Dynamics and Causes of Facultative Polyandry in Saddle-Back Tamarins (Saguinus Fuscicollis) (Peru).

Abstract

Cooperative polyandry, where a single breeding female lives with two or more males who mate with her and subsequently help care for her young, occurs in a small but diverse array of animals, yet its causes are not well understood. In my dissertation research I studied the dynamics and causes of cooperative poly and ry in wild saddle-back tamarins living in Amazonian Peru. This species has unusually variable mating patterns: some groups are polyandrous, whereas others are monogamous, or even, rarely, polygynous. My seven-year study of this population showed that the ultimate cause of poly and ry in this species is, most probably, the high cost of raising infants. Lone male-female pairs are unlikely to be able to raise infants, and normally, adults either mate monogamously and are helped with infant care by nonreproductive helpers (older offspring) or mate polyandrously, with both males aiding with infant care. Demographic data showed that offspring exhibit helping behavior for some of the same reasons as do nonreproductive helpers in many bird species. The interactions between adult group members suggested that (1) the social relationships between polyandrous males are primarily affiliative and cooperative rather than aggressive, and (2) adult males make a greater investment than do females in the relationships between males and females. I suggest that polyandrous males are rarely aggressive towards each other because they benefit from each others' presence, and that the behaviors that increase a male's proximity to the female and increase her readiness to copulate with him have evolved as a form of indirect male-male competition for matings. Adults of both sexes contributed substantially to offspring care. Males carried infants twice as much, on average, as did females, and also groomed young more, spent more time in vigilance behavior, and remained in closer proximity to juveniles than did females, who performed all the energetically costly lactation. These results confirm the suggestion, based on descriptions of parental care patterns in some cooperatively polyandrous bird species, that cooperative and noncooperative forms of poly and ry are very different, since in noncooperative poly and ry males perform all or almost all of the offspring care.