Wandering albatrosses are typical representatives of bird monogamy. It is well known that this graceful flesh gliding opportunity is privately reserved for life, and between long sea flights, season after season, the same bird couple will breed. But in rare cases, a pair of albatrosses will "divorce" – a term used by ornithologists for situations where one partner leaves an albatross while the other remains in a flock. Divorce rates vary widely throughout the bird world, while drifting albatrosses have relatively low divorce rates.
Still, these large bird drifters will part ways. Scientists at MIT and the Woods Hole Oceanographic Institution (WHOI) have found that, at least for one particular population of wandering albatrosses, whether a pair of albatrosses divorces comes down to one important factor: personality.
In a recent study published in the journal Biological Letters, the team reported that the chances of divorce for an albatross couple were strongly influenced by the male partner's "boldness." The bolder and more aggressive the male, the more likely the couple is to stay together. The more shy the male, the greater the chances of the couple divorcing.
The researchers say their study is the first to link the personality and divorce of wildlife species.
Stephanie Jenouvrier, senior author of the study and associate scientist at WHOI's FLEDGE lab and seabird ecologist said: "Bold males were previously thought to be more aggressive and more likely to lead to 'conjugal discord', and they were more likely to risk switching partners to improve future reproductive outcomes. On the contrary, we found that shy people divorce more because they are more likely to be forced to 'divorce' by more competitive intruders. This may not be an isolated case, and personality is expected to influence divorce rates in many species, but in different ways. "
Lead author Ruijiao Sun, a graduate student in the MIT-WHOI joint program and MIT's Department of Earth, Atmospheric and Planetary Sciences, said the new evidence on the link between the personality and divorce of wandering albatrosses may help scientists predict population resilience.
"The wandering albatross is a vulnerable species," Sun said. "Understanding the impact of personality on divorce is important because it can help researchers predict consequences for population dynamics and implement conservation efforts."
Co-authors of the study include Joanie Van de Walle of WHOI, Samantha Patrick of the University of Liverpool, and Christophe Barbraud, Henri Weimerskirch and Karine Delord of CNRS-La Rochelle University in France.
The new study focused on a population of wandering albatrosses that regularly return to the southern Indian Ocean island of Possession for breeding. This population has been the focus of long-term research, even dating back to the 50s of the 20th century, when researchers have been monitoring each breeding season of these birds and documenting individual pairings and breakups over the years.
This particular population favors male individuals more than female individuals because the feeding grounds of female albatrosses overlap with fishing vessels, where they are more likely to be accidentally caught by fishing lines as bycatch.
In earlier studies, Sun analyzed data from this long-term study and found a strange pattern. Those individuals who divorce are more likely to do so again and again.
"Then we wondered what drives divorce and why some albatrosses divorce more often," Jenouvrier said. "In humans, you also see this repetitive pattern of divorce, related to personality. And the wandering albatross is one of the rare species where we have both demographic and personality data. "
The personality data comes from an ongoing study that began in 2008 and was led by co-author Patrick, who has been measuring individual personalities in the same population of wandering albatrosses on Possession Island. In the study of animal behavior, personality is defined as consistent behavioral differences exhibited by individuals. Biologists primarily measure an animal's personality as a gradient between shyness and boldness, or less to more aggression.
In Patrick's study, the researchers measured the boldness of albatrosses by measuring how responsive birds respond to humans approaching their nests, about 5 meters away. Rate the bird according to its reaction (a bird that does not respond gets zero and is the most shy, while a bird that raises its head or even stands up can give a high score and is the most daring).
Patrick conducted multiple albatross personality assessments of the same habitat over many years. Sun and Jenouvrier wondered if personality could have anything to do with their chances of divorce.
Shy birds
In their new study, the team used data from demographic and personality studies to see if any particular patterns emerged between the two. They applied a statistical model to both datasets to test whether the personality of individuals in a pair of albatrosses influenced the fate of the pair of albatrosses.
They found that for female albatrosses, personality had little to do with whether the bird was divorced or not. But among males, the pattern is clear: those identified as shy males are more likely to divorce, while bold males spend more time with their partners.
"Divorce doesn't happen very often," Jenouvrier said. "But we found that the more shy the bird, the more likely it is to divorce."
But why? In their study, the group came up with an explanation that ecologists call "forced divorce." They point out that in this particular population of wandering albatrosses, males far outnumber females and are therefore more likely to compete with each other for mates. As a result, males who have already formed a mate may face a third "invader" – a male who is vying for a partner's position.
"When a third intruder competes, shy birds may back off and give up their mates, while bold individuals are aggressive and guard their mates and secure their partnership," Sun explains. "That's why shyer individuals may have higher divorce rates."
The team is planning to expand their work to study how personality influences larger population changes and evolution.