When it comes to love and companionship in the animal kingdom, sometimes anything goes.
There is the baby hippopotamus who, when separated from his herd, found company in a century-old tortoise at a Kenyan animal sanctuary. The odd pair became inseparable. At another Kenyan sanctuary, a fierce lioness repeatedly adopted young animals she should have been more interested in eating -- oryxes, which are a type of antelope. And a children's book glorified the fine but strange romance between a bull moose and a Hereford cow on a Vermont farm in the 1980s.
If birds of a feather supposedly flock together, what's to explain such odd pairings? In fact, animal behaviorists say they're not as rare as you might think.
"It happens all the time," said David Barash of the University of Washington in Seattle. "I don't think there's anything that truly defies biological explanation."
Take the hippo and the tortoise. While the bond between the animals is certainly unusual, some researchers point out the year-old hippo sought out the tortoise's company only after losing his herd as waters ran dangerously high during the Dec. 26 tsunami in a river that drains into the Indian Ocean.
The hippo, whom park managers called Owen, became dehydrated and was brought to a sanctuary in Mombasa, where he soon found "Mzee," a 120-year-old tortoise who shared little in common with the hippo apart from a dull, grayish color.
At first Mzee hissed at the young newcomer, but soon the Aldabran tortoise was eating and sleeping with the hippo and acting as the calf's mother, even though Mzee is a male tortoise. Owen, meanwhile, treated the old tortoise like a parent, licking his face and following him everywhere.
What could inspire such an odd bond?
"In this case, it sounds like 'any port in a storm,' " said Katherine Houpt of Cornell University's College of Veterinary Medicine in Ithaca, N.Y. "Animals that live in groups are likely to become attached to anything, whether it's a person or another animal. When they're scared, it's more likely to happen."
The fact that both animals were limited to the confines of the sanctuary may also have played a role.
"When you look at sanctuaries and zoos, animals don't always have access to their usual crowd," said Joanne Oliva-Purdy, an applied animal behaviorist based in Leadville, Colo. "Then you start to realize how flexible their behavior is."
Animal behaviorists describe a critical period when strong bonds or intense learning happens as imprinting. During this time two mates or parents and young (from goslings to humans) form the bond that persists throughout most of their lives. It's what makes ducklings fall in line behind their mother, prompts a male sparrow to defend its mate and inspires a mother grizzly to fight to the death to protect her young.
While these bonds normally form at critical times such as a youth's birth or at the start of a mating season, sometimes they are created unexpectedly and in unexpected combinations.
Biologists are still scratching their heads over a lioness's behavior in Kenya's Samburu National Park in 2002. Normally a fierce predator, the full-grown lioness adopted a series of five baby oryxes, an African antelope that lions usually feast upon. After each "adoption," the lioness protected the young oryx, slept by it and guarded it from other lions.
The lioness grew increasingly thin since she was constantly guarding her young and not hunting. But her natural instincts did apparently kick in when one of the young oryxes she had adopted died naturally in the night -- and she ate it.
Some wildlife experts tried to explain the behavior by saying perhaps the lioness couldn't have her own cubs and thus sought out the mothering role. Others suggested she may have had a mental disorder.
Barash chalks it up to chance.
"Imprinting doesn't only occur when a baby attaches to a parent," he said. "The mother will imprint on the baby as well. Maybe the lioness was exposed to an oryx at a critical time."
Barash recounted another case where a sparrow started feeding a large goldfish. The goldfish would swim to the edge of its pond and hold its mouth open, whereupon the sparrow would deposit its catch of the day.
"It was a happy coincidence for the fish," he said.
Sometimes happy coincidence leads to odd romance. In 1986, thousands of people flocked to Shrewsbury, Vt., to observe the apparent liaison between Jessica, a Hereford cow, and a huge male moose dubbed Bullwinkle. For 76 days, the moose never left Jessica's side and the two were often observed nuzzling affectionately.
More recently, a yearling moose and a 20-year-old horse became close pals last summer at a farm outside Groton village in Vermont.
Some might be inclined to regard such pairings as romantic and convention-defying -- like Romeo and Juliet or the Duke and Duchess of Windsor. But biologists like Barash see the quirky matchups -- and love, in general -- through a more practical lens.
"There is a self-serving component to love and companionship," he said. "From a biological perspective it's a mechanism whereby natural selection has gotten critters to care for each other. Whatever works, works."