And sure enough, their androgen levels dropped.But when they ate their own eggs, they were able to quickly recover those hormone levels and mate again.It’s like they’re saying, “I’m not ready to be a parent yet, so I’m just gonna eat my own babies and try again later.”
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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 sure enough, their androgen levels dropped. But when they ate their own eggs, they were able to quickly recover those hormone levels and mate again. It’ When male blennies have a small clutch of eggs, they may engage in filial cannibalism and eat their own young to get back into the mating world. On the other hand, burying beetles generally lay too many eggs for the limited food source they’ve provided, so they may eat some of their straggler babies to prevent sibling rivalry. Similarly, male Japanese giant salamanders have been documented eating eggs to prevent the spread of a fungal infection. These amphibious fathers have been observed fanning the eggs with their tails to increase oxygen in the nest and clearing out debris that makes its way into their den. If mold still finds its way to the eggs, they may eat the infected eggs to protect their offspring. Plus, you can get 20% off an annual Premium subscription at Brilliant.org/SciShow.
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.
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. Plus, you can get 20% off an annual Premium subscription at Brilliant.org/SciShow. At Brilliant.org/SciShow you can embark on your own math adventure! Plus, you’ll get a free 30-day trial and 20% off an annual premium Brilliant subscription.
Thanks for watching! [OUTRO]