Phylogenetic analysis of twinning in callitrichinae

Heteropaternity of twins in ring-tailed lemurs (Lemur catta)

OBJECTIVES

This project investigated paternity among 14 sets of twins born into a colony of ring-tailed lemurs (Lemur catta) on St. Catherines Island, GA. Female L. catta commonly mate with >1 male, and twins of different sexes confirm that dizygotic twinning can occur in this species. However, no study has previously evaluated twins using genetic data to measure the proportion of mono versus dizygotic twinning in L. catta, and no study has tested for heteropaternity (sirehood by two different males) in this species. We predicted that L. catta would show a predominance of dizygosity and evidence for heteropaternity.

MATERIALS AND METHODS

Paternity was determined for 28 infants (14 sets of twins) via paternity exclusion analyses using 10 polymorphic microsatellite markers, and data were collected on sexual behavior across four mating seasons during the first cycle of mating in each year, which overlapped with the conceptive periods of these infants.

RESULTS

All twins were found to be dizygotic, as evidenced by dissimilar multilocus genotypes between the two infants of each twin pair. Heteropaternity was found in 3 of 14 (21% of) twin pairs, whereas the remaining 11 pairs of twins (79%) shared the same sire. In one case of heteropaternity, one sire was a resident male, and one sire was an extra-group male. An extra-group male also sired one of the same-sire twin pairs. All other sires of twins were group residents.

DISCUSSION

Female multiple mating, which is common to many primates including L. catta, can be viewed as an adaptive reproductive strategy whose function-in addition to fertility assurance or stimulating sperm competition-may be to increase the genetic diversity of a female's offspring, even among infants born in the same birth event. Therefore, during the course of primate evolution, the adaptive benefits of heteropaternity would be expected to have contributed to positive selection for female promiscuity.

Parallel episodes of phyletic dwarfism in callitrichid and cheirogaleid primates

The Callitrichidae are the smallest anthropoids, whereas the Cheirogaleidae include the smallest of all primates. Using species-level analyses, we show that these are derived conditions; both neonatal and adult body mass decreased in a gradual, phyletic manner in parallel across callitrichids, and across cheirogaleids. We identify lineages with particularly rapid decreases and highlight the pygmy marmoset, Callithrix pygmaea, as a phenotypic outlier. The life-history traits associated with body-mass reduction in each clade suggest that the convergent evolution of small body size was achieved by changes in different ontogenetic stages. Body-size reduction in callitrichids appears to be almost exclusively due to alterations in prenatal growth rate, whereas body-size reduction in cheirogaleids may have been largely due to reduced duration of growth phases. Finally, we use these results to discuss some of the debates surrounding the evolution of Homo floresiensis and suggest potential parallels between the evolution of H. floresiensis and callitrichids.

Different ovarian responses to potential mates underlie species-specific breeding strategies in common marmoset and Goeldi's monkey

Callitrichids are cooperative breeders, characterized by obligate twinning, extensive paternal care, and monopolization of reproduction by the dominant female. This is the case in the common marmoset, and in common marmoset groups of more than one adult female, subordinate females are typically acyclic consistent with infertility. However, one callitrichid, Goeldi's monkey, gives birth to singletons and exhibits low paternal care. Given these reproductive traits of Goeldi's monkey, we hypothesized that there would not be suppression of ovarian activity. To test this hypothesis, we applied non-invasive endocrine methods in a step-wise experiment with laboratory groups of both species. In each species, six pairs of sisters were studied alone, in visual contact with an unrelated male and in a polygynous trio with the male, and urine samples were collected for determination of oestrogen titres reflecting ovarian activity. Common marmoset sister pairs exhibited a marked difference in social status: during the study 5 of 6 dominant females conceived but only 1 of 6 subordinate females; the remaining 5 subordinates were acyclic at the end of the study, and instances of ovulation typically resulted in aggression. Goeldi's monkey sister pairs showed no status differences: in all pairs, however, both sisters exhibited a temporary cessation of ovarian cyclicity on trio formation, followed by ovulation and conception. We conclude that these marked differences in ovarian responses reflect the differences in inter-female competition for paternal caregiving resources. In common marmosets with high inter-female competition, suppression of ovulation functions to reduce aggression received by subordinate females; in Goeldi's monkey with low competition, temporary cessation of ovulation could facilitate female choice.

Costs of twins in free-ranging white-bellied spider monkeys (Ateles belzebuth belzebuth) at Tinigua National Park, Colombia

A female spider monkey (Ateles belzebuth belzebuth) that gave birth to twins was studied during 13 months, and her activity budget and diet were compared to those of females with single offspring and females with no offspring to assess selective pressures that could influence litter size. We recorded qualitative information on the development and social interactions of the twins and three other single infants. Emi, the female that had the twins, had the highest proportion of resting time and the lowest proportion of feeding and moving time compared to other adult females and males. Emi also had the lowest average daily travel distance and relied more heavily on flowers and leaves than other group members who included a higher proportion of fruits in their diets. These results suggest that twins are energetically costly to spider monkeys because of the direct energy allocated into raising and carrying the twins and, also, because these costs have direct effects in the ability of a mother to obtain high-quality resources (e.g., fruits). The twins had a slower rate of physical and social development as well as a smaller body size compared to same-aged singletons by the end of the study. Thus, the extended periods of maternal care and the costs associated with rearing and carrying offspring are some of the factors that influence the evolution and maintenance of a litter size of one in most anthropoid primates.