Zoologists from Wake Forest University have discovered unusual behavior in Nazca boobies (Sula granti): the females of these birds openly mate with various neighbors, yet their offspring are sired exclusively by their regular partners. The study revealed that females enjoy complete sexual freedom thanks to their physical superiority over males and maintain control over paternity through a strict schedule of encounters.

Behavioral Traits of Nazca Boobies

Most monogamous birds experience a conflict of interest: males guard their mates to avoid raising another male’s chicks, while females, if they cheat, do so secretly. Nazca boobies, which inhabit the Galápagos Islands, have long puzzled scientists. Despite being monogamous, these birds actively and openly engage in sexual activity with other males. However, genetic tests show that there are no extra-pair chicks in their nests.

Research Process

To unravel this paradox, scientists spent 74 days in a colony of 10,000 boobies on Española Island in the Galápagos archipelago, recording over 1,600 mating events. At the same time, DNA tests were conducted and the individual ovulation cycles of each bird were analyzed. The results of the study were published in the journal PLOS One.

Main Findings

Observations showed that about 23% of all matings were with partners outside the pair bond. These encounters were voluntary and often took place in front of the social partner, who showed no aggression. However, genetic purity of the offspring was ensured by a strict schedule: females visited neighbors weeks before nesting, but as ovulation began (4–8 days before laying an egg), they stopped these visits and mated only with their “husband.”

Reasons for Female Sexual Freedom

First, females are physically larger than males (on average by 13%) and have a stronger bite, which prevents males from forcing them. Second, there is a sex ratio imbalance in the population: because of their larger size, females are more likely to die of starvation before fledging, so among adults, there are three males for every two females. As the authors note, “males may be motivated to accept any female and tolerate any of her behavior.”

Why Do Females Mate with Other Males?

The researchers proposed the “address book” hypothesis: sex with neighbors does not serve a reproductive function but acts as a tool for social interaction. By maintaining intimate ties with several nearby males, a female is essentially testing and “reserving” potential partners for the next year, should she decide to change her current mate.

Serial Monogamy in Boobies

Boobies practice serial monogamy: a pair forms a strong bond to raise a chick during the current season, but there are no guarantees for the future. Between seasons, “divorces” are common: females, being a scarce resource in the population, tend to leave partners who have exhausted themselves caring for offspring in favor of “fresher” candidates.