When it comes to the birds and the bees, sometimes Mother Nature slips some surprises.
Species don't always stick to their own kind when getting down to business. The results are often compromised offspring who can't reproduce, but occasionally can. And when they do, some biologists worry they can pose a threat to existing species.
Take Kekaimalu, the hybrid offspring of a false killer whale and a bottlenose dolphin who lives at Sea Life Park in Hawaii. Last week, park officials announced that the "wholphin" gave birth to a healthy female calf. It was the third birth for the 19-year-old wholphin and a combination of Kekaimalu's mixed genes and those of a male Atlantic bottlenose dolphin.
Are combinations like wholphins natural? Some biologists argue that anything that's possible in nature is natural and that interspecies mating has long played a role in evolution. Others, however, say such couplings are often the unhealthy result of human interference and, if unleashed in the wild, can threaten the rigor of existing species.
"Species are adapted to specific conditions," said Susan Haig, a wildlife ecologist with the U.S. Geological Survey and a professor at Oregon State University. "So I think it's important to maintain the integrity of species."
Whether or not they pose a threat, hybrid mammals continue making appearances both in controlled settings, such as zoos and farms, and in the wild.
"Some people think that what we've got is what we've got and anything new in the mix is bad," said David Hull, a professor emeritus of philosophy and biology at Northwestern University in Evanston, Ill. "But evolution is still in motion and always will be."
Breeding between species is rampant among plants and less complex animals that lack a backbone. It's only among the higher mammals, the ones as Hull says, "most people pay attention to," where it remains fairly rare.
Hybrid mammals may be rare, but they are memorable. There is the liger, a hybrid of a female tiger and male lion (alternatively known as a tigon, if the mother is the lion). The hybrids have manes like a lion, the sleek bodies and stripes of a tiger, and the 1,000-pound heft of a lion.
Kekaimalu, the Hawaii "wholphin," has a mixed charcoal hue of the dolphin's light gray and the false killer whale's black coloring. She's also larger than the standard dolphin since her false killer whale father weighs some 1,500 pounds more than her dolphin mother. (Incidentally, Kekaimalu's parents are famous for another reason besides their surprise coupling -- they were featured in the Adam Sandler movie "50 First Dates.")
The mixed species have generally been the product of closed environments, such as zoos and aquariums, and the young are usually infertile. Outside such settings, people have long tinkered with interspecies mating to create animals ideally suited for specific tasks.
To create work animals with greater strength and agility, people have bred mules (sterile combinations of horses and donkeys) and zorses (a horse-zebra hybrid). And in an effort to create a terror-fighting tool, Russians have experimented with breeding combinations of jackals and husky dogs to form a hybrid with elite bomb-sniffing abilities.