Factors influencing infant sex ratio in howler monkeys (Alouatta spp.): A literature review and analysis

• Published in: American journal of physical anthropology Vol. 172; no. 1; pp. 48 - 57
• Main Authors: Dias, Pedro A. D., Montero Domínguez, Irma L., Rangel Negrín, Ariadna
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01-05-2020; Wiley Subscription Services, Inc
• Subjects: Alouatta - growth & development; Alouatta - physiology; Animals; competition; Daughters; Demography; Dispersal; Female; infant sex ratio; Infants; Literature reviews; Male; Monkeys & apes; Parity; Population growth; Primates; Rainfall; Ratios; Relatedness; sex allocation; Sex Ratio; Sons; Tactics; rainfall; competition; sex allocation; infant sex ratio; sex ratio; population growth
• ISSN: 0002-9483; 1096-8644; 2692-7691
• DOI: 10.1002/ajpa.24035

Objective
Frequency‐dependent selection is expected to maintain infant sex ratios around parity over evolutionary time. However, over ecological time periods, infant sex ratios vary, and it has been proposed that this variation may reflect adaptive processes. In primates, there are consistent patterns of variation in infant sex ratios, although their adaptive significance remains contentious. In addition to design issues, contrasting results could have derived across primates from variation in the fitness benefits accrued through sons or daughters associated with the specific social, ecological, and demographic context of populations. Thus, different sex allocation tactics could occur within species over time and space.
Methods
We reviewed the literature to describe variation in infant sex ratio in howler monkeys (genus Alouatta) and to examine whether such a variation could be associated with adaptive sex allocation. We found 26 studies that provided data for this review. These studies yielded 96 infant sex reports, corresponding to 1,477 sexed infants.
Results
Infant sex ratio across howler monkey species tends to parity, but females produce more sons under high group densities and more daughters when rainfall increases.
Discussion
Based on these results, as well as on information on howler monkey dispersal patterns, demography, and within‐group genetic relatedness, we speculate that, depending on population growth stage, sex allocation is explained by (a) local resource enhancement, that is, more cooperative philopatric daughters are produced when populations are growing; and (b) local resource competition, that is, more dispersing sons are produced when populations are saturated. Thus, there is evidence suggestive of adaptive variation in infant sex ratios in howler monkeys.