Perched in the Galápagos Islands, David Anderson, a research professor and professor of biology emeritus at Wake Forest University, has uncovered new insights into the mating behavior of Nazca boobies. His recent study, published in PLOS One and titled “Female sexual agency and frequent extra-pair copulations, but no extra-pair paternity, in Nazca boobies,” challenges traditional ideas about avian monogamy.
Anderson observed that female Nazca boobies engaged in numerous conspicuous sexual encounters with multiple males during a 74-day observation period. Despite this promiscuity, their eventual nesting partners appeared unbothered by these liaisons.
“Many seabird species have long-term pair bonds, and they have these extended courtship periods and shared parental care,” Anderson said. “You don’t expect to see females just running back and forth trying to copulate with so many males. So yeah, that’s a huge surprise. This mating system has some similarity to the lek arrangement as far as females’ sexual agency.”
The study documented that most females had several partners during the breeding season, with one female having up to 16 partners. Anderson described this as highly unusual for seabirds: “That’s just mind-blowing for a seabird,” he said. “Many of these female boobies are really freewheeling it when it comes to sexual behavior.”
According to Anderson, the power dynamic within the colony contributes to this behavior. Female Nazca boobies are larger and stronger than males, and their lower numbers make them more sought after by males. This imbalance allows females to mate openly with other males without facing consequences from their primary partner.
Nazca booby females can display “whatever sexual behavior is best for them, and there’s nothing the males can do about it,” Anderson explained. “The males are afraid of the females, and also won’t risk alienating a female since there are so few of them.”
However, when ovulation begins and females are ready to breed, their sexual activity outside the main partnership decreases sharply. Genetic analysis revealed that despite frequent mating with multiple partners prior to breeding season, only the main partner fathers offspring.
“She’s copulating with other males in the lead up to the breeding season, but genetic data showed that they’re never the father of her children,” Anderson said. “This reconciles evidence that females are shopping around, but it never results in fertilized eggs in the end. These flings are sex, but not reproduction.”
“This is sort of pushing the envelope on the females controlling their own destiny in reproduction and males just accepting the outcome,” he added.
Anderson’s research continues a tradition of studying family dynamics among birds because their observable behaviors offer insight into complex social structures within animal populations.
He plans further investigation into why female Nazca boobies engage in multiple matings if only one male ultimately fathers their chicks: “Why are these females doing it, if it’s not leading to a fertilized egg?” he said. “We would very much like to know the answer to that.”
