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

Abstract

Prior research on cooperative breeders has considered correlations between group reproductive output (GRO) and the number of individuals in each age-sex class, but without controlling for uneven sampling efforts, the underlying effects of group size, and pseudoreplication at the group and species levels. Among callitrichids, age-sex classes do not provide meaningful categories, as individuals within an age-sex class can demonstrate varying reproductive development due to reproductive dominance of a few individuals per group. This study re-assesses the drivers of GRO in callitrichids by a) conducting a meta-analysis of published studies of callitrichid group composition; b) determining a novel method to assign developmental class based on reproductive morphology; and c) utilizing a multistep modelling approach to assess whether any sex-based developmental class predicts both the presence and the numbers of surviving offspring among free-ranging saddleback (Leontocebus weddelli) and emperor tamarins (Saguinus imperator) in Peru. The meta-analysis utilizing a historical dataset revealed that adult females and group size, but not the number of adult males is significantly correlated with GRO. Statistical models of the new dataset revealed that only mature males predicted if groups had any infants at all, but that the number of surviving infants was predicted by mature females and group size. Thus mature males appear to be necessary for groups to raise any infants, but mature females and a larger group size increase group reproductive output overall..

A multiyear survey of helminths from wild saddleback (Leontocebus weddelli) and emperor (Saguinus imperator) tamarins

“…Three callitrichines at this site, the saddleback tamarin, emperor tamarins, and the more cryptic Goeldi's monkey (Callimico goeldii; Watsa et al, 2012), share forest habitat with eight other primate species including three species of Cebidae, and two species each of Atelidae and Pithecidae, as well as owl monkeys (Aotus nigrifrons; Watsa, 2013). At EBLA, both S. imperator and L. weddelli have average group sizes of 5 (range of 3-8) individuals and group compositions are similar (Watsa, Erkenswick, & Robakis, 2017;Watsa et al, 2015).…”

“…This finding is consistent with observations of blood parasites in these primates (Erkenswick, Watsa, Gozalo, et al, 2017), and may be a consequence of low group size variation (3-8 inds.). Some callitrichids can occur in larger groups, for example, Callithrix at 15 members (Pontes & da Cruz, 1995;Watsa et al, 2017), but it is also possible that group size and parasite diversity cannot be linked within the Callitrichidae. Alternatively, the majority of parasite taxa in this study are trophically transmitted, rather than directly transmitted, which may not be influenced by group size to the same extent.…”

Vocal Transmission of Breeding Status May Facilitate Dispersal in a Cooperative Breeding Primate

“…Breeding status was determined using a model based on morphological and behavioral observations (see Watsa et al, 2017 for a detailed description of the methodology). In brief, primary breeding females were initially identified by nipple lengths that indicated parity, and primary breeding males were identified according to whether researchers had noted copulations during behavioral observations.…”

“…During mark-recapture events, morphological measurements-including vulva length/width, testicular length/width, and body mass-were taken on all individuals. These measurements from individuals with known breeding statuses were used to train a model which then assigned breeding status to the rest of the individuals in the population based on variation in nipple lengths, body mass, vulvar index and testicular volume (Watsa, Erkenswick, & Robakis 2017).…”

“…Often, secondary breeders are the undispersed offspring of the social group (Nievergelt et al, 1999). The remainder of a group consists of non-breeders, or sexually immature offspring of the primary breeders (Koenig & Dickinson, 2016;Watsa et al, 2017).…”