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As a SciShow viewer, you can  keep building your STEM skills with a 30 day free trial and 20%  off an annual premium subscription at Brilliant.org/SciShow. So why do animals eat their own babies?

I mean, sure, it might be a  dog-eat-dog world out there, but a dog-eat-puppies world?  That– it just seems too dark. But it turns out that, at  least in certain species, filial cannibalism is totally a thing. While most cases do end in death, there are some animals that feast  upon their young without killing them.

Which is a silver lining, I guess? Now, the tendency to chow down  on your own offspring might seem not just gruesome, but illogical. After all, parents put a lot of time  and energy into making those babies.

But there are a number of very good reasons animals resort to such measures. And get ready! Because we’re  gonna talk through five of them. [INTRO] Although clearly not ideal  for the babies that get eaten, filial cannibalism is often a  beneficial strategy for parents if their offspring don’t have a great  chance of survival in the first place.

Take long-tailed sun skinks, for example. Usually, the females are terrible  parents by human standards. After laying their eggs, they  hightail it out of there, and leave the nest unprotected.

But one particular population of these lizards, located on Taiwan’s Orchid Island, encounters a lot of egg-eating snakes. So those females have evolved  a bit of parental instinct. A mother will guard her eggs for at least a week, which significantly increases her  babies’ likelihood of survival.

But it turns out these eggs face another unexpected predator on occasion: the skink that put all that  effort into laying them. A study published in 2008  used both wild observations and field experiments to observe how trespassers affected these Orchid  Island animals’ nest-guarding behavior. In the experiments, either  a non-threatening lizard or an egg-eating snake was  released into each nest.

And they let the trespassers  in either once or 3 times a day to see if that extra stress  would make a difference. Most of the moms ignored  the harmless lizard intruder no matter the frequency, knowing  their eggs weren’t at risk. But when an egg-snatching snake was introduced, she’d violently protect her clutch by  straight up attacking the intruder.

Which, you know, thanks, mom! However, if the snake attacks were too frequent, she often resorted to a more desperate response. She gobbled up her own eggs before  the snake could come back again.

That might seem like an  overly melodramatic reaction, Like, “If anyone’s going to  eat my eggs, it’s gonna be me!” It’s a bit of cry of desperation. But that skink is likely  weighing the costs of defending her existing eggs over and over again,  and realizing it’s a losing battle. Instead of letting a snake benefit  from her failed nesting attempt, she can recover some of her spent  energy by eating her own eggs.

That way, she gets the  boost she needs to try again with a new brood of eggs,  hopefully this time without a persistent predator in the neighborhood. But it’s not always a super stressful environment that drives filial cannibalism. In the case of barred-chin  blennies, it may just be that they’re eager to give mating another go, instead of raising their current brood.

For this species of fish, the  male blennies are the caregivers, tasked with watching over fertilized eggs. But this responsibility actively impedes  their ability to make more babies. A male blenny can only  successfully mate when he has high enough levels of androgens,  which are hormones that are tied to sexual characteristics and fertility.

But androgen levels drop as soon  as daddy’s got a clutch of eggs to watch over. It looks like  the mere presence of eggs prevents a male blenny from getting it on again. For example, in one set of  experiments, researchers gave eggs to males that hadn’t spawned on their own.

And their bodies would still kick into parent mode and they couldn’t mate, anymore. In another experiment, when researchers removed the eggs from a caregiving male, their hormone levels spiked back up and they had brand new clutches to  protect in as little as one day! Eager little beavers!

So what if a wild blenny dad  makes a bunch of babies with mom, but he thinks that bunch isn’t big enough? If he wants to make more blenny juniors, he either has to wait for his current  clutch to hatch, or… you know? Although in some cases, the  males didn’t eat all the eggs.

Instead, they were caught  spitting eggs out of their nest! It appears that male blennies are  just that hasty in getting eggs out of their presence ASAP, so they  can get back into the mating world. So in the case of blennies,  filial cannibalism kicks in when the clutch of eggs is too small.

But in other animals, they might do the opposite, eating their young only when  their clutch is too big. Burying beetles are appropriately named, because they completely bury the carcass of either a small mammal or bird to be  used as their very own nursery. After that bit of interior decorating,  they lay their eggs near the carcass.

When the babies hatch, they can wriggle  right on over and start feeding. The thing is, these beetles  generally lay too many eggs for the limited food source they’ve provided. Having lots of babies right  from the start helps compensate for those that are going to be lost to predators, or any other causes of death along the way.

But the nursery has a finite food source, so if the brood is too successful  and the numbers aren’t whittled down, there’s a risk of there being too much competition between all the offspring. Lucky for the parents, the eggs  don’t all hatch at the same time. So to prevent any sibling rivalry  over that one precious buried corpse, they’ll eat some of their straggler babies.

