Infidelity, Siblicide, and Survival of Blue-Footed Boobies

Sexual infidelity is almost universal in the “monogamous” pairs of blue-footed boobies, since females tend to have sexual relations with “extra” males, outside of the relationship with their “official partner,” explained the emeritus researcher of UNAM, Hugh Drummond Durey.

The above is one of the conclusions of four decades of intense observation on Isla Isabel (Nayarit) by the team of researchers from the Ecology Institute (IE) of the National University, headed by Drummond Durey, Sergio Ancona Martínez and María Cristina Rodríguez Juárez, who with the participation of numerous students and volunteers have ringed 23 thousand offspring to document the complete lives of the individuals.

The researcher from the Animal Behavior Laboratory highlighted in an interview: What we have witnessed is a committed monogamy for five months, a time in which the male and female together dedicate themselves to producing and incubating a clutch and feeding the nest day and night. However, during the courtship stage, both of them usually copulate clandestinely with other individuals of the boobies' colony, generally with their neighbors.

He pointed out that boobies, which can be considered an index of the health of the marine ecosystem of the Pacific, are the most studied vertebrate species in Mexico and, possibly, in Latin America. Therefore, researchers consider it their responsibility to take advantage of population, demographic, and behavioral data to address important issues of ecology, natural resources, global climate change, and the evolution of social behavior.

Regarding social behavior, the most outstanding results were captured in the book “Blue-footed Boobies: Sibling Conflict and Sexual Infidelity on a Tropical Island," published by Oxford University Press. Based on nearly a hundred articles in scientific journals, the book communicates to the general public numerous discoveries about biological family conflict. It describes and explains, in terms of evolutionary theory, the fascinating world of lethal conflict between siblings, problems and deceptions between females and males, and between parents and offspring.

To top it off, the author concludes with a chapter based on the psychological and anthropological literature on such conflicts in Homo sapiens, where the author proposes that some convergences between the two species stem partly from the same processes of evolution by natural selection. In humans, social learning and culture influence all behavior, but in some domains, biological tendencies also play a role.

The research team has found that, despite the almost universal infidelity of blue-footed boobies, only four percent of offspring are not from the “official father.” It is suggested that females control the relative use of male ejaculates, and their general (unconscious) policy is not to give paternity to the extra male.

It is conceivable that their infidelity functions to ensure fertilization of eggs when the official male is infertile, to facilitate mate switching, or to inhibit infanticide by neighboring males. Their infidelity is nearly universal in socially monogamous bird species, and its adaptive function(s) remains one of the greatest puzzles in evolutionary biology, the author said.

The phenomenon of siblicide is even more fascinating and dramatic, the author said. Siblicide commonly lays extra eggs that can only be hatched if sardines and anchovies (their prey) are abundant. If El Niño occurs, a chick must be eliminated through lethal competition among them, orchestrated by their mothers.

By laying and hatching their eggs at four- to five-day intervals, the mothers confer on the first to hatch the ability to aggressively dominate its sibling, limit its food intake and, in extreme cases, starve it to death or drive it out of the nest. Some of the young manage to infiltrate a neighboring nest, but most die under a shower of bites from the neighbors, who do not tolerate the reproductive parasites, the researcher said.