Thanks to Brilliant for supporting this SciShow List Show!
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.
So if you’re looking to get university level explanations
for the math version of group theory, you can find it in this Brilliant course.
But if you’re just dipping your toes into math,
Brilliant also offers introductory courses like Algebra I,
Geometry Fundamentals, and Introduction to Linear Algebra.
You can choose your own math adventure at Brilliant.org/SciShow
or by clicking the link in the description down below.
You’ll get a free 30 day trial and 20% off
an annual premium Brilliant subscription.
Thanks for watching!
[OUTRO]