Animals That Mate for Life: Swans, Albatrosses and the Truth

What does “mate for life” really mean?

Biologists separate two kinds of monogamy:

• Social monogamy means a male and a female live together, share a territory and raise young as a pair. This is what most people mean by “mating for life.”
• Genetic monogamy means all of the pair’s young really are fathered by the male partner. DNA tests have shown that this is much rarer than social monogamy.

Many animals are also serially monogamous: they keep one partner for a breeding season, then find a new one the next year. Most songbirds in gardens across Europe and North America live this way.

How common is monogamy in animals?

Monogamy is common in birds and rare in mammals. Around 90% of bird species are socially monogamous, because chicks often need two parents to feed and protect them. In mammals, only about 3–9% of species form pairs. Female mammals produce milk, so males can often leave the work of raising young to the mother.

Even in socially monogamous birds, extra-pair matings are common. Studies of many songbird species have found that, on average, roughly one chick in ten is fathered by a male outside the pair, although the share varies enormously from species to species.

1. Swans

Swans are the classic symbol of lifelong love. Mute swans usually pair at two to four years old and often stay together for many years, defending a territory and frequently reusing the same nest. The male, called a cob, and the female, called a pen, share the work of guarding the nest and the cygnets, and if one parent dies while the young are small, the other carries on raising them alone.

Swans are not perfect partners, however. Black swans of Australia pair for life but have a divorce rate of about 6%, and genetic studies have found that around a third of their broods include cygnets fathered by a male outside the pair. Learn more in do swans mate for life?

2. Albatrosses

Albatrosses are among the most faithful birds known. Young birds spend years courting before they settle down. Laysan albatrosses perform elaborate dances with up to 25 ritual moves, including bill-clacking, sky-pointing and bowing, and pairs form bonds that usually last the rest of their lives. Since an albatross can live for more than 50 years, these partnerships can be very long.

The wandering albatross, which has the largest wingspan of any living bird, mates for life and breeds only every other year, because raising one chick takes almost a year. Pairs spend months apart at sea and reunite at the same nest site.

Albatross pairs do sometimes split up. In black-browed albatrosses on the Falkland Islands, a long-term study found that divorce, usually rare, became more frequent in a year when sea temperatures were unusually warm and food was harder to find. On the Hawaiian island of Oahu, where females outnumber males, about 31% of Laysan albatross pairs are made up of two females, which raise chicks together.

3. Bald eagles

Bald eagles usually keep the same mate for life and return to the same nest year after year, adding fresh sticks each season. Their nests can become enormous; the largest recorded tree nests of any animal were built by bald eagles, up to about 4 m (13 ft) deep, 2.5 m (8.2 ft) wide and weighing as much as a ton. If one eagle dies, the survivor usually finds a new partner. See do bald eagles mate for life?

4. Gray wolves

A wolf pack is a family: usually a breeding pair and their offspring from the past one to three years. The breeding pair often stays together until one of them dies, and the two lead the pack’s travels and hunts. In most packs only this pair breeds. The gray wolf is one of the few large mammals in Europe and North America that lives in such lasting pairs.

5. Beavers

Beavers live in family colonies made up of an adult pair, their newborn kits and the young from the year before. The pair builds and maintains the lodge and dams together and may share a territory for many years. Their family life is a clear case of social monogamy, though genetic studies have found that some kits are fathered by males from outside the pair. Meet the North American beaver in its profile.

6. Gibbons

Gibbons are small apes of the forests of Southeast Asia, southern China and northeastern India, and unlike their larger relatives they usually live in pairs. Mated pairs sing loud duets each morning, often joined by their young, to defend their territory. Gibbons often keep the same mate for life, but field studies have recorded both extra-pair matings and occasional “divorces,” so their reputation for perfect fidelity is overstated.

7. Prairie voles

The prairie vole, a small rodent of North American grasslands, is famous in neuroscience. Males and females form lifelong pair bonds: they nest together, huddle, groom each other and share the care of their pups. Studies of prairie voles helped show how the hormones oxytocin and vasopressin act in the brain to create these bonds; their close relatives, meadow voles, do not pair up.

Yet prairie voles are not sexually faithful. Both males and females will mate with other voles when they get the chance, which makes them a textbook example of social monogamy without genetic monogamy.

8. Shingleback lizards

The shingleback, or bobtail, is a stout, slow-moving skink from southern and western Australia, and one of very few lizards known for lasting pair bonds. Each spring, from about September to November, the same male and female find each other again before the breeding season, and a long-term field study recorded pairs reuniting every year for up to 20 years. The pair then stays together for weeks before mating, with the male following the female closely and keeping watch for danger. Once mating is over they separate until the next spring.

9. Other faithful partners

A few other animals stand out for their fidelity:

• Azara’s owl monkeys of South America live in pairs in which the father does much of the infant carrying, and DNA studies have found them to be genetically monogamous.
• California mice form lasting pairs in which the male helps raise the pups, and they are a model species for research on partner fidelity.
• Termites have a king who stays with the queen for life, unlike ants and honeybees, whose males die soon after mating.
• Seahorses of some species keep the same partner through a breeding season, greeting each other with a daily dance.

Divorce in the animal world

Animal “divorce” means that two living partners split and pair with new mates. It is more common than many people think. A pair that fails to raise young is more likely to split, which suggests that animals switch partners when it improves their chances of breeding.

Penguins show the full range. Some species, such as gentoo penguins, often keep the same partner for several seasons, but in emperor penguins about 85% of adults find a new partner each year. Find out more in do penguins mate for life?

Myths and facts about lifelong love

Myth: animals that mate for life die of a broken heart when their partner dies. Grieving behavior has been described in some birds and mammals, but most widowed swans, eagles and wolves eventually take a new partner.

Myth: monogamous animals never cheat. DNA evidence shows that true genetic monogamy is rare, even among famously loyal species.

Fact: some pairs really do last for decades. Albatrosses, swans and shinglebacks can stay with the same partner for 20 years or so, longer than many animals live.