Phylogenetic approach to the study of tamarin and marmoset social systems

Female Dispersion Is Necessary, but Not Sufficient, for Pairbonded Monogamy in Mammals

Explanations for the evolution of social monogamy in mammals typically emphasise one of two possibilities: females are overdispersed (such that males cannot defend access to more than one female at a time) or males provide a service to the female. However, the first claim has never been formally tested. I test it directly at three levels using population-level data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygynous genera. Second, using two indices of territorial defendability, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5–10 females if they ranged solitarily. Finally, I use a model of male mate searching strategies to show that the opportunity cost incurred by pairbonded males is typically 5–10 times the reproductive success they actually obtain by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable. At present, the evidence is undecided as to whether mitigating predation or infanticide risk is the primary function, but estimates of their impacts suggest that both are in fact plausible.

What isn't social tolerance? The past, present, and possible future of an overused term in the field of primatology

In the past four decades, the term social tolerance has been utilized to describe, explain, and predict many different aspects of primates' sociality and has been measured with a large range of traits and behaviors. To date, however, there has been little discussion on whether these different phenomena all reflect one and the same construct. This paper opens the discussion by presenting the historical development of the term social tolerance and a structured overview of its current, overextended use. We argue that social tolerance has developed to describe two distinct concepts: social tolerance as the social structure of a group and social tolerance as the dyadic or group-level manifestation of tolerant behaviors. We highlight how these two concepts are based on conflicting theoretical understandings and practical assessments. In conclusion, we present suggestions for future research on primate social tolerance, which will allow for a more systematic and comparable investigation of primate sociality.

Social complexity and the fractal structure of group size in primate social evolution

Compared to most other mammals and birds, anthropoid primates have unusually complex societies characterised by bonded social groups. Among primates, this effect is encapsulated in the social brain hypothesis: the robust correlation between various indices of social complexity (social group size, grooming clique size, tactical behaviour, coalition formation) and brain size. Hitherto, this has always been interpreted as a simple, unitary relationship. Using data for five different indices of brain volume from four independent brain databases, we show that the distribution of group size plotted against brain size is best described as a set of four distinct, very narrowly defined grades which are unrelated to phylogeny. The allocation of genera to these grades is highly consistent across the different data sets and brain indices. We show that these grades correspond to the progressive evolution of bonded social groups. In addition, we show, for those species that live in multilevel social systems, that the typical sizes of the different grouping levels in each case coincide with different grades. This suggests that the grades correspond to demographic attractors that are especially stable. Using five different cognitive indices, we show that the grades correlate with increasing social cognitive skills, suggesting that the cognitive demands of managing group cohesion increase progressively across grades. We argue that the grades themselves represent glass ceilings on animals' capacity to maintain social and spatial coherence during foraging and that, in order to evolve more highly bonded groups, species have to be able to invest in costly forms of cognition.

Prenatal Androgen Effects as a Proximate Mechanism Underpinning Variation in Social Behavior Among Female Nonhuman Primates

While the role of ecological factors in shaping primate social systems has been a central focus for decades, less attention has been given to phylogenetic relationships and the potential role of underlying proximate mechanisms. This study aimed to investigate the relationship between one such proximate mechanism, prenatal androgen effects (PAEs), and aspects of social behavior in female nonhuman primates using the 2D:4D ratio as a proxy for PAEs and phylogenetically controlled methods. In general, female 2D:4D ratios were highest in monogamous species (