The mixed-up world of hybrid animals

Deep in the Amazon rainforest live two green birds. The snow-capped manakin, has a splash of white on its head. The opal-crowned manakin looks very similar. But this species’ crown can appear white, blue or red depending on the light. It’s “like a rainbow,” says Alfredo Barrera-Guzmán. He is a biologist at the Autonomous University of Yucatán in Mérida, Mexico.

Thousands of years ago, these two species of birds started mating with each other. The offspring initially had crowns that were dull whitish-grey, Barrera-Guzmán suspects. But in later generations, some birds grew yellow feathers. This bright color made males more attractive to females. Those females may have preferred mating with yellow-capped males rather than snow-capped or opal-crowned males.

Eventually, those birds became separate enough from the two original species to be their own, distinct species: the golden-crowned manakin. It’s the first-known case of a hybrid bird species in the Amazon, he says.

Usually, different species don’t mate. But when they do, their offspring will be what are called hybrids.

The molecules of DNA in each of an animal’s cells hold instructions. These guide what an animal looks like, how it behaves and the sounds it makes. When animals mate, their young get a mixture of the parents’ DNA. And they can end up with a mixture of the parents’ traits.

If the parents are from the same species, their DNA is very similar. But DNA from different species or species groups will have more variations. Hybrid offspring get more variety in the DNA they inherit.

So what happens when the DNA of two animal groups mix in a hybrid? There are many possible outcomes. Sometimes the hybrid is weaker than the parents, or doesn’t even survive. Sometimes it’s stronger. Sometimes it behaves more like one parent species than the other. And sometimes its behavior falls somewhere in between that of each parent.

Scientists are trying to understand how this process — called hybridization (HY-brih-dih-ZAY-shun) — plays out. Hybrid birds may take new migration routes, they found. Some hybrid fish appear more vulnerable to predators. And rodents’ mating habits may affect what their hybrid offspring can eat.

Wise to hybridize?

Hybridization happens for many reasons. For instance, the territory of two similar types of animals may overlap. This happens with polar and grizzly bears. Members of the two groups of animals have mated, producing hybrid bears.

When the climate changes, a species’ habitat can shift to a new area. These animals may encounter other, similar species. The two groups may mate by accident. For instance, researchers have found hybrids of southern flying squirrels and northern flying squirrels. As the climate warmed, the southern species moved north and mated with the other species.

When animals can’t find enough mates from their own species, they may select a mate from another species. “You have to make the best out of the situation,” says Kira Delmore. She is a biologist at the Max Planck Institute for Evolutionary Biology in Plön, Germany.

Scientists have seen this happen with two antelope species in southern Africa. Poachers had thinned out the populations of giant sable antelope and roan antelope. Later, the two species bred with each other.

People can unwittingly create opportunities for hybridization, too. They might put two closely related species in the same enclosure at a zoo. Or as cities expand, urban species may increasingly encounter rural ones. People may even set loose animals from other countries, accidentally or on purpose, into a new habitat. These exotic species now may encounter and mate with the native animals.

Many hybrid animals are sterile. That means they may be able to mate, but they won’t create offspring. For example, mules are the hybrid offspring of horses and donkeys. Most of these are sterile: Two mules can’t make more mules. Only a horse mating with a donkey can make another mule.

Biodiversity is a measure of the number of species. In the past, many scientists assumed that hybridization wasn’t good for biodiversity. If many hybrids were produced, the two parent species could merge into one. That would reduce the variety of species. That’s why “hybridization was often viewed as a bad thing,” Delmore explains.

But hybridization sometimes can boost biodiversity. A hybrid might be able to eat a certain food that its parent species cannot. Or maybe it can thrive in a different habitat. Eventually, it could become its own species, like the golden-crowned manakin. And that would increase — not decrease — the variety of life on Earth. Hybridization, Delmore concludes, is “actually a creative force.”

Going their own way

Hybrids can be different from their parents in many ways. Appearance is just one. Delmore wanted to know how hybrids might behave differently than their parents. She looked to a songbird called the Swainson’s thrush.

Over time, this species has split into subspecies. These are groups of animals from the same species that live in different areas. However, when they do encounter each other, they can still breed and produce fertile young.

One subspecies is the russet-backed thrush, which lives on the west coast of the United States and Canada. As its name implies, it has reddish feathers. The olive-backed thrush has greenish-brown feathers and lives farther inland. But these subspecies overlap along the Coast Mountains in western North America. There, they can mate and produce hybrids.

One difference between the two subspecies is their migration behavior. Both groups of birds breed in North America, then fly south in winter. But russet-backed thrushes migrate down the west coast to land in Mexico and Central America. Olive-backed thrushes fly over the central and eastern United States to settle in South America. Their routes are “super different,” Delmore says.

The birds’ DNA contains instructions for where to fly. Which directions do hybrids get? To investigate, Delmore trapped hybrid birds in western Canada. She placed tiny backpacks on them. A light sensor in each backpack helped record where the birds went. The birds flew south to their wintering grounds, carrying the backpacks on their journey.

The next summer, Delmore re-captured some of those birds back in Canada. From the sensors’ light data, she figured out what time the sun had risen and set at each point along the bird’s journey. The length of the day and timing of midday differs depending on location. That helped Delmore deduce the birds’ migration paths.

Some hybrids roughly followed one of their parents’ routes. But others didn’t take either path. They flew somewhere down the middle. These treks, though, took the birds over rougher terrain, such as deserts and mountains. That could be a problem because those environments might offer less food to survive the long journey.

