Monogamy is seen in less than 10 percent of mammal species, and
birds don't appear to be so true, according to the researchers.
In 2011, the world learned of a shocking celebrity breakup - the sudden unexplained separation of Bibi and Poldi, two 115-year-old Galapagos tortoises at the Happ Reptile Zoo in Austria.
After nearly a century of living together, the female Bebe got fed up: one day she bit Poldy on the shell until it bled, and continued to attack him until zoo staff moved her partner into a separate enclosure.
In the wild, Galapagos tortoises are not monogamous, so it should come as no surprise that Bibi and Poldi's bond lasted so long, but their pair never produced offspring. Attempts at reconciliation were unsuccessful.
Photo: theguardian.com
"We feel like they can't see each other anymore," lamented zoo director Helga Happ at the time.
Why do ruptures happen
Among humans, this question has spawned ballads, provided rich fodder for novels, and continues to intrigue scientists.
Of course, to break up, one must first come together. In social monogamy,
animals live together and form strong bonds known as mating bonds, although sexual fidelity is a separate issue. Among mammals, humans are one of the exceptions: social monogamy is observed in less than 10% of the other mammals.
According to Professor Simon Griffith, an evolutionary ecologist at Macquarie University, this low rate is due to the difference in parental investment between males and females.
In most mammalian species, parental care comes primarily from the female, who invests enormous resources in carrying and providing milk for her offspring.
"In many mammals, the father shows no parental care," says Griffith. "He can guard or hold a territory, but . he can't provide the offspring with everything they need. In birds, it's completely different. The father can take care of the offspring almost as well as the female in terms of delivering food. That's why birds have more frequent partnerships than mammals."
The mating bonds of birds
According to Prof. Raul Mulder, an evolutionary ecologist at the University of Melbourne, before the advent of methods to establish paternity, there was evidence that birds as a group were largely sexually monogamous.
"If you analyze whether a particular species of bird mates, how long they mate, and how long they stay together, and classify all known birds, you get over 90 percent," Mulder says.
Photo: theguardian.com
But after genetic testing methods were developed, scientists began to realize that birds were not as true as previously thought - that social and sexual monogamy don't necessarily go hand in hand. Mulder's work with the beautiful painted robin showed that 76% of chicks born in a couple's nests had other fathers. This astounding figure was surpassed only by the Australian magpie, with 82%.
In general, Griffith said, Australian birds breed less frequently than European species because strong partnerships are needed to survive in a variable environment. In the northern hemisphere, the timing of the breeding season is predictable and depends on the length of the day, but in Australia the decision to breed also depends on climatic factors.
"In some years there's not a lot of rain, nothing grows, and the birds and animals that live there can't breed," Griffith says of Australia's arid zone. "Propagation is a much more complex decision on an individual level, but if there are good partnerships, you can optimize that decision together."
Among birds, the model of monogamy is the wandering albatross, which can live up to 50 years and usually mates with one partner throughout its life.
"It takes this bird so long to form mating bonds," said Dr. Ruijao Sun, a researcher at the University of California, Santa Barbara. "If an individual loses its mate, it takes years for it to bond with a new one and start breeding again. Wandering albatrosses have only one egg each breeding season, but one parent always has to sit on the nest to protect the chick and incubate it, but they also need to forage for food ... so they have to alternate."
"It takes two to raise chicks," says Sun, who believes long-lived species such as the wandering albatross benefit more from strong mating bonds. "Each time they breed, they hone their behavior - they coordinate much better with each other, which makes breeding easier."
Species with shorter lifespans, on the other hand, may abandon their partner to maximize breeding opportunities. Even so, Sun estimates that the divorce rate for wandering albatrosses is about 10 percent. By comparison, the king penguin, which is sexually monogamous with a partner, divorces at a rate of about 80%.
Causes of divorce in birds
Other factors that can push animals toward divorce - what scientists call mate switching - include high mortality rates and skewed sex ratios. Both increase competition for mates, creating a temptation for those in the minority to settle down with someone more attractive.
Photo: theguardian.com
Research is emerging that climate crisis may also play a role in divorce. Studying snow petrels that nest in rock crevices in Antarctica, Sun and her co-authors found that the number of snow days during the breeding season is directly related to the frequency of breakups. Too much snow fills the nests and freezes the eggs, leading to incubation failure.
"
They may either leave their previous nest or abandon their partner," Sun says, adding that
stress from constant snow removal can cause the birds to "
blame their partner for failures more than they normally would."
The authors of the paper predict that the shrinking sea ice in the face of climate change will also affect future survival rates by changing the sex ratio. "There will be many males in the population and fewer females willing to mate with them," Sun says.
Stress and partner blaming probably also play a role in the Falkland Islands, where unusually warm water temperatures have been linked to higher divorce rates among black-browed albatrosses. "Environmentally driven divorces," the researchers suggested at the time, "may thus represent an unobserved consequence of global change."