This morbid behavior has been studied  in these beetles since the 1980s, when pairs of the beetles  were bred in a lab setting. In one study from 2013, they were carefully monitored  with video recording, and larvae were weighed  periodically to track their growth. And it turned out that the  offspring that were slower to hatch also showed slower growth rates,  and were more likely to be killed.

So it looks like they’re not  only eating their babies to cut down on sibling rivalry,  but selectively doing so, weeding out the least likely  to survive to help ensure their healthiest offspring thrive instead. This tough love approach  to parenting is applicable beyond just keeping numbers in check. In some cases, it’s a matter  of removing bad apples before they spoil the whole batch.

Male Japanese giant salamanders  can commit to caring for their eggs and offspring  for as long as seven months… which feels like forever compared  to some of these other examples. But being a parent is tough, and these amphibious fathers have been documented eating eggs to prevent the  spread of a fungal infection. Admittedly, we’re not talking  large sample sizes, here.

Just a few nests over a few studies. But these observations have been critical to uncovering this species’s brooding behavior. And these studies have found that  caretaker males do a lot of cleaning to try to keep things as safe and  healthy for their offspring as they can.

This includes fanning the eggs  with their tails to increase oxygen in the nest, and clearing out debris  that makes its way into their den. By removing wayward organic matter, these salamanders reduce the ingredients that freshwater molds need to take hold. But if mold still finds its way to the eggs even after all those housekeeping  efforts, there’s still plan B.

Just gobble up the infected. At nests found in two different rivers, salamander dads were observed eating whiter eggs that were either dead or infected with water mold. The adults seem to be totally fine  munching on this kind of mold, but for aquatic eggs,  infection is a serious threat.

Since this mold can spread  quickly if left unchecked, these males are likely doing a huge favor to the overall brood by taking  out any infected eggs early. And it appears the cleaning-up  cannibalism doesn’t stop at the egg stage! One male was observed eating  a dead baby salamander too so the removal of dead or  ill offspring appears to be an important habit throughout  this long parental stage.

If they had opposable thumbs, I’d  ship them a “Number One Dad” mug. Although maybe just sharpie  an asterisk on the end. While a lot of parents are  clearly just straight up eating their own eggs and babies, there are some species that munch  on their young without killing them.

Take the appropriately nicknamed Dracula ants who, as you might have guessed, drink  the blood of their wee babies. Well, it’s not exactly blood. Like all insects, ants have hemolymph, their own version of circulating fluid, that enables them to store  and use water and chemicals required for their day to day functions.

This larval hemolymph feeding has been documented in a number of different ant  species, where adult ants, typically the queens, feed directly  on the inner fluids of larvae. And they do so by puncturing  them right on the back using their chomping mandibles. Luckily for the babies, these  wounds close up quickly, although not without leaving scars.

While the jury’s still out on how these ants evolved such a unique feeding habit, the why is probably to create a  consistently available food source. In a weird way, it makes sense to  use larvae as a fresh food reserve that can keep you satiated whether or not your latest foraging attempt was successful. And a study from 2019 of one  Japanese hemolymph-sucking species took a deep dive into their  somewhat horrifying habit.

While there are exceptions  if the colony is starving, the queen is the only adult in this species who takes part in this not-so-deadly,  not-quite-bloody feast. And even then, they found that the  queen only chows down on her babies once her colony is established  and the population is doing well. Up until then, she feeds more  on prey she’s not related to.

But once her colony is big enough, she depends exclusively on  the hemolymph of her babies. And it turns out, these babies are  quite literally built for this task. Each larva has specialized structures,  with little cracks built into them, that make drinking its inner  juices easier for Her Majesty.

Just a little pinch of her  mouthparts and she’s tapped in. Now, the larvae do eventually  grow out of this role. So they’re not doomed to life  as a proverbial blood bag.

But hey, they’re not human. I don’t  know! Maybe they wouldn’t even mind.

Maybe that would be like a  fulfilling life for them! In some cases, filial cannibalism is an example of “desperate times call for desperate measures.” In others, it’s about setting up  a convenient and consistent meal. It might always seem heartless to us, but this family-first cannibalism  could be the deciding factor in ensuring offspring have the best start at life.

At least the offspring that  the parents decide not to eat. Thanks to Brilliant for  supporting this SciShow List Show! Scientists seem to have a few theories about why a group of animals starts acting more  like predators and less like family.

But that’s a totally  different kind of group theory from the course offered at Brilliant.org/SciShow. Brilliant’s Group Theory course is all about math! This interactive online learning  platform has thousands of lessons to choose from in science,  computer science, and math, including this one created in partnership with a University of California,  Berkeley PhD in mathematics.

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