Primigravidity, body weight, and costs of rearing first offspring in rhesus macaques.

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Body size is associated with menarche and ovarian function, but the relationship to first conception is rarely examined. We conducted a longitudinal investigation of rhesus macaques, Macaca mulatta, to determine the effect of differences in body weight on both the age at first conception and survivorship of first progeny. Young females who became pregnant weighed significantly more than peers who remained barren, with weight changes for females who successfully raised offspring no different than those for females who did not produce offspring. Infant mortality among primiparae was not significantly greater than that among multiparae, although nearly twice as high. First-born males tended to have lower survivorship than first-born female offspring. We suggest that reproductive costs encountered by primiparous females are more likely to be modulated by immature neuroendocrine function than by inexperience, small body size, or infant suckling patterns. We conclude that body size influences probability of first conception, socioendocrine factors mediate the likelihood of infant survival, and primiparous production of male progeny seems to exert a greater reproductive cost than does production of female progeny.