The mating game: ligers, zorses, wholphins, and other hybrid animals raise a beastly science question: what is a species?

Science World, Jan 24, 2003 by Sharon Guynup

What has a mane like a lion, the sleek muscular body of a tiger, stripes and spots, and weighs up to 1,000 pounds? Answer: A liger. The punch line sounds like a joke, but ligers--produced by a female tiger mating with a male lion--are actual animals and one of the world's more bizarre-looking hybrids, or mixed animal species.

If these ferocious cats met in the jungle, a tiger would probably not choose to visit a pride of lions; a raucous brawl--not romance--would be the more likely result. But with little choice in captivity--like an open zoo--the odd coupling may occur. In the wild, animals rarely interbreed for one potent reason: The offspring are usually infertile, or unable to reproduce--which can spell extinction for a species. "Infertile offspring don't pass on their genes [hereditary instructions in all cells] to the next generation," says University of Maine biologist Judith Rhymerat.

But even more threatening to species preservation are hybrids that can reproduce. For example, over the past decade Midwestern barred owls have pushed westward to the Pacific coast where they've settled in the forest habitat of endangered spotted owls--and bred with them to create sparred owls. "It's a nasty situation," says Susan Haig, a wildlife ecologist at Oregon State University.

Hybrids can result in loss of genetic diversity, she explains, and there's no protection for them under the 1973 Endangered Species Act. By traditional species' definition--in which organisms with common traits breed to create fertile offspring--they shouldn't be mating: Sparred owls could trigger the Northern spotted owl's extinction.

BREEDING BARRIERS

While ligers are rare, some animals in captivity are deliberately interbred for greater strength or endurance, like mules (horse + donkey) and zorses (horse + zebra). They're also interbred for food, like the beefalo (cow + buffalo) and different types of catfish and trout. Russians crossbreed dogs with jackals to create a hybrid whose superior sense of smell, for example, is put to the test sniffing out bombs in Moscow's airports.

But why don't distinct wild animal species--like lizards and frogs, or cougars and elephants--mate of their own accord? The answer: Nature imposes breeding barriers, safeguards to protect individual species and help them adapt to their environment. Animals evolve, or develop unique traits over time, to ensure their survival. So specific genes that help a species adapt to a particular climate, eat what's on the local menu, and fight off neighborhood predators, are passed on to the next generation. Mixing genes through interbreeding can eliminate survival traits--or result in infertile offspring.

To produce fertile offspring, scientists think chromosomes (cell structures that house all the genes) from both a mother and father may need to pair off evenly during meiosis, a process of cell division that produces sex cells. For the hardy mule, for example, this is impossible, since its father--a donkey--has 62 chromosomes and its mother--a horse--has 64. When the two animals mate, each contributes half its chromosomes to the mule. In turn, the mule is almost always sterile because it inherits a total of 63 chromosomes, a number that can't divide into pairs (see diagram, below).

Sometimes the main breeding obstacle is a simple difference in habitat or breeding area--one species may fare better in thick jungles, another in wide-open spaces. And even if separate species do mate--and a female's egg successfully fertilizes, or fuses, with a male's sperm--the parental genes must partner perfectly to develop a healthy embryo (living organism in its earliest stages of development). "Genes need to turn on and off at the right time, in the right places--millions of times--in order to form limbs and other body parts," notes Eric Hallerman, a geneticist at Virginia Polytechnic Institute and State University. "If they don't, the embryo dies or becomes grossly malformed--and then dies." The off-and-on gene sequence isn't the same in all species, because different species possess different genes--which means they don't coordinate properly.

Besides infertility, blindness, faulty hearts, and brief life spans are routine disorders for many hybrids. Case in point: When a 400-pound Atlantic bottlenose dolphin and a 4,000-pound false killer whale mated off the coast of Hawaii, their wholphin offspring died at age 5, decades younger than the average 40- to 50-year life span of its parents.

RULE BREAKERS

Many of today's newly created creatures would confuse 18th-century Swedish naturalist Carolus Linnaeus, who developed the Linnaean taxonomic, or classification, system for the natural world. Within this system, taxonomists have identified and grouped about 2 million plant and animal species based on similarities and differences. But how exactly do you define a species? "That's one of the biggest questions in science," says Rhymer. "It's what everyone is arguing about."

Traditionally, a species is a group of organisms that share at least one unique characteristic, can interbreed to produce fertile offspring, and rarely reproduce with organisms of another species. But what to make of fertile hybrids like the sparred owl? "The old definition of a species doesn't really work today," Rhymer says. "We know of related species separated by millions of years that still have the ability to reproduce successfully."