Why infanticide in animals

While humans may be uncomfortable with killer mum jokes, even with regard to other species, perhaps we can at least better understand the fact that, yes, infanticide does happen. But why? She gives the example of marmosets, tree-dwelling primates found in South America. In , a study described the killing of a one-month old marmoset , the daughter of the dominant female, by the second breeding female in the group. This female made the attack while she herself was pregnant.

Later, she gave birth to twins and appeared to become the leader of the group. While the marmoset appears chillingly cold-blooded to us, the move was clearly made out of a primal urge to secure the best possible future for her offspring. Huchard and Dieter Lukas at the University of Cambridge recently published a study of female infanticide in mammals.

The work is yet to be peer-reviewed, but it has helped to document the prevalence of killer mums in mammalian societies. Females will kill whenever they can assess clear benefits from doing so. Interestingly, the frequency and distribution of infanticide seems to suggest that there is a correlation between infanticide risk and social organization in mammals and primates. Several explanations have been proposed for the existence and evolution of infanticide in non- human primates.

The benefits of infanticide for male non-human primates, and its costs to females, probably vary across mammalian social and mating systems [7] and between different primate species as well.

Infanticidal behavior can be adaptive because of nutritive benefits individuals gain from cannibalism, conservation of resources by eliminating competitors and paternal manipulation in which parents terminate investment in particular offspring by killing and often consuming them [8].

The prevailing adaptive explanation for the behavior, however, has been the sexual-selection hypothesis according to which males increase reproductive opportunity by killing unrelated, unweaned offspring, thus hastening the mother's next ovulation, at which time the infanticidal male can mate with her [9].

Alternate nonadaptive explanations such as social-pathology and side effect hypotheses have also been proposed. Males in primate groups often exploit infants to which they are not related in ways that lead to the death of the infant. Maternal infanticide, although very rarely observed in non-human primates, has been considered as a way of paternal manipulation to end investment in certain offspring.

However, the study indicates that the mothers likely did not kill their offspring to exploit the meat. On the other hand, the evolution of nonparental male infanticide has often been explained by the sexual selection hypothesis, which posits that infanticide improves male reproductive success by shortening the interbirth intervals of the mothers of the killed offspring. The sexual selection hypothesis is supported by findings that indicate that infanticide mostly occurs in social species, less in solitary species, and least in monogamous species since according to the sexual selection hypothesis, infanticide would be most adaptive in stable bisexual groups where a few males monopolize reproduction over short periods of time [7].

In social species where a few males monopolize reproduction over short periods, killing the offspring of males who had previously been monopolizing reproduction would be adaptive for the individuals committing infanticide. From the newcomers' standpoint, "there's no sense in spending energy or resources raising the previous males' cubs," Stanford says, since the new males are most likely unrelated.

In the game of life, the prize goes to the individuals who have the most reproductive success and pass on the most genes—a task best accomplished by raising your own offspring or helping to raise those of your relatives.

Assisting unrelated individuals adds nothing to your reproductive scorecard. This type of infanticide is found in almost every primate species, including chimpanzees, gorillas—and, as much as we would like to deny it, humans. Male bonobos are one of the few great apes who have not been seen killing infants. This is probably because female bonobos are the dominant members of their societies, making it risky for the males to attack any youngsters.

Also, bonobos happily mate with everyone in their community. Thus, males aren't readily able to identify which kids are theirs. A common counter-reproductive strategy of females in many animal societies is to confuse males about which if any kids they've fathered.

It doesn't always work. Male bottlenose dolphins, for instance, remember which females they've mated with. When a male dolphin encounters a strange female with a young calf, he'll do his best to separate the pair and will then severely injure or kill the youngster by bashing it and heaving it through the air.

If the infant dies, the mother will become fertile in a few months—giving the killer male a chance to father her next calf. If the infant lives, the mother won't be receptive for another three to four years—a long time from a male's standpoint. In the game of life, it doesn't pay to wait for her to rear her kid, especially if you know it's not yours. Better to get rid of it. Zoos generally try to prevent killings by males by carefully managing the reproductive events of the animals in their care.

But sometimes animals behave in unpredictable ways. That's what happened in when, as visitors looked on, an adult male chimpanzee bashed and killed his sister Gracie's three-month-old baby at the Los Angeles Zoo. The zookeepers had kept Gracie away from the rest of the troop for three months after she gave birth, giving her time to bond peacefully with her infant. All seemed to be going well, and the keepers decided to slowly reintroduce the pair to their community.

The other chimpanzees welcomed back the pair, and peered with curiosity at the new infant—the first baby chimp born at the zoo in 13 years. But one day, without warning, Gracie's brother snatched the infant from her arms and dashed around the enclosure slamming her against the ground and walls. Despite Gracie's cries and protests, he wouldn't give back the now-dead baby, and it wasn't possible for the keepers to intervene.

But as far as I can tell, it did not involve any mismanagement by the zoo. It wasn't caused by the chimpanzees being in captivity. According to the rules of reproductive success, it would have made the most sense for Gracie's brother to protect his niece because they were related. If his niece grew up and had babies of her own, they would also carry some of his genetic legacy.

His reproductive success would be enhanced. Perhaps it wasn't intentional. Perhaps it was simply because he was a young male, prone to making exuberant displays, Stanford says. The zoo's keepers and visitors were heartbroken.

And the staff did its best to help Gracie by giving her the body of her baby to grieve over in a separate room. For sloth bear Khali's third cub, life is on an upswing. Her eyes were still tightly shut when the keepers took her from her mother, but they opened to the world on January The zoo has yet to name the cub.