The liger, offspring of a male lion and a tigress (a male tiger and a lioness produce a tigron), is just one of many examples of cross-species hybrids.
Sheep can breed with goats, horses with donkeys, and chickens with peafowl. Indeed, many of our garden and crop plants were created by crossing different wild species.
Do animals cross breed in the wild?
Captive or domesticated animals may have little choice about who they breed with, but interbreeding occurs naturally, too. Ligers and tigrons were reportedly born in the wild when the parent species’ ranges overlapped in Asia. Polar bears and grizzlies are apparently colour-blind when it comes to love.
The European edible frog is a naturally occurring hybrid between the pool frog and marsh frog. And in Britain the various species of Dactylorhiza orchids interbreed so promiscuously that the multitude of hybrids makes identification a taxing task, even for experts.
All of which would seem, at first glance, to challenge the very notion of species, generally understood to be groups of organisms that breed only among themselves (though it is not always this simple).
But crossbreeding doesn’t necessarily conflict with this definition. For a start, most hybrid offspring are sterile: structural differences between the chromosomes inherited from each parent make it impossible for them to produce viable eggs or sperm.
Also, those hybrids that are fertile tend to be biologically weak, because of a dilution of the specialised adaptations of both parents. Either way, few hybrids leave descendants, so the integrity of the parental species is maintained.
How does nature try and prevent interbreeding?
But while interbreeding is not necessarily disastrous for a species, it is still a waste of precious effort and resources for individual animals or plants. Little surprise, then, that many species have evolved mechanisms to avoid it.
Closely related species may have distinctive songs and courtship displays that reduce the odds of mistaken identity. Among insects, species that look identical to the untrained eye often have distinctively shaped genitalia that only permit coupling with members of the same species.
For example, males of the closely related ground beetles Carabus maiyasanus and C. iwawakianus have penile projections that fit snugly into specialised pouches in the vaginal walls of females of the same species. Should a male mate with a female of the wrong species, the projections break off, tearing the female’s reproductive organs and sometimes even killing her.
But nature’s best efforts to avoid crossbreeding are not fail-safe. And there is little doubt that human activity is presenting new opportunities for interbreeding as we move species around the globe, bringing together closely related species that don’t have isolating mechanisms.
Ruddy ducks imported here from the USA have dispersed to mainland Europe, breeding with Endangered white-headed ducks in Spain; a controversial and expensive cull of ruddy ducks in Britain was launched to reduce the threat.
Sometimes, however, crossbreeding can be a creative evolutionary force. While most hybrids are biological dead ends, occasionally they will be endowed with a novel genetic combination that hits the reproductive jackpot.
A rare example among animals is a hybrid of two species of fruit-fly in the USA whose larvae eat blueberries and snowberries respectively. Following the 19th- century introduction of a species of honeysuckle, the hybrid Lonicera fly was able to flourish because its particular genetic mix allowed it to feed on the berries of this new untapped resource.
A new species of monkey flower discovered recently in Scotland evolved only in the past 20 years or so as a result of interbreeding between two species introduced from the Americas. In fact, hybridisation is a major driving force of plant evolution – perhaps half of all the world’s plant species were created in this way.
Main image: Liger © Getty Images