Savage siblings.

If you trade insults with your brother or sister every waking moment, you might think you have the worst sibling relationship on Earth. Think again! Some members of the animal kingdom take sibling rivalry to the max. They kill their brothers and sisters to survive!

Murdering a sibling, or siblicide, may seem cruel. Fortunately it's not the most common behavior in the animal kingdom. In fact, many animals have amicable sibling relationships (see "Family Bonds," p. 10). But when there's a short supply of food, some birds, insects, fish, and mammals snare their share by killing their closest competitors -- their sibling

Scientists have known for years, for example, that great egret chicks peck their youngest sibling to death and even toss it over the side of the nest. And some bee-eater birds use sharp hooks on their bills to stab their siblings.

Now, recent studies show that some animal parents actually "promote" siblicidal behavior! Why would parents be so heartless? Because siblicide may help the parents keep their genes alive.

When parents have babies, the offspring inherit the parents' genetic material. But for genes to live on, the offspring must be fit enough to survive and reproduce themselves. And under certain circumstances -- say, when food is in short supply -- fewer offspring may be the parents' best bet.

Great egrets and cattle egrets, for example, often have enough food for only two chicks, although a mom egret typically bears three. By sometimes allowing their older children to kill the youngest, the parents guarantee that they raise two well-fed, strong chicks who have an excellent chance to mature and reproduce.

In fact, new research shows that the two oldest cattle egret chicks are destined for success even before birth, says behavioral ecologist Douglas Mock. Mock and colleague Hubert Schwabl recently discovered that the first egg to form inside the mother egret always gets the highest dose of the hormones, or chemical messengers, that trigger aggressive behavior. The second egg in line gets the same dose. But egg number three gets only about half the amount. With less tendency to be aggressive, the youngest chick is less able to defend itself against its more aggressive siblings.

So why do egrets even bear a third chick? The youngest chick serves as "insurance," Mock explains. If something happens to either of the older chicks -- say a gull swoops down and carries one away -- then the youngest chick can take its place. The parents still end up with two chicks to pass on their genes.

SIBLINGS FOR LUNCH

Some amphibians solve the food-shortage problem in a more direct way: They eat their siblings.

Most spadefoot tadpoles feed on algae. But sometimes, a tadpole eats another tadpole. This nutrient- and hormone-rich meal makes the tadpole grow and changes its appetite. The cannibal tadpole develops enormous jaws and gorges exclusively on other tadpoles.

The cannibal tadpoles are usually careful to eat only nonrelatives, says biologist David Pfennig. They nip at tadpoles that swim by and conduct chemical "taste tests' to determine if they are kin. If the large-jawed. tadpoles accidentally swallow a sibling, they immediately spit it out.

But if food is scarce, the cannibals cease to discriminate. Then they eat any tadpole that crosses their path, whether it's a sib or not.

Cannibalistic siblicide can even begin before birth. Sand tiger sharks, for example, eat their brothers and sisters inside their mother's womb!

A female sand tiger shark produces 400 to 500 embryos at a time. While still in the womb, these embryo sharks grow razor-sharp teeth, says marine biologist John Wourms. The embryonic sharks start to eat other embryos. Within a few months, three to four dominant sharks engage in a life-or-death struggle until only one survives. By the time it is born, the sole-surviving shark pup has become an experienced predator.

TWIN TROUBLE

Spotted hyena cubs don't have to kill their siblings for food. They get a continuous supply of milk from their mother. But same-sex twins always fight to eliminate a potential rival for mates or social rank in the clan.

This competition starts soon after the hyena twins are born. While hidden inside its burrow, the older cub clamps down on its younger twin's neck and violently shakes the cub. But the older twin is not always the victor, says biologist Laurence Frank. The younger one often fights back, initiating a fierce battle that can last for weeks. In the end, the weaker sibling may die from badly infected wounds and even malnourishment. The victor emerges from the burrow, ready to join the hyena clan.

Just remember, siblicide is usually a last resort in times of desperation. Mock says it's as if animals know conditions aren't going to get any better and "think," "One of us is going to have to die and it's not going to be me!"