Imagine leaving the kids in the care of their dad and returning to be told: "Sorry, honey, I seem to have eaten some of the kids." It sounds like a terrible nightmare, but that nightmare plays out all the time in the animal kingdom. Eating the kids might seem like a counterproductive strategy when it comes to passing along your genetic legacy, but infanticide, sometimes accompanied by cannibalism, is actually quite widespread, occurring in a range of species, from fish to mongoose to lions, spiders and birds.

“Why would an animal parent kill and sometimes eat the kids? That's a question that inspires the research of evolutionary biologist Hope Klug.”

Why would an animal parent kill and sometimes eat the kids? That's a question that intrigues and inspires the research of evolutionary biologist Hope Klug at the University of Tennessee at Chattanooga. Why does filial cannibalism – eating the kids – evolve?

"For a very long time people thought it was all about energetic need," says Klug. In other words, scientists assumed that if parents ate their kids, it was likely because they couldn't find enough food ... and the babies presented a convenient snacking option to make up for lost energy. If that was the case in fish, for example, where dads often rear the young alone, "you’d expect fathers to eat more of their offspring when they were in either poor condition or really needed the food," explains Klug. Instead, the evidence that’s emerged is mixed. Some studies don’t find this relationship at all.

In recent years, Klug and others have developed alternative explanations. Adult fish, for example, sometimes find themselves caring for more eggs than can feasibly survive, and in that situation "you might expect males to eat some of the eggs to thin them out," says Klug. Males may also snack on eggs of lower quality, such as those infected by fungus.

Sand goby dads sometimes choose to eat their slow-developing progeny so they don't miss out on new mating opportunities. Image: Asbjørn Hansen, Flickr

In sand gobies (Pomatoschistus minutus), the fate of the eggs comes down to timing. In this tiny fish species that Klug studies in the Baltic Sea, males build nests that they almost completely cover in sand, with just a tiny entryway in the front. After courtship, fish dads get a clutch of eggs to fertilise and look after, deposited by females on the nest ceiling. Dads fuss over the eggs for about ten days, fanning them to keep them oxygenated and protecting them from egg-eating invaders. And then they start again (and again), with several clutches of new eggs for about six weeks. While they're caring for all these eggs, the male fish are missing out on new mating opportunities ... so if any of the eggs take too long to develop, the impatient dads might choose to eat them rather than wait, Klug has discovered.

Climbing out of the water and into the world of land mammals, you might be surprised to hear that even the adorable-looking mongoose is not above ruthless baby-killing behaviour. A recent study in Uganda by Michael A. Cant and Sarah J. Hodge from the University of Exeter, with colleagues at the Universities of Cambridge and Liverpool John Moores, involved a series of experiments over seven years in a wild population of about 400 banded mongoose (Mungos mungo).

In this species, multiple females, but mostly those four years and older, contribute young to a communal litter, breeding about four times a year. The scientists had been puzzled about how reproduction was suppressed in younger females, who are capable of reproducing, but often don't. They wondered whether, within the highly social mongoose society, the older (more socially powerful) females used the threat of infanticide to deter the younger females from breeding.

In banded mongoose societies, dominant females are known for killing litters that don't include their own pups. Image: Reinhold Stansich, Flickr

Through a series of experiments that used contraceptive injections to block reproduction in some females but not others, the researchers uncovered evidence to support that theory: in banded mongoose society, dominant females do indeed kill litters that don’t include their own pups. 

Mongoose moms apparently cannot recognize their own offspring, but they have come up with a clever way to ensure they kill only the litters from other moms: they kill any pups born before, but not after, their own. 

But the story doesn’t end there ... the younger, subdominant females have developed a counterstrategy to protect their own offspring: they synchronize the birth of their pups to coincide closely – usually on the same day – with the birthing of pups by the older dominant females. With so many pups born close together, it becomes impossible for the older females to differentiate which pups belong to them, and which pups don’t. Killing pups at random just isn’t worth the risk ... so all the babies stay safe (take that, you malicious mongoose mamas).

Research by Klug, Hodge and others is shedding light on the 'sense' behind this seemingly senseless behaviour, but there are more chapters to be written on the science of infanticide. It’s one of the many mysteries just beginning to be unravelled about nature’s most extreme measures.

Top header image: Helmut Schwarzer, Flickr