Why some animals evolved to immolate

A male musk ox can weigh up to 800 pounds and charge at speeds of more than 30 miles per hour. During the breeding season, these furry critters from the Arctic Circle smash into each other head-on, then stab their opponents with their huge, razor-sharp horns.

Furthermore, over the course of his 10 to 12 years of life, a single male musk ox can rack up some 2,100 blows to the head.

How do muskox survive these blows without their brains turning to mush?

“People have always assumed that animals that butt heads, like muskox and bighorn sheep, are somehow immune to head injuries,” says Nicole Ackermans, a neuroscientist at the Icahn School of Medicine at Mount Sinai. in New York (USA). “Like they have magic horns, or something.”

(Related: The 12 rarest animals in the world)

But when Ackermans started reading the scientific literature, he discovered that no one had studied whether these North American herbivores suffer brain damage from their head-banging lifestyle. So she and her colleagues acquired muskox and bighorn sheep brains through a combination of field expeditions, donations from subsistence hunters and captive research herds.

“We found a specific pattern in all of our specimens that closely resembled early chronic brain trauma in a human,” says Ackermans, who led a recent paper on the findings, published in the journal Acta Neuropatholgica.

The new research could be critical to better understanding brain injuries in humans, Ackermans says, because bovids (animals like oxen and sheep) have folded, wrinkled brains that are more like ours than, say, mice. , whose brains are smooth.

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It’s also proof that evolution can lead a species down surprisingly self-destructive paths. And in this regard, musk oxen are far from alone.

Do not die

For their research, Ackermans and colleagues stained the brains of three muskox oxen and four bighorn sheep with biomarkers. These chemicals can reveal patterns of traumatic brain injury that are often associated with human conditions, such as Alzheimer’s disease and chronic traumatic encephalopathy (CTE). In this case, the scientists were specifically looking for something known as the tau protein.

“When neurons are damaged, whether through aging, genetic problems, or mechanical impact, they tear apart, and this protein breaks down and forms into clumps,” Ackermans explains. “And if you see them in specific patterns, you can tell if it’s a normal brain, or aging, or Alzheimer’s, or possible trauma.”

Unfortunately, the biomarker approach did not work as well in sheep brains, although they did show signs of tau accumulation. However, the brains of the musk oxen lit up with tau like a Christmas tree.

At first glance, it might not make sense that a natural behavior such as head banging could be so detrimental. But it’s the long-term game that matters, says Ackermans.

“Every year a musk ox will butt heads a lot, but if they breed even once successfully, that’s all it takes,” he says. “What’s fostered, evolutionarily, is just not dying.”

It probably helps that male muskox live less than 15 years and females between 15 and 23 years, he says. So while tau proteins accumulate throughout the life of animals, they may never accumulate to a point where they can cause conditions like Alzheimer’s or other dementias.

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“Their lives are not that complicated,” says Ackermans. “So potentially they only survive long enough to do what they need to do.”

And even if they did develop those conditions, who would know? “There’s no behavioral scale for the musk ox. So we can’t say they don’t get a little forgetful,” she says.

Next, Ackermans wants to study various species of woodpeckers to see if they have brain trauma from their head-banging behaviors. The only other study that looked at bird brains found some evidence of tau, but “it wasn’t really in any specific pattern,” he says.

mating to death

In a way, musk oxen are an interesting parallel to certain marsupials, says Diana Fisher, a mammal ecologist at the University of Queensland’s School of Biological Sciences in Australia.

Small and carnivorous, antecines are a genus native to mainland Australia and Tasmania. In recent years, they have made headlines for the way males practice semelparity, that is, a single burst of reproductive production, followed by programmed death. The females of Antechinus they can live for two to three years or more. But males rarely last more than 11 months.

“They have a very hectic mating season,” says Fisher. Mating fights can last 12 to 14 hours, after which each male will attempt to mate with as many females as he can, leading to his death.

“The collagen in their skin breaks down, their intestines break down and they bleed internally,” explains Fisher. “They become very susceptible to parasites and disease, and their immune systems fail.” In a matter of weeks, they will be dead.

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“This is all quite unusual for mammals,” says Fisher, who typically survive long enough to experience multiple mating seasons.

Suicide breeding is most common in insects, fish, plants, and arachnids: when another Australian native, the redback spider, mates, the male places himself in the female’s mouth.

“That discourages the female from continuing to mate,” Fisher says, “because she’s busy eating.”

self-destructive insects

A similar, though slightly different, dynamic occurs in large colonies of social insects.

When a European bee stings a soft-skinned attacker, such as a bear, it dies when its sting lodges in the victim’s skin. A blaster ant can rip its abdomen in two while defending its nest from attackers. And in some species of termites, the older workers can become suicide bombers.

But how does it make sense to kill yourself, evolutionarily speaking?

“Easy,” says Thomas Seeley, a Cornell University biologist and author of the life of the bees, in an email. “Workers achieve genetic (evolutionary) success not by reproducing themselves, but by helping their mother, the queen of the colony, to do so.”

(Related: Learn why insects are disappearing at an alarming rate).

“One form of this help is colony defense,” he explains.

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