After Short Interbirth Intervals, Monkeys Have Higher Infant Mortality

Unusually short or long interbirth intervals (IBIs) are associated with increased risks of infant mortality in humans. However, further research is needed to determine the extent to which this relationship holds more broadly among primates. Using Cox proportional hazards models, we examined the effects of atypical IBI on infant survivorship using a large demographic dataset (n = 15,852) of callitrichine monkeys (marmosets, tamarins, and lion tamarins) from zoo and laboratory populations. In seven of the nine species studied, infants born after short IBIs had significantly higher risks of mortality than infants born after longer IBIs. Taken together, these results suggest that fundamental constraints may underlie the relationships between elevated infant mortality and atypical birth spacing in human and nonhuman primates

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

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, Callithrix jacchus, Cebuella pygmaea, Saguinus imperator, Saguinus oedipus, Leontopithecus chrysomelas, Leontopithecus chrysopygus, Leontopithecus rosalia, and Callimico goeldii) from zoo and laboratory populations spanning 80 years (1938-2018). Through this comparative approach, we found several lines of evidence that litter size and sex composition may impact fitness. Singletons have higher survivorship than litter-born peers and they significantly outperform litter-born individuals on two measures of reproductive performance. Further, for some species, individuals born in a mixed-sex litter outperform isosexually-born individuals (i.e., those born in all-male or all-female litters), suggesting that same-sex competition may limit reproductive performance. We also document several interesting demographic trends. All but one species (C. pygmaea) has a male-biased birth sex ratio with higher survivorship from birth to sexual maturity among females (although this was significant in only two species). Isosexual litters occurred at the expected frequency (with one exception: C. pygmaea), unlike other animals, where isosexual litters are typically overrepresented. Taken together, our results indicate a modest negative effect of same-age sibling competition on reproductive output in captive callitrichines. This study also serves to illustrate the value of zoo and laboratory records for biological inquiry