Sibling Rivalry

Scientists recently discovered what makes Nazca boobies (seabirds that live on the Galapagos Islands) kill their siblings. Yeah, that’s right. These birds practice what scientists call obligate siblicide. Females lay two eggs several days apart. If only one egg hatches, the little booby will grow up to be a happy, well-adjusted adult booby. But, if the second egg hatches, the first-born booby will knock its sibling out of the nest where it will freeze or starve to death. (The booby parents don’t intervene, probably because they can’t support more than one chick anyway.)

So what’s the fuel behind this siblicidal rage? Testosterone. Scientists found that all Nazca boobies are born with elevated levels of testosterone. But when a chick kills its sibling, its testosterone levels shoot even higher. Though the hormone surge is temporary, it seems to have a lasting effect on the boobies. As adults, boobies that killed their siblings tend to beat-up or molest unguarded chicks. Freaky, eh?

What’s even freakier is that Nazca boobies aren’t the only animals that kill their siblings to survive. Sandtiger sharks actually eat their siblings in the womb. Somewhere between four and six months of gestation (around the halfway point) sandtiger embryos grow teeth and develop the ability to swim. Then, the strongest embryo in each uterus (female sandtiger sharks have two) kills and eats its wimpier siblings. With its pesky siblings out of the way, the winning embryo has plenty of room to grow within its very own uterus. And so, after a 9 to 12 month gestation period, the female gives birth to two three-foot long pups. At birth, the pups are a little too big and a little too vicious—they are experienced killers, remember—to tempt predators. That’s a big advantage for the survival of the pups, but a major disadvantage for the survival of the species. Most sharks give birth to at least 12 pups at a time so the sandtiger’s brood of two is rather puny in comparison. And in an ocean of uncertain survival, a puny brood is a one-way ticket to endangered species status.

There’s another endangered species affected by this whole sibling rivalry thing. Tasmanian devils don’t kill their siblings directly. For them, it’s more of a race for survival. Ya see, female devils give birth to 30 or more young at a time. The young must make their way from the mother’s vagina to her pouch where they will attach to one of her nipples for 100 days. Here’s the catch: she only has four nipples. Those that claim a nipple survive and those that don’t die. Needless to say, competition for a spot is fierce.

Now, now, before you get too downtrodden about the behavior of the young’uns in the Animal Kingdom, remember that life’s not all bad: cue the quintessentially cute harp and hooded seal pups. These guys don’t need to race (or kill…or eat) any siblings to survive. They just need to be fat and fuzzy—and to get fat, they need to eat. So, hooded seals nurse for four days—harp seals nurse for about two weeks—and gain up to 7kg (15lbs) per day thanks to their mother’s super-fatty milk.

And so my friends, cuteness still prevails in some parts of the animal kingdom. (Sigh) All is right in the world.