Do animals really mate for life?

It’s no surprise how lovebirds got their name, and how that name became slang for an affectionate human couple. Mated pairs of these African parrots feed, cuddle, groom each other, and become distressed when separated. Their behavior is summed up even better by their Spanish name: inseparables.

Lovebirds are unusual not just because of their shameless PDA, but because they choose one mate for life. While more than 90 percent of birds pair up in the practice of “social monogamy,” most choose a new mate every year or every few years. Besides lovebirds and some other parrots, only a few birds actually mate for life, including swans, eagles (shout out to our favorite bald eagle team: Jackie and Shadow), and albatrosses. Mating for life is even less common in non-avian animals.

You might expect that it would be more practical to keep the same partner year after year than to have to find a new one every breeding season. But it’s not always the most effective plan, especially if the animal couple doesn’t stay together year-round.

From an evolutionary standpoint, sticking with one mate is not in all animals’ best interests. Although some animals have lifelong partners, most prefer different breeding strategies. And far from the perfect picture of two lovebird soulmates, even monogamous animal relationships include just as many complications as human ones.

Mating for life has many advantages

In birds, “social monogamy is linked to biparental care, the male and the female caring for the offspring together,” says Dr. Bart Kempenaers, Director of Ornithology at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany. For bird parents, “it’s advantageous to stay together because you have to coordinate this care. With more experience breeding together, this coordination might be easier.”

Teamwork for long-term partners isn’t limited to co-parenting. Kempenaers notes that some birds, such as cardinals, partner year-round, not just during the breeding season. When there’s no offspring to rear, the male and female cardinal defend their territory together.

Mammals and other animals can also derive various benefits from a lifelong partnership. In addition to sharing parental care, wolves, coyotes, and foxes hunt and defend territory with their partners, while pairs of beavers maintain their dams and lodges together. Breeding pairs of French angelfish (one of the only fish that mate for life) don’t care for their young at all, but stick together to patrol their feeding grounds.

What makes some birds more likely to stay together

In birds, Kempenaers identifies longevity as one major influence on whether a species mates for life. “To stay together, both partners have to live to the next breeding season,” he says. And since “larger birds, like albatrosses, live much longer than a small bird like a chickadee” on average, says Kempenaers, the birds which mate for life are mostly large-bodied and long-lived. Albatrosses, which mate for life, can live over 50 years; chickadees, which do not, live only two or three years in the wild.

Great blue herons do not mate for life. While they are monogamous during a single breeding season, they choose new partners every year. Image: NurPhoto / Contributor / Getty Images / Ronen Tivony

Small animals also have more predators to contend with than large ones, decreasing their chances of surviving from one breeding season to the next. Many of the birds and mammals that mate for life, such as eagles and wolves, have few natural predators.

There are also other factors which influence mating habits. For two partners to stay together long-term, they have to be able to consistently find each other every breeding season. Birds that mate for life are generally those that either stay together year-round, or return to the same places at the same time every year. They may even use the same nesting site every year, which biologists call “site fidelity.”

Bird couples can “divorce”

The breakup of a pair of birds is often called “divorce,” but Kempenaers cautions that this term “suggests that it’s a decision by one or both partners to stay together or not, and that’s not necessarily the case.”

Some birds that mate for life, such as cranes and swans, migrate together in family units. However, in most migratory birds, partners travel separately. This can lead to birds choosing a new mate out of necessity more than incompatibility with the previous partner.

“If one of the partners comes back [from migration] but the other partner isn’t back yet, then this individual faces a dilemma,” says Kempenaers.

“It doesn’t know if its partner is still alive. The breeding season is happening, it’s ready to breed. So this bird might decide to mate with another individual rather than wait for the partner who might never show up.” Even if the previous year’s partner shows up late, Kempenaers notes, the one who got there first will typically stay with its new partner.

For birds, seizing the opportunity to breed comes before human notions of fidelity. A study of mute swans found that their low rate of “divorce” was three times higher in pairs that did not breed successfully than in pairs that did (nine percent versus three percent).

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Animals also “cheat” on each other

Social monogamy in animals doesn’t necessarily mean sexual monogamy. Even for species that mate for life, there may be survival benefits to teaming up with one partner while continuing to mate and have offspring with others.

This behavior is common across bird species. In some cases, says Kempenaers, the “social father” cares for the offspring, “but he isn’t the father of any of them” genetically, or at least some of them. A study of black swan pairs found at least one baby fathered by a different male in up to 40 percent of nests. Similar behavior has been observed in socially monogamous mammals, such as wolves and gibbons.

“If you ask why it happens, or what the advantages are, it’s always important to distinguish between the male side and the female side,” says Kempenaers. Males who mate with many females might be trying to father as many offspring as possible. However, says Kempenaers, “it is more difficult to understand” why female birds often take the initiative by leaving their nesting territory for liaisons with neighboring males.

This might ensure that the female’s own genes get passed on, just in case her nesting partner is infertile. It might even bring in extra parental care from the other males, as has been observed in some birds, such as the blue tit. But the exact drivers for this behavior remain a mystery.

There’s still much we don’t know about animal mating habits

Kempenaers explains that scientists are still working to define what better cooperation actually looks like in partners that stay together long-term. Other existing questions include how different mating systems evolve and why they can vary so much, even among species that have a lot in common.

Kempenaers was part of a 2023 study on the Alaskan breeding grounds of a shorebird called the long-billed dowitcher. Dowitchers are relatively large, long-lived birds that practice social monogamy. “Both the male and the female are essential for reproduction,” says Kempenaers, sharing tasks like incubating eggs.

One might expect a bird with these characteristics to return to the same mate and nesting site, at least for several years in a row. This is what similar birds in the same environment as the dowitchers do. Yet dowitchers choose a new mate and a new place to nest every year. “We found this really puzzling,” says Kempenaers. “If all the others come back to the same place, why don’t the dowitchers do this?”

To find out, researchers put satellite trackers on dowitchers and followed their movements throughout the year. They found that “the females leave much earlier than the males from the breeding grounds, and they winter at different places,” says Kempenaers. Crucially, once they have mated, “they never meet up again.” But why dowitchers behave this way when other socially monogamous birds in the same environment do not “remains a bit of an enigma,” he adds.

In nature, fidelity has benefits for some, but not for others, though what those benefits are is not always perfectly clear. And while it’s tempting to see animal relationships in human terms, monogamy in the animal kingdom often looks very different from human romance.

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