Thanks to Brilliant for
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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
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