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

Abstract

In many birds and mammals, the size and sex composition of litters can have important downstream effects for individual offspring. Primates are model organisms for questions of cooperation and conflict, but the factors shaping interactions among same-age siblings have been less-studied in primates because most species bear single young. However, callitrichines (marmosets, tamarins, and lion tamarins) frequently bear litters of two or more, thereby providing the opportunity to ask whether variation in the size and sex composition of litters affects development, survival, and reproduction. To investigate these questions, we compiled a large dataset of nine species of callitrichines (n = 27,080 individuals; Callithrix geoffroyi,

After short interbirth intervals, captive callitrichine monkeys have higher infant mortality

“…Secondly, family members other than the parents care for offspring ( Tardif, 1996 ; Tardif and Ross, 2009 ). One therefore might expect that callitrichines have low infant mortality thanks to cooperative care, but it appears that frequently birthing twins and larger litters imposes the costs of higher infant mortality due to direct or indirect sibling conflict ( McCoy et al., 2019 ). Relatedly, callitrichine mothers experience a postpartum estrus where they can ovulate and conceive soon after giving birth (unusual among primates ( French et al., 2002 )).…”

“…We constructed Cox Proportional Hazards models of survivorship using the “coxph” function in the R package “survival” ( Therneau and Grambsch, 2000 ). We included litter size as a predictor variable, because singletons have higher survivorship than litters ( McCoy et al., 2019 ), litter number (i.e., what number litter it was for a given dam) to control for parity, and the IBI category (short, normal, or long). We also clustered births by dams to account for non-independence of siblings born to the same mother.…”

“…To better understand these reproductive processes across primates, we analyzed a large dataset (n = 15,852) of births with known IBIs in nine species of callitrichine monkeys—an American primate clade that routinely produces litters (of fraternal twins and triplets) and relies on cooperative care to raise offspring ( Haig, 1999 ; Tardif, 1996 ; Tardif and Ross, 2009 ). We derived these data from demographic records of zoo and laboratory populations spanning 80 years (1938–2018 ( McCoy et al., 2019 )). Do callitrichine infants suffer higher mortality after extreme (shorter or longer) IBIs?…”

“…We compared the survivorship profiles of individuals born after short, normal, and long IBIs ( Figure 2 ; see STAR Methods ; Data S1 ) by constructing Cox proportional hazards models. In these models, we also included variables for litter size because singletons have higher survivorship than litters ( McCoy et al., 2019 ) and litter number (i.e., what number litter it was for a given dam) to control for dam age and parity. We also controlled for dam identity ( Figure 2 ; Data S1 ).…”

Factors influencing the survival of wild cotton‐top tamarin (Saguinus oedipus) infants

“…This skewed sex difference at birth and survival rates should be interpreted with caution given the large number of infants (N = 86) of undetermined sex. Male biased litters are commonly observed in callitrichids both in managed care and in the wild (Baker & Woods, 1992;McCoy et al, 2019;Poole & Evans, 1982;Ross et al, 2007;Savage et al, 1996Savage et al, , 2009Snowdon et al, 1985;Yamamoto, 2005). However, there does not appear to be a consistent bias toward male survival.…”