Growing Up Tamarin: Morphology, Reproduction, and Population Demography of Sympatric Free-ranging Saguinus fuscicollis and S. imperator

The goal of the present study was to examine group structure, reproductive success and individual growth and development to assess two sympatric Saguinus spp. These data are evaluated in light of the reproductive system proposed for these callitrichids---strict polyandry, cooperative breeding, reproductive suppression, and single reproducing females---in the context of future genetic studies. As such, I attempted to achieve three specific goals---first, an examination of morphological variation between two sympatric callitrichid species; second, an evaluation of age and reproductive status per individual; and third, a comparison of group compositions, mating systems and dispersal patterns of both species to the expected callitrichid reproductive pattern.;I monitored two sympatric species of callitrichids in southeastern Peru at a field site named CICRA over the course of three years---57 animals in 7 groups of saddleback tamarins (Saguinus fuscicollis), and 36 animals in 6 groups of emperor tamarins (S. imperator). I used an annual capture and release protocol to individually identify all animals, and recorded 19 measures of morphology and dental condition for all subjects. Behavioral data on mating, dispersal, and monthly group-compositions were also obtained, with an emphasis on one focal group per species. I used dental morphology to estimate age for animals 1 y old via molar occlusal wear. I assessed changes in morphology with age and sex for both species, comparing these measures using Mann-Whitney U tests, and used reproductive morphology to assign breeding status to adults, identifying primary, secondary, and non-breeders for both sexes. I then identified the predominant mating systems based on the number and sex of breeding adults in a group for both species.;Age-structures predicted by dental wear did not differ significantly between species, or between the sexes for each species. Further sampling of younger adults in the study population is required to conduct population viability analyses. Adult Saguinus imperator are significantly heavier than adult S. fuscicollis, with and without pooled sexes. Among S. imperator, significantly thicker limbs and not an overall increase in body size account for this increased weight. No sexual dimorphism was recorded for either species, save in upper arm lengths among S. fuscicollis (slightly longer in females) and waist circumferences in S. imperator (slightly larger in females, but unrelated to pregnancy).;There are no significant differences between the species in the size of their genitalia or scent glands, despite differing markedly in their physical appearance (i.e shape and pigmentation). No significant effect of month was found on testicular volume, vulvar indices or suprapubic gland areas (Kruskal-Wallis rank sum test, p ≥ 0.05). Suprapubic scent gland areas are significantly higher among females than males for both species (e.g. in S. fuscicollis, gland area =267.5 +/- 143 mm for females and 117.1 +/- 72.4 mm for males). Vulvar indices explain ca. 70% of variation in suprapubic gland area for females of both S. fuscicollis (R2 = 0.70, P ≤ 0.001) and S. imperator (R 2 = 0.76, P ≤ 0.001), while testicular volumes explain suprapubic gland areas only among male S. fuscicollis (R2 = 0.63, P ≤ 0.001). Male S. imperator have undifferentiated glands in general, unrelated to age or breeding status. While vulvar indices initially appear to grow faster than testicular volumes in both species, by 1.5 years of age, males have fully developed genitalia while females still appear underdeveloped. Morphological scores assigned to genitalia and glands encompass their range of variation, and can be used to distinguish infants from adults, but not other age classes (scores are too variable among adults).;Twinning and strong birth seasonality were observed in both species, with overlapping birth peaks influenced by the environment occurring during the wet season (ca. September to March). Primary breeding males had higher testicular volumes than secondary breeding males. Secondary breeding females had lower vulvar indices and suprapubic scent gland areas than primary breeding females, and required 2--3 y to acquire secondary breeding status compared to 1 y for males.;Immigration events are significantly less common than emigration events, with breeding females having extended tenures in both species. Cold fronts known as friajes create breeding vacancies among groups, which were filled by individuals from outside the group. Overall, Saguinus imperator appears to be more similar in terms infant survivorship and mean reproductive output to both the CICRA and Cocha Cashu populations of S. fuscicollis, than it is to S. mystax at other long-term study sites. However, S. fuscicollis has a slightly higher reproductive output likely achieved by its polygynandrous mating system, maintained by reduced reproductive suppression, in which multiple females breed successfully in a group. (Abstract shortened by UMI.).