Do mothers prefer helpers or smaller litters? Birth sex ratio and litter size adjustment in cotton‐top tamarins (Saguinus oedipus)

Abstract

Sex allocation theory has been a remarkably productive field in behavioral ecology with empirical evidence regularly supporting quantitative theoretical predictions. Across mammals in general and primates in particular, however, support for the various hypotheses has been more equivocal. Population-level sex ratio biases have often been interpreted as supportive, but evidence for small-scale facultative adjustment has rarely been found. The helper repayment (HR) also named the local resource enhancement (LRE) hypothesis predicts that, in cooperatively breeding species, mothers invest more in the sex which assists with rearing future offspring and that this bias will be more pronounced in mothers who require extra assistance (i.e., due to inexperience or a lack of available alloparents). We tested these hypotheses in captive cotton-top tamarins (Saguinus oedipus) utilizing the international studbook and birth records obtained through a questionnaire from ISIS-registered institutions. Infant sex, litter size, mother's age, parity, and group composition (presence of nonreproductive subordinate males and females) were determined from these records. The HR hypothesis was supported over the entire population, which was significantly biased toward males (the “helpful” sex). We found little support for helper repayment at the individual level, as primiparous females and those in groups without alloparents did not exhibit more extreme tendencies to produce male infants. Primiparous females were, however, more likely to produce singleton litters. Singleton births were more likely to be male, which suggests that there may be an interaction between litter size adjustment and sex allocation. This may be interpreted as supportive of the HR hypothesis, but alternative explanations at both the proximate and ultimate levels are possible. These possibilities warrant further consideration when attempting to understand the ambiguous results of primate sex ratio studies so far.

A comparative study of litter size and sex composition in a large dataset of callitrichine monkeys

“…All but one species in our sample exhibited male‐biased BSR, although three species did not significantly diverge from the expected 1:1 BSR ( C. jacchus, C. pygmaea , and S. imperator ; Table ). Biased birth sex ratios are typical in mammals (Clutton‐Brock & Iason, ; Faust & Thompson, ; Thogerson et al, ), including callitrichines (e.g., L. rosalia , Rapaport, Kloc, Warneke, Mickelberg, & Ballou, ; S. oedipus , Boulton & Fletcher, ; C. jacchus , Poole & Evans, ; but see Rothe et al, ). Some of the callitrichine species surveyed here, though, exhibited skews of relatively large magnitude (Figure a; Table ).…”

Modeling Developmental Class Provides Insights into Individual Contributions to Infant Survival in Callitrichids

“…One of the primary arguments for the presence of helpers, typically unrelated adult males or natal subadults, is that they alleviate the cost of rearing energetically expensive twin offspring that constitute over 80 % of all births in callitrichids (save Callimico) (Tardif 1997;Wislocki 1939). Alloparenting behaviors by helpers benefit offspring survival, and thus increase group reproductive output (GRO) (Bales et al 2000;2001;Boulton and Fletcher 2015;Garber 1997;Heymann 2000;Koenig 1995). By investing in the care of offspring, helpers could incur indirect fitness benefits if they are related to the biological parents; they also benefit the group by providing increased vigilance and protection from predators, or access to valuable resources (for reviews see Bales et al 2000;French 1997;Tardif 1997).…”

Exposure to male-dominated environments during development influences sperm sex ratios at sexual maturity

“…; Johnson et al. ; Silk and Brown ; Boulton and Fletcher ; Song et al. ), although these findings are more directly related to resource competition rather than mate competition per se (but see Saragusty et al.…”