Another group of hybrids took the olive-backed thrush’s route south. Then they returned via the russet-backed thrush’s path. But that strategy might also cause problems. Normally, birds learn cues on their way south to help them navigate back home. They might notice landmarks such as mountains. But if they return by a different path, those landmarks will be absent. One result: The birds migration might take longer to complete.

These new data might explain why the subspecies have remained separate, Delmore says. Following a different path may mean that hybrid birds tend to be weaker when they reach the mating grounds — or have a lower chance of surviving their yearly journeys. If hybrids survived as well as their parents, DNA from the two subspecies would mix more often. Eventually these subspecies would fuse into one group. “Differences in migration could be helping these guys maintain differences,” Delmore concludes.

Perils of predators

Sometimes, hybrids are shaped differently than their parents. And that can affect how well they avoid predators.

Anders Nilsson recently stumbled onto this finding. He is a biologist at Lund University in Sweden. In 2005, his team was studying two fish species named common bream and roach (not to be confused with the insect). Both fish live in a lake in Denmark and migrate into streams during winter.

To study their behavior, Nilsson and his colleagues implanted tiny electronic tags in the fish. These tags allowed the scientists to track the fish’s movements. The team used a device that broadcast a radio signal. Tags that received the signal sent back one of their own that the team could detect.

At first, Nilsson’s team was interested only in roach and bream. But the researchers noticed other fish that looked like something in between. The main difference was their body shape. Viewed from the side, the bream appears diamond-shaped with a taller middle than its ends. The roach is more streamlined. It’s closer to a slim oval. The third fish’s shape was somewhere between those two.

“To the untrained eye, they just look like fish,” Nilsson admits. “But to a fish person, they are hugely different.”

Roach and bream must have mated to produce those in-between fish, the scientists thought. That would make those fish hybrids. And so the team began tagging those fish, too.

Fish-eating birds called great cormorants live in the same area as the fish. Other scientists were studying the cormorants’ predation of trout and salmon. Nilsson’s team wondered if the birds were eating roach, bream and hybrids as well.

Cormorants gobble fish whole. Afterward, they spit out unwanted parts — including electronic tags. A few years after the researchers had tagged the fish, they visited the cormorants’ nesting and roosting sites. The birds’ homes were pretty gross. “They throw up and defecate all over the place,” Nilsson says. “It’s not pretty.”

But the researchers’ search was worth it. They found a lot of fish tags in the birds’ mess. And the hybrids appeared to fare the worst. For their efforts, the team found 9 percent of the bream tags and 14 percent of the roach tags. But 41 percent of the hybrids’ tags also turned up in the nests.

Nilsson isn’t sure why hybrids are more likely to be eaten. But perhaps their shape makes them easier targets. Its diamond-like shape makes bream hard to swallow. The roach’s streamlined body helps it quickly swim away from danger. Since the hybrid is in between, it may not have either advantage.

Or maybe hybrids just aren’t very smart. “They could be sort of stupid and not react to the predator threat,” Nilsson says.

Picky mating

Just because scientists find hybrids doesn’t mean the two species will always breed with each other. Some animals are choosy about which mates they’ll accept from another species.

Marjorie Matocq studied this question in rodents called woodrats. Matocq is a biologist at the University of Nevada, Reno. She started studying California’s woodrats in the 1990s. Matocq found these creatures interesting because they were very common, but scientists knew so little about them.

In a recent study, her team focused on two species: the desert woodrat and Bryant’s woodrat. Both live in the western United States. But desert woodrats are smaller and inhabit dry areas. The bigger Bryant’s woodrats live in shrubby and forested areas.

At a site in California, the two species overlapped. The animals here were mating and producing hybrids, but Matocq didn’t know how common this was. “Is it just a chance accident, or is this happening all the time?” she wondered.

To find out, the researchers brought woodrats to their lab. They set up tubes shaped like a T. In each experiment, the scientists placed a female desert woodrat or Bryant’s woodrat at the bottom of the T. Then they put a male desert woodrat and a male Bryant’s woodrat in opposite ends of the top of the T. The males were restrained with harnesses. The female could then visit either male and decide whether to mate.

Female desert woodrats almost always mated with their own species, the scientists found. These females may have avoided Bryant’s woodrats because those males were bigger and more aggressive. Indeed, the males often bit and scratched the females.

But the female Bryant’s woodrats didn’t mind mating with male desert woodrats. Those males were smaller and more docile. “There wasn’t as much danger,” Matocq observes.

The researchers suspect that many wild hybrids have a desert woodrat father and a Bryant’s woodrat mother. That could be important because mammals, such as woodrats, inherit bacteria from their mothers. These bacteria stay in the animal’s gut and are called their microbiome (My-kroh-BY-ohm).

An animal’s microbiome may affect its ability to digest food. Desert and Bryant’s woodrats likely eat different plants. Some of the plants are toxic. Each species may have evolved ways to safely digest what they chose to eat. And their microbiomes may have evolved to play a role in that as well.

If true, hybrids may have inherited bacteria that help them digest the plants that Bryant’s woodrats typically consume. That means these animals might be better-suited to dine on what a Bryant’s woodrat eats. Matocq’s team is now feeding different plants to the parent species and their hybrids. The researchers will monitor whether the animals get sick. Some hybrids might fare better or worse depending on their mix of DNA and gut bacteria.

What’s exciting about hybrids is that you can think of each one “as a little bit of an experiment,” Matocq says. “Some of them work, and some of them don’t.”