Climate change swells divorce rates in one famously loyal bird species

The albatross bird is a species that is known to be monogamous. However, researchers found that these monogamous bird pairs ‘divorced’ more often nowadays based on environmental factors.

Scientific American explains the purpose of the albatross “marriages” as having a “practical purpose: staying with the same partner builds trust, which is essential as the pair alternates between lengthy foraging trips and egg-incubation duties.”

In a study published by the Royal Society, scientists define “divorce” as “a strategy used to correct for suboptimal partnerships … informed by measures of previous breeding performance.”

Albatross pairs stay together when they successfully reproduce, and tend to split when they do not. Yet, the researchers write, “The environment affects the productivity and survival of populations, thus indirectly affecting divorce via changes in demographic rates.”

The study focuses on going through 15 years of data on “the long-lived black-browed albatross (Thalassarche melanophris) as a model organism, [testing] the hypothesis that environmental variability directly affects divorce.”

The researchers say they have looked at data going back more than a decade: “Since 2003, we have collected capture-mark-recapture data on black-browed albatrosses breeding on New Island, Falklands, home to approximately 15,500 pairs.”

They have also, “Starting from the 2008 season, ... collected GPS tracking data during incubation and brooding, which enabled us to quantify the oceanic areas most intensely used by albatrosses (electronic supplementary material).”

According to the Guardian, “Albatrosses, some of the world’s most loyally monogamous creatures, are ‘divorcing’ more often – and researchers say global heating may be to blame.”

The researchers wrote in their study that they found that “divorce rate varied across years (1% to 8%). Individuals were more likely to divorce after breeding failures.”

They went on to say, however, “regardless of previous breeding performance, the probability of divorce was directly affected by the environment, increasing in years with warm sea surface temperature anomalies (SSTA). Furthermore, our state-space models show that warm SSTA increased the probability of switching mates in females in successful relationships.”

The scientists looked at two environmental factors – warm sea surface temperature anomalies (SSTA) and wind intensity (WIND): “Specifically, SSTA was found to have a strong negative effect on the probability of breeding and on breeding success, whereas WIND positively affected breeding success.”

The Guardian explains SSTA’s effect as “For seabirds, warmer waters mean less fish, less food and a harsher environment. Fewer chicks survive. The birds’ stress hormones increase. They are forced farther afield to hunt.”

Albatrosses have been the subject of study for a long time, and “There are all these things we think of as being super-duper human,” Dr Graeme Elliot, principal science adviser at New Zealand’s department of conservation, who has been studying albatrosses in the country’s waters for three decades, tells the Guardian.

Why? Because the birds live for 50-60 years, they have what the Guardian calls “a long, awkward teen phase” as they learn how to seduce a mate through dance, and take years-long trips away from home as they mature. They also mate for life, and loudly celebrate when greeting a partner after a long absence, the Guardian reports.

The research, co-authored by Francesco Ventura, suggests that “our state-space models show that warm SSTA increased the probability of switching mates in females in successful relationships.”

The researchers point out that “For the first time, to our knowledge, we document the disruptive effects of challenging environmental conditions on the breeding processes of a monogamous population, potentially mediated by higher reproductive costs, changes in phenology and physiological stress.”

They conclude with “Environmentally driven divorce may therefore represent an overlooked consequence of global change.”

Francesco Ventura, researcher at University of Lisbon and co-author of the Royal Society study, tells the Guardian that the researchers were surprised to learn that warmer waters were associated with unusually high rates of albatross couples splitting up, even when the lack of fish were accounted for.

Albatross divorce was usually predicted by a reproductive failure, Ventura goes on to say. If a pair failed to produce a chick, they had a higher chance of splitting up. Less food for birds could lead to more failures. But the researchers, the Guardian reports, “were surprised to find that even when they accounted for that, higher water temperatures were having an extra effect – pushing up divorce rates even when reproduction was successful.”

According to the Guardian, Ventura suggests two possible reasons for divorcing albatross even when reproduction was successful: “one that warming waters were forcing the birds to hunt for longer and fly further. If birds then failed to return for a breeding season, their partners may move on with someone new. Added to that, when waters are warmer and in harsher environments, albatross stress hormones go up. Ventura said the birds may feel that, and blame their partner.”

Speaking to Scientific American, Ventura says ““Previous successful females are the ones that are most affected by this [warming].” He goes on to say “They divorced more often, when in theory they should have remained together with their previous partner.”

Scientific American calls it a possible manifestation of what Ventura calls the “partner blaming hypothesis,” in which the female conflates the stress caused by environmental conditions with poor performance by her partner.

 “We propose this partner-blaming hypothesis – with which a stressed female might feel this physiological stress, and attribute these higher stress levels to a poor performance of the male,” Ventura tells the Guardian.

Meanwhile, Elliot tells the Guardian that “[Albatross] numbers are plummeting [in different locations]”. The Guardian reports that “The populations of wandering albatrosses that he studies were now declining at rates of 5-10% every year, and had been dropping since around 2005. Those dropping numbers come as a result of less prey, warming seas, and increasing encounters with tuna line-fishing boats, which accidentally catch and kill the birds.”

Elliot adds that dropping population numbers were resulting in more homosexual couplings: “We’re getting male-male pairs amongst the birds on Antipodes Island [to the south of New Zealand], which we haven’t had before,” he says. “A few percent of the boys are pairing up with another boy because they can’t find a female partner.”

While the Royal Society study focuses on the albatross population at the Falkland Islands, where it was not in decline, and where divorce was not catastrophic, Ventura says, the same dynamics could apply to other albatross populations, and have a more damaging effect where bird numbers were more fragile: “If we’re talking about a population with a much lower number of breeding pairs, that disruption of a bond might definitely induce some disturbance in the regular breeding processes,” he tells the Guardian.

Ventura hypothesizes that similar patterns may manifest in other seabird populations and possibly among some monogamous mammals, highlighting a potentially overlooked consequence of climate change, Scientific American reports.

According to Natasha Gillies, a researcher at the University of Liverpool in England, who studies seabird breeding behavior and was not involved in the new study, this could have “profound” impacts for smaller populations of birds by decreasing breeding options, Jack Tamisiea writes for Scientific American.

 “If you have a situation where increasing sea-surface temperature is leading to higher divorce rates, that reduces breeding success for the population as a whole,” she says. “Ultimately, you’re sending fewer albatrosses out into the world, and that’s going to impact the population more widely.”

Elliot tells the Guardian that he hopes that people take a step to protect the albatross from these environmental threats – particularly climate change, and tuna fishing. “We kind of need an international campaign to save these birds,” Elliot says. “If we don’t turn it around, they’ll go extinct.””