THEY seemed the perfect couple. Growing up in the same neck of the woods, they went their separate ways in adolescence, before reuniting and hitting it off. But after many years together, they seemed to just drift apart. Divorce soon followed.
This sad story may be all too familiar to many, but there is one important difference. The parties in this case aren’t human. They are birds.
For years now, behavioural biologists have been aware that some animals – birds in particular – choose to pair up for years on end, often raising offspring together. Some researchers have also become fascinated by the fact that these couples can break up. Although scientists are generally loath to anthropomorphise, there are times when even they can’t resist. This explains why the biological literature is full of studies analysing the factors that drive animal “divorce” in all manner of species, from albatrosses to penguins, beavers to seahorses.
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Recently, as we have learned more about this, a surprising conclusion has emerged: under the right circumstances, animal divorce can be so beneficial that it might be an evolved trait. But on the flip side, we have also come to realise that divorce rates have the potential to spiral out of control so that viable breeding colonies may decline, dwindle and disappear. What’s more, research has even identified a worrying new factor that might unexpectedly contribute to a sharp rise in this sort of divorce-driven crisis: climate change.
Many animals will never divorce, for the simple reason that it is unusual for them to form monogamous pairs. , for instance. From an evolutionary perspective, this rejection of monogamy makes sense: animals typically stand to have more offspring that have the qualities needed to thrive by mating with multiple partners. As such, researchers have long wondered why monogamy exists – particularly among birds, where . A 2018 study pointed to . For example, if feeding offspring is so challenging that it requires the work of two adults, then males and females should benefit from remaining together after mating. Long-lived animals may also benefit from monogamy because it allows them time to perfect the art of rearing offspring together. This can be useful if environmental conditions deteriorate, because a long-term couple may be able to draw on their shared experience to cope.
To complicate things, we now know that there are different ways for animals to be monogamous. Towards the end of the 20th century, DNA testing showed that many “faithful” pair-bonding animals bend the rules: one partner may . Because of this, biologists now distinguish between genetic monogamy – in which animals live in pairs at least some of the time and breed only with their partner – and social monogamy, in which two animals cohabit and even raise offspring together, but may mate with other partners.
Both genetically and socially monogamous relationships can end in divorce – and break-ups aren’t always triggered in the same way. For the lined seahorse (Hippocampus erectus), which is usually genetically monogamous, divorce appears to be the result of a memory lapse. A 2021 study found that if, after mating, seahorse pairs were housed in separate tanks, the female lost her preference for her male partner. When she was reintroduced to him after the birth of their offspring, alongside two other male seahorses, she seemed to have forgotten the pair bond and was as likely to mate with any of the three males.
In other cases, there is an active agent involved. Take the socially monogamous Eurasian beaver (Castor fiber). A 2017 study suggested that those in south-east Norway : couples tend to break up after this time together on average. Often, divorce was triggered by the appearance of a new, younger rival – either male or female – that disrupted the relationship.
This phenomenon is sometimes called forced divorce, and a study published a few months ago shows it occurs in albatrosses too. Male wandering albatrosses (Diomedea exulans) living on the Crozet Islands in the southern Indian Ocean that were judged by biologists to be “shy” were .
“Just like with humans, there [is generally] a cue or trigger for divorce, something that causes them to decide, hey I’m going to leave my partner,” says Antica Culina at the Ruder Boskovic Institute in Zagreb, Croatia, who has devoted part of her career to studying pair-bonding in animals.
According to Culina, the main cue is reproductive success, or lack of it: if you fail to rear offspring with a partner then you are more likely to split up. But recently, biologists have begun to recognise that it isn’t just this that can prompt a monogamous pair to part.
Every September in the Falkland Islands, some 500,000 pairs of socially monogamous black-browed albatrosses (Thalassarche melanophris) gather to breed and raise chicks. Francesco Ventura at the Woods Hole Oceanographic Institution, Massachusetts, and his colleagues tracked 424 female albatrosses over a 15-year period, recording whether or not they remained faithful to their partners.
They found that, as expected, divorce was most often prompted by breeding failure. Each female lays just one egg, and if it failed to hatch couples were over five times as likely to split up. Ventura’s team also discovered a correlation between divorce and sea surface temperatures. For instance, between 2012 and 2016, sea surface temperatures weren’t unusually high and divorce rates hovered below 4 per cent. But they rose to 8 per cent in 2017 when the sea surface around the islands was warmer than usual.
