Nature's Rules For Killing Your Siblings

We may earn a commission from links on this page.

All of you dreading the trip home to your family this holiday season should remember it could be worse. You could be part of a species where killing your siblings is just part of the typical family dynamic.

It's Never Too Early to Get the Upper Hand

Life, it is often said, is not fair. It's especially unfair if you are a younger sibling. Unless there are mitigating circumstances, you are almost never on the "cide" end of a siblicide. Birds are the unquestioned champions of siblicide. All over the world, they lay their eggs in different stages and produce, in successive weeks, an older sibling and a younger sibling. Some even stack the deck, producing two older siblings which will pair up against the single younger sibling and drive it out of the nest. (Bald eagles do this. Be sure to mention it to anyone who tells you that killing your sibling is somehow un-American.)

Insects also commonly engage in siblicide on a first-come-first-served basis. Every queen bee only got to be royalty because she hatched early and toured the cells that had developing queens, destroying them all. (If she doesn't get to them before they hatch, the prospective queens battle it out.) Parasitic wasps make it a point never to pass up a chance to be reprehensible, so their larvae often kill and cannibalize each other as well.

Sharks kill each other before they're even born. Sand tiger sharks give birth to live young. While they're still in the womb, shark embryos attack and cannibalize their siblings.

There are very rare exceptions to the older-sibling-lives rule. Macaroni penguins lay an initial small egg when they are first nesting. It's soon followed by a larger, more robust egg. The younger chick is generally larger and healthier than its older sibling, and drives the older chick out of the nest. Still, unless younger siblings have a definite physical advantage, the smart money's on age and experience. If any younger siblings want to win this fight you have to make it a point to get up early and sharpen your knife.

Know Why You Do It

There are two kinds of siblicide; obligate and facultative. Obligate siblicide is, of course, obligatory. In nearly every family, someone is going to end up dead. Facultative siblicide is murder that only happens when things go south for the family.

Avian and insect siblicide is almost always obligate. That's why one egg is so much older than the other. If you engage in obligate siblicide and you get caught, blame your parents. It technically is their fault. Masked boobies, a Galapagos species notorious for siblicide, lay eggs that, under perfect conditions, only hatch about sixty percent of the time. That's two-thirds of the success rate of other birds. They need to lay two eggs to keep their numbers up, but can't support both children. Obligate siblicide means, evolutionarily speaking, that birds are so bad at laying viable eggs that it's worth the energy to produce a redundant egg, only to let one of the resulting offspring die if both eggs hatch.

If you find yourself in a situation that calls for facultative siblicide, you can still blame your parents most of the time. As long as the parents bring in food, facultative siblings are a big, happy family. If there's a food shortage, siblings will compete for food. Once their weight drops too far, they'll start attacking each other. This is a good opportunity for younger siblings to get in the ring. Make sure you're first in line for food every time your parents feed you. If the food supply looks like it's dipping, start planning. Remember, there's no such thing as too much food.

Scientists aren't always sure which siblicides are facultative and which are obligate. Spotted hyenas, one of the few mammals with well-documented instances of siblicide, are the subject of much speculation. Some scientists believe that the hyenas engage in obligate siblicide, but only in the case of same-sex litters. A bunch of females will kill each other, and a bunch of males will as well, but females and males don't tend to turn on each other. Others believe that siblicide in hyenas is purely facultative. When the food supply dips they turn on each other, but not otherwise. (Each group of scientists does admit that the struggle for dominance in hyenas begins immediately and can be fairly bloody, so siblings take note. You don't just have to go for the kill. You have to dominate your opponent at every opportunity.)

Shark siblicide introduces an interesting twist — clan rivalry. A tiger shark can have litters of pups who share the same mother, but have many different fathers. Scientists found that, at the beginning of a pregnancy, there are a roughly equal number of pups from each father. As the pregnancy wears on, certain fathers' pups disappear. Towards the end of the pregnancy, one father's pups have eaten nearly all of their half-siblings. The sharks are playing out genetic warfare in the womb. If this is happening in your family — blame your father.

Get Your Parents On Your Side

But blame is only for if you get caught. Beforehand, it's important to get a parent on your side. Even in species that are regularly siblicidal, parents sometimes keep their offspring from killing each other. In one study, scientists switched blue-footed booby and masked booby eggs. Blue-footed chicks normally only kill their siblings when their body weight drops to seventy-five percent of normal. Masked chicks always try to kill each other. Sometimes the masked booby parents even stand over one chick, keeping it from maneuvering to defend itself so the other chick can drive it out.

When the researchers switched the eggs, the relatively loving blue-footed chicks engaged in siblicide half the time. The masked booby chicks, when raised by the blue-footed booby parents, dropped their siblicide rate from one hundred percent to eighty percent. If that's not a testament to the importance of parenting, then what is?

Normally, getting a parent on your side means being nice to that parent, but let's face it, you didn't read an article on siblicide because you were willing to be nice. Instead, be needy. The fact is, parents will pay attention to whatever sibling makes a big display of needing them the most. Our best example of this comes from those most famous avian juvenile delinquents — the cuckoos. The Hodgson's hawk cuckoo rolls its competition out of the nest, like any other good bird. When its adoptive parents come back to the nest to feed it, it opens its mouth and spreads its wings. On each wing is a red spot. The red throat of a baby bird is what catches a parent's attention. They aim food at the red spot, and the chick with reddest spot is the one that gets the most food. Parent birds, fooled by the red spot, will actually leave food on the cuckoo's wings, because they only see that needy red. The cuckoo doesn't have to butter anyone up, or look cute. All it has to do is demand to be fed three times, and it gets a meal for three. Demand all the attention, and sooner or later, you'll get it. Perhaps from the police.

Top Image: Alex Popovkin

[Via Siblicide in the Spotted Hyena, Animal Cannibalism, Why Shark Embryos Eat Each Other Up In Utero, Siblicide in Nature.]