Like most humans, elephants are born one at a time, and older sister elephants babysit their younger siblings while the mother is away.
Steve Jenkins wrote about them and other animal siblings with surprising attributes in a children's book, called Sisters and Brothers: Sibling Relationships in the Animal World. One species of armadillo, for instance, almost always enters the world as an identical quadruplet, Jenkins explains.
And then there are the naked mole rats, which can have hundreds of siblings, all from one mother called the queen. In their network of narrow underground tunnels, the younger or lower status sibling has to make way for the older one.
Some animal siblings are unusually helpful to each other. Take European shrews, which are so tiny that a litter of them can fit into a teaspoon. To keep from getting lost, they move in a caravan, with the mother leading and the children following along, grabbing onto the sibling in front of them with their teeth.
Asian small-clawed otters also tend to stick together. The Smithsonian National Zoological Park in Washington, D.C., is home to a group of six brothers from two different litters. In the wild, these otters stay together in a family group of up to 15 for as long as the parents are there to keep the group together.
Smithsonian biologist Erika Bauer said that the otter brothers spend almost all their time together, and get along pretty well, except for the occasional scuffle around feeding time. They even sleep in a big pile, and when they want to move the location of their sleeping nest, they work together: "They pull the grasses down, and carry it around. They look ... very industrious," Bauer says.
There is, of course, a darker side of animal sibling relationships. Cattle egrets, a species of heron, are known to practice siblicide when the parents are away from the nest hunting for insects and fish.
"One of the chicks as it gets stronger will actually kill its sibling and throw it out of the nest," explains bird curator Dan Boritt. The motive seems to be to get all the food caught by the parents. "Even though it sounds quite cruel, if you had to split your limited resources amongst two, three, four chicks," he says, "chances are none of those would survive."
Lions boast a special form of sister and brotherly alliance: sisters stay together all their lives in groups called prides, and brothers form coalitions when they set off to find other prides to conquer.
At the National Zoo there are some new additions to the pride: seven lion cubs, now a few weeks old. They haven't made a public appearance yet, since they're still getting their shots, but big cat expert Craig Saffoe has spent time with them.
"They all play together, they wrestle and everybody's biting each other's tails," he said. "I'm not a big fan of the "C" word, but they are really, really cute."
Now as any biologist will adamantly tell you, we humans must resist the urge to anthropomorphize these animals with our own human stories of love and bickering. So think of these tales as extreme metaphors, for the next time you build a metaphorical a nest with your sibling, or feel like pushing them out of one.