Why some animals evolved to sacrifice themselves?

A male musk ox can weigh up to 800 pounds and charge at speeds in excess of 30 miles per hour. During the breeding season, these shaggy Arctic Circle behemoths collide face first, then stab their opponents with their huge, sharp horns.

In addition, a single male musk ox can take about 2,100 blows to the head over the course of their 10 to 12 year lifespan.

All of this begs the question, how do musk oxen survive these attacks without turning their brains into mush?

“People have always assumed that animals that headbutt, such as musk oxen and bighorn sheep, are somehow immune to head injury,” says Nicole Ackermans, a neuroscientist at the Icahn School of Medicine at Mount Sinai in New York. “Like they have magic horns or something.” (Read about five times that evolution ran ‘reverse’.)

But when Ackermans started reading the scientific literature, she found that no one had investigated whether these North American herbivores suffered brain damage from their head-banging lifestyle. So she and her colleagues acquired the brains of musk oxen and bighorn sheep 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 was very similar to early chronic brain trauma in a human,” said Ackermans, who led a recent paper on the findings. published in the magazine Acta Neuropathy.

The new research could be crucial to better understand brain injuries in humans, Ackermans says, because cattle (animals such as oxen and sheep) have folded, wrinkly brains more like ours than, say, mice, whose brains are smooth. .

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

‘Just don’t die’

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

“When your neurons get damaged, be it from aging, genetic problems or mechanical impact, they get torn, and this protein breaks down and forms into clumps,” explains Ackermans. “And if you see them in specific patterns, you can see if it’s just a normal brain, or aging, or Alzheimer’s, or possibly trauma.”

Unfortunately, the biomarker method didn’t work very well on the sheep brains, although they did show signs of tau build-up. The musk ox brain, however, lit up with tau like a Christmas tree.

At first glance, it may not make sense that natural behaviors, such as headbutting, could be so harmful. But it’s about the long game, says Ackermans. (Read how climate change is causing musk oxen to freeze to death.)

“Every year a musk ox is headbutted many times, but if they reproduce successfully even once, that’s all you need,” she says. “What is evolutionarily encouraged is simply not to die.”

It probably helps that male muskoxen live less than 15 years and females 15 to 23 years, she says. So even if tau proteins build up throughout the animals’ lives, they should never build up to a point where they can cause conditions like Alzheimer’s disease or other forms of dementia.

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

And even if they did develop those conditions, who would know? “There is no behavior scale for musk ox. So we can’t say they don’t feel a little forgetful,” she says. (Read how animals can get dementia.)

Next, Ackermans wants to study different species of woodpeckers to see if they show brain trauma from their head bangs. The only other study that looked at the brains of birds found some evidence of tau, but “it wasn’t really in a specific pattern,” she says.

Mate to Death

In some ways muskoxen are an interesting parallel to certain marsupials, says: Diana Fisher, a mammalian ecologist at the University of Queensland School of Biological Sciences in Australia.

Small and carnivorous, antechinuses are a genus native to mainland Australia and Tasmania. In recent years, they’ve made headlines for the way the males practice semelparity — or a single burst of reproductive output, followed by programmed death. Antechinus females can live two to three years or more. But the males rarely last longer than 11 months. (Read about five animals that mate themselves to death.)

“They have a very hectic mating season,” Fisher says. Breeding periods can last 12 to 14 hours, after which each male will try to mate with as many other females as possible – leading to his demise.

“The collagen in their skin disintegrates, their gut disintegrates, and they have internal bleeding,” says Fisher. “They become very susceptible to parasites and disease, and their immune systems fail.” They’ll be dead in a few weeks.

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

Suicidal reproduction is more common in insects, fish, plants and arachnids: When another native Australian spider, the redback spider, mates, the male will place itself in the female’s mouth.

“That stops the female from mating further,” Fisher says, “because she’s busy feeding.”

Self-destructing insects

In large, social insect colonies, a similar but slightly different dynamic takes place.

When a European honeybee stings a soft-skinned attacker, like a bear, it dies when its sting lodges itself in the victim’s skin. An exploding ant can tear its belly in half while defending its nest from attackers. And in some termite species, older workers can turn themselves into suicide bombers.

But how does suicide make sense from an evolutionary point of view?

“Easy,” says Thomas Seeley, a biologist at Cornell University and author of The life of 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.” (Find out why insects are disappearing at an alarming rate.)

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

“Some researchers call this a ‘superorganism,'” Alice Laciny, an entomologist who works on exploding ants at the Natural History Museum Vienna, said in an email. “So an ant colony or beehive is more like one big animal, with the queen representing the reproductive organs. The little workers are numerous and need only small amounts of resources to breed, so they kind of resemble the body’s cells.”

As with the musk oxen, our violent, self-destructive behavior of the worker ants seems worth the cost, as long as it leads to reproduction.

“In this system, protecting her queen and sisters, even through self-sacrifice if necessary, is how an ant worker can protect and pass on her genes,” Laciny says.

A Mother’s Ultimate Sacrifice

Another form of sacrifice in the animal kingdom is the effort some mothers make to give their young a fighting chance.

After they are born, some species of legless amphibians become known as: caecilians literally eat their mother’s top layer of skin as their first meal. And African social spiders take it a step further, with some females allowing their young to practice matriphagy — or killing and eating their own mother. (Watch baby spiders eat their mother alive.)

Giant octopuses may just be the ultimate self-sacrificing mothers. women can watch over their eggs for an incredible four years – during which time they don’t even eat.

“Inevitably, the females will deplete their entire body reserves and die while guarding the eggs,” Fisher says.

“You do feel sorry for them, but that’s how many species get the best offspring success in the next generation.”

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