Ventura and his colleagues, who published their findings in 2021, also noticed something else. During these warmer years, which are thought to be a consequence of climate change, .
Misinformed decisions
“It’s well established that one of the main reasons that albatrosses divorce is related to breeding success,” says Ventura. “However, what we found was that regardless of this success or failure, and regardless of previous reproductive outcome, as sea temperatures increased, so did the probability of divorce.”
Why? Ventura says that when seas are warmer there is less food available, meaning albatrosses have to travel further to get a meal. In seabirds, food shortages are known to raise levels of a stress hormone similar to cortisol. It is possible, he thinks, that a stressed and hungry female will suspect her partner is somehow to “blame” for her hunger – even though the reality is there is simply less food available. In other words, says Ventura, the environment may be misinforming decisions about whether to stay or go.
Recent research suggests this may be bad news for animals. Last year, Brian Lerch at the University of North Carolina, Chapel Hill, and his colleagues published a mathematical model of animal divorce. This revealed that there are .
The model assumed divorce is triggered by females who want to raise their reproductive success. It concluded that the more variability there is between male-held territories or habitats – defined, for instance, by the availability of food or abundance of predators – the more likely it is that females will divorce their partners. This is because it is difficult to raise offspring on a poor territory, and so females stuck on one will be keen to upgrade by hitching up with a male on a better territory. Crucially, because of natural death rates across the colony, there should always be the possibility of those better territories becoming available, which makes divorce potentially worthwhile for the female.
“We found that the more that bad territories get worse or good territories get better, the more beneficial and likely it is that animals will divorce,” says Lerch.
By implication, if climate change temporarily reduces the availability of food, as it did around the Falkland Islands, then females may conclude they are on a poor territory and that divorce is the right move.
While divorce can sometimes be beneficial to the individual, the model showed that as rates rise, it reduces the reproductive success of the colony as a whole. Lerch suspects this is because breeding pairs that have been together for many years have plenty of experience to draw on, so they have a higher chance of raising offspring successfully than a pair attempting to raise offspring together for the first time. “Our model showed that the total reproductive success of the population is likely to decrease as divorce rates increase,” says Lerch.
Colonies in decline
This suggests that if climate change prompts a surge in break-ups across a population – and particularly if it breaks up experienced breeding pairs that have the best chance of raising offspring successfully – breeding colonies may go into decline.
The good news for the black-browed albatrosses on the Falkland Islands is that the breeding population is robust. There is no immediate likelihood that a spike in divorce triggered by warmer seas will lead to the demise of this breeding colony, says Ventura. But for animal colonies elsewhere that are already shrinking, high divorce rates triggered by climate change could be serious, he says.
“There are sea birds that breed out on isolated colonies that are already among the most threatened vertebrates,” says Ventura. “A disruption in normal breeding processes could be much more problematic for these populations.”
A declining colony of Magellanic penguins (Spheniscus magellanicus) near Punta Tombo in southern Argentina may be vulnerable. These monogamous birds are picky when it comes to choosing a nesting site, and usually head straight to the same spot year after year. The penguins prefer to nest in bushes or burrows, which provide cover and protection from predators and the weather. Those with the best-protected nests are more likely to raise chicks.
A study published last year showed that than it had been the previous year. If climate change leads to die-back of the bushes protecting nests, then divorce rates will surge.
“The breeding failure might initiate a kind of negative feedback,” says Eric Wagner at the University of Washington in Seattle, who authored the study. If pairs divorce and females find new partners, he says those new pairings may experience breeding failure because of a lack of experience raising offspring together. This will trigger even more divorce.
“The expectation is that access to food, and access to high-quality habitat, is likely to decrease in most species as a consequence of climate change,” says Lerch. “There’s good reason to expect that it will lead to more divorce in a lot of populations.”
One computer model of an animal breeding colony suggested that, if either males or females are encouraged to , viable populations have the potential to disappear – a process researchers dub evolutionary suicide.
All of which provides more evidence of the importance of acting to reduce the severity of climate change. We already appreciate that lowering carbon emissions will help humans and natural ecosystems avoid the more damaging effects of global warming. It turns out that action might also spare some animals from the ordeal of divorce.
