In the summer of 2020, scientists in Pennsylvania caught sight of something no one had ever seen before: a bird that looked like a rose-breasted grosbeak but sang like a scarlet tanager.
Upon closer analysis, the animal was later determined to be a hybrid, the offspring of a mating event between two separate species.
“When I saw it, I said, ‘Oh my god!’” remembers Bob Mulvihill, ornithologist for the National Aviary in Pittsburgh. Mulvihill caught the bird and extracted a blood sample to study the hybrid animal’s genes.
While some animal hybrids are well known, like the mule, this case was more unusual due to the different colors of each bird species. Rose-breasted grosbeaks are black and white with a red patch on their chest, while scarlet tanagers are brilliant orange and black. (Read more about never-before-seen colorful bird hybrid.)
The birds aren’t exactly cousins, or even close relatives. Mulvihill suspects the species may be separated by more than 10 million years of divergent evolution.
Even stranger is the fact that these species coexist across much of their North American ranges, leading researchers to wonder why no one had ever seen evidence of cross-breeding before.
“Was this just a case of Romeo and Juliet?” he laughs.
In the modern era of gene sequencing and genetic analysis, hybrid animals have new relevance and may hold clues that clarify the mysteries of evolution.
What are hybrid animals?
Genetically, a hybrid animal is the result of interbreeding between divergent lineages, says Erica Larson, an evolutionary biologist at the University of Denver.
Most non-scientists would take that to mean breeding between two different species, but it can also include subspecies or even populations within a species that are distinguishable from one another based on certain traits or characteristics.
“They might breed at different times of the year, or they might have behavioral differences that make them less likely to mate,” Larson says. “But then if they do mate, they might make hybrids that are totally fine.”
One instance of this may be the spotted skunk, which scientists recently divided into seven species, some of which look almost identical and live in the same areas, but mate and give birth months apart.
“Another great example is coral,” says Larson. “A lot of corals release their gametes at a very particular time. So all of these species are physically in the same place, and maybe they could form a hybrid,” but they miss the chance by spawning hours or days apart.
In captivity or a lab, however, those sorts of natural barriers are less of an obstacle to successful hybridization.
What animals can hybridize?
One of the most well-known examples of hybridization is the so-called liger, a cross between a male lion and a female tiger. Ligers were elevated to pop culture fame after a reference in the 2004 film Napoleon Dynamite, but fewer people have heard of the tigon, the offspring of a female lion and a male tiger.
Both pairings are extremely unlikely to happen in the wild because lion and tiger ranges almost never overlap. The same is true for the cama, a llama and dromedary camel cross, that live on opposite sides of the Atlantic Ocean, but have been bred by researchers.
Equids can be especially prone to hybridize. Donkeys and horses can breed to create mules, while zebras and horses make zorses or other combinations.
In 2019, scientists proved for the first time that narwhals sometimes hybridize with belugas, resulting in a narluga. There are also at least 20 reports of various species of dolphins and whales producing hybrids in both the wild and captivity.
And it’s not just mammals. There are also documented cases of hybridization between timber rattlesnakes and western diamondback rattlesnakes, Cuban and American crocodiles, Russian sturgeon and American paddlefish, cutthroat trout and rainbow trout, as well as in various insects, such as ants, bees, wasps, and termites. Plants are especially capable at hybridizing and are thought to do so at even higher rates than animals.
One of the most populous hybrid species on Earth may be modern humans, which carry genetic signs of hybridization with other, ancient hominins, such as the Neanderthals and Denisovans. (Read how you may have more Neanderthal DNA than you think.)
Are animal hybrids rare?
While hybrids seem unusual, it’s likely that many are quite common.
For instance, Mulvihill says it’s probably no coincidence that hybridization is well documented in birds, where a whopping 10 percent of the more than 10,000 known species are known to hybridize. The reason why may be there’s a voluminous number of birders snapping photos of interesting sightings and then posting them to forums, birding association pages, or smartphone apps such as iNaturalist.
“Certainly, hybridization is pretty well known in butterflies, too,” says Mulvihill. But unlike birders, few casual observers of butterflies can easily detect signs of hybridization in the insects.
Some scientists also believe that certain hybrids may become more common in the future. For instance, as climate change shrinks Arctic sea ice, polar bears are expected to spend more time on land, where they may encounter brown bears expanding northward. If they mate, they can create hybrids known as pizzlies or grolar bears. (Read why scientists expect more hybrids as the Arctic warms.)
Are animal hybrids good or bad?
Hybridizing doesn’t always spell genetic doom. But it may not produce stronger species, either.
For example, ligers are prone to health problems, such as rapid growth and heart problems. (Learn more about ligers and other big cat hybrids.)
Also, the parent species may sport incompatible genetic differences, such as different numbers of chromosomes. This is one reason hybrids are often sterile, and a non-reproducing offspring can limit a parent’s success extending its gene pool.
“They have one less chance to pass on their genes to a future generation,” says Larson.
Hybridization can be troublesome if one or both parent species is in danger of extinction. This is because when a species’ genetics becomes rare, the hybrid’s new combination of genetics can threaten its existence by replacing it. This is called genetic swamping, and it’s why hybridization with coyotes is one of the many threats currently afflicting red wolves in the southeastern United States.
However, hybridization can also introduce beneficial genes, such as pesticide resistance, says Larson. If those genes help the hybrid survive and reproduce, such benefits can become widespread in a population. This is what scientists call adaptive introgression.
“But I think most of the time, it’s probably not beneficial or detrimental,” says Larson. “Most of the time, it probably doesn’t do anything.”
With advances in genetic tools, scientists can now look at a hybrid’s genome and easily identify interloping genes that may have originated somewhere else. And this means every hybrid is a window into how evolution creates new species.
“When you have two species whose genomes have undergone independent evolution for hundreds of thousands of years, and then you bring them back together, and you mix up those genomes in the form of a hybrid,” says Larson, “you get to understand what works and what doesn't.”