It’s a brutal reality of the animal kingdom—one that seems to defy logic and even basic survival instincts.
Infanticide, the killing of young offspring, is disturbingly common among mammals. Lions do it.
Baboons do it.
Even adorable-looking lemurs and mice have been observed turning against their own kind in this shocking way.
But why?
Why would a species weaken its own population by killing its future generation?
For decades, scientists have struggled to answer this unsettling question.
The prevailing theory has been that dominant males kill the young of their rivals to eliminate competition and force females back into fertility, giving them a chance to pass on their own genes.
But new research suggests a more complex battle is at play—one where females have developed an ingenious counter-strategy to fight back.
And the repercussions of this evolutionary arms race are far more widespread than we ever imagined.
A Cold, Calculated Strategy
To get to the bottom of this phenomenon, researchers Dieter Lukas from the University of Cambridge and Elise Huchard from the University of Montpellier analyzed infanticidal behavior in over 200 mammal species.
Their findings, published in Science, confirmed what many suspected—infanticide is often a strategic move driven by reproductive urgency.
In species where males frequently kill offspring, dominance is often fleeting.
A powerful male may lead a group for only a short period before being overthrown by a stronger rival.
Since most female mammals will not mate while nursing, a new alpha male cannot afford to wait months or even years for offspring that aren’t his to grow up.
Instead, he eliminates them, forcing the females to return to estrus and giving himself a chance to father his own young before his rule ends.
“The males don’t manage to stay dominant for very long, so when they can mate with the females, they need to do it as quickly as possible,” Huchard explained in an interview with New Scientist.
From an evolutionary standpoint, the logic is ruthless but effective.
But what if females weren’t just passive victims in this game?
What if they had their own way of ensuring their babies survived?
Promiscuity as a Defense Mechanism
Here’s where things get really interesting.
Lukas and Huchard found that females in infanticidal species have developed an incredibly effective defense—promiscuity.
By mating with as many males as possible in a short time frame, females make it impossible for any one male to be certain of his paternity.
The result?
Males become hesitant to kill infants because there’s a chance they could be eliminating their own offspring.
This tactic, known as paternity confusion, has been observed in a wide range of species, from primates to rodents.
And it works.
The study found that in species where promiscuity rates were high, infanticide rates dropped significantly.
But the evolutionary battle doesn’t end there.
Just as females have developed countermeasures, males have evolved a new weapon of their own.
Bigger Testicles, More Sperm, Greater Odds
If males can no longer ensure reproductive success by killing off rival offspring, how do they adapt?
The answer is unexpected: they grow bigger testicles.
It turns out that in species where paternity confusion is common, males have evolved larger testicles and produce more sperm to increase their chances of fertilizing an egg.
The Madagascar mouse lemur is a prime example—during breeding season, its testicles swell to five to ten times their normal size.
This ensures that in a competitive mating environment, its genes have a better chance of surviving.
“In species in which infanticide occurs, testis size increases over generations, suggesting that females are more and more promiscuous to confuse paternity,” Lukas noted in a press release.
This trend is seen across multiple species.
Among primates, for instance, bonobos—a species that engages in high levels of promiscuity—have far larger testicles compared to chimpanzees, their close relatives, who still practice infanticide.
Can Infanticide Be Eliminated?
While it might seem like this evolutionary arms race will continue indefinitely, there’s evidence that it has already led to significant shifts in mammalian behavior.
Some species, such as bonobos, appear to have eliminated infanticide altogether since diverging from their chimpanzee ancestors.
This suggests that under the right conditions, sexual selection and reproductive strategies can override even deeply ingrained behaviors.
The study of infanticide and the counter-strategies against it offers a fascinating glimpse into the relentless push-and-pull of evolution.
While the idea of male mammals systematically slaughtering their rivals’ offspring may be disturbing, the female-led response of strategic promiscuity and reproductive trickery reveals an equally powerful force at play.
In nature, survival isn’t just about brute strength—it’s about outwitting, outlasting, and adapting in ways we’re only beginning to understand.
One thing is clear: the battle of the sexes in the animal kingdom is far from over. And the next move?
That might be up to evolution itself.
Sources: New Scientist, Phys.org
