main and his colleagues report in the journal "Scientific Reports" that up to 8.8 percent of the brown bear's genetic makeup is obviously polar bears. "The genetic material of the mini-power plants known as mitochondria in their cells was apparently confiscated by the polar bears from a female brown bear 150,000 years ago," says Janke.

Hereditary traits that bring the recipient an advantage

In 2005, a young bear was caught in northern Cambodia, which, according to its physique and genetic make-up, must have been a mixture of a black bear and a sun bear. Janke and colleagues also found traces of intimate encounters between the seven bear species that have taken place over the past five million years in the genetic makeup of the collar bears, lip bears, sun bears and spectacled bears.

“Such hybrids occur roughly every ten to 100 years and leave traces in the genome,” concludes Janke from these analyzes. That doesn't sound like much at first. In a few hundred thousand or even millions of years, however, a lot accumulates. Eduardo Eizirik from the Pontifical Catholic University of Rio Grande do Sul in the Brazilian city of Porto Alegre and colleagues in the journal "Science Advances" also report this on the big cats lion, leopard, jaguar, tiger and snow leopard, which obviously also ignore the boundaries between the species. In these animals, the researchers found hereditary traits that had brought another species and that brought the recipient an advantage.

Without the genetic make-up of other species, the jaguar would not be as strong

These hereditary characteristics influence, for example, the development of the head or the limbs, the metabolism, the reproduction, the color and pattern of the fur or the sensory organs. The jaguar has two genes that obviously come from another big cat like the leopard. Both hereditary traits may have played a role in the development of the jaguar head, which is strikingly strong compared to the other big cats. With its teeth, a jaguar can bite twice as hard as a lion.

Its teeth penetrate the skull of many mammals, while other big cats suffocate their prey or break their necks. This super bite was particularly important when, at the end of the last ice age, many large mammal species that the jaguar had previously captured became extinct in South America. In this situation, the big cats could resort to the well-armored caimans and turtles, which other big cats would grit their teeth at. Without the genetic makeup of other species, the jaguars would have had a bad hand.

Diethard Tautz and colleagues from the Max Planck Institute for Evolutionary Biology in Plön found a similar pattern in a study in mice discussed with colleagues on the Internet platform “BioRXiv”. In these rodents, similar to bears and big cats, there is an exchange between species and subspecies.

Resistance to infection

"Such hybridizations obviously occur much more frequently than we had previously assumed," says Tautz. Hereditary properties that give resistance to certain infections stand out in particular. In such cases, the advantages of crossing species boundaries are obvious.

How do such mixtures come about? After all, the more changes accumulate in the genome between two species, the longer both have gone their own way. After a while, the differences are so great that hybrids, like mules, can hardly have offspring of their own. If the development continues, at some point there will be no more hybrids.

Researchers on Heligoland are observing the formation of a new species. The house mouse, Mus musculus domesticus, arrived there around 400 years ago and had to change its diet. After all, there was practically no agriculture and the mice switched from grain to animal proteins, for example in dead birds.

Exchange of genetic material, even if the pairing is unsuccessful

Presumably, the communication between the animals, which seem to be talking with a kind of ultrasound language, also changed. Perhaps a mouse that comes to Helgoland from the mainland today no longer understands its conspecifics there, which are now listed as a separate subspecies Mus musculus helgolandicus. Of course, if you don't understand him anymore, it is difficult to persuade a partner to mate.

In rare cases it works. Perhaps the female will then throw mixed breeds. If that fails, the male's sperm cells are quickly broken down in the female's body. “The genome in it is, however, very stable,” says Tautz. If a mating with a partner from the own group occurs during this time, this naked genetic material from the other group could slip into the egg cell during fertilization and then be incorporated into the genetic material there. Different species can also exchange genetic material if the pairing between them is unsuccessful. So it is not surprising when researchers find a few percent in the genome of many species that come from a different species.

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