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

Abstract

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. K E Y W O R D Sinfant sex ratio, competition, population growth, rainfall, sex allocation, sex ratio

Evaluation of the potential of the observation of Alouatta palliata for local conservation and development in fragments of rainforests: the case of the Isthmus of Tehuantepec, Mexico

“…A second factor here is the proportion of sub-adults to each adult female or male (BF 5). If the sub-adults that reach sexual maturity at 42 months are male, they may present natal emigration; that is, abandoning the troop to try to form their own group due to the proportion of the sexes in their original one (Days et al, 2020).…”

Female-biased birth sex ratio in a female dispersal primate suggests local resource competition

“…Most research on BSR and its biases has thus far been performed in birds [22,23], invertebrates [24] and mammals [15]. Within mammals, the bulk of the data stems from the order Primates [14,[25][26][27] with an emphasis on cercopithecine primates for which most data are available. This uneven sample likely causes an overrepresentation of species with male-biased dispersal [28,29].…”

Evaluación del potencial ecoturístico para conservar el hábitat fragmentado de Alouatta palliata: el caso de una cuenca hidrográfica en el Sur de Veracruz

“…Los subadultos machos que alcanzan su madurez sexual a los 42 meses pueden presentar una separación de su grupo originario y abandonar la tropa para buscar formar su propia tropa, afectando a la proporción sexual de la tropa de origen (Duarte- Días, 2007;Días et al, 2011;Días et al, 2020). Si los subadultos que alcanzan su madurez sexual son hembras, se mejora la proporción por ser reproductivas para el macho alfa (Glander, 1980;Young, 982;Rodríguez-Matamoros et al, 2012).…”

“…Los estudios poblacionales permiten conocer si las poblaciones se encuentran en expansión o en extinción. Días et al (2020), demostró que, dependiendo de la etapa de expansión o reducción de la población, la asignación del sexo se explica por los recursos disponibles de alimentación. De acuerdo con los resultados de estos autores, cuando las condiciones son adecuadas para la supervivencia de la población se producen más hembras y, en el caso contrario, se produce un mayor número de machos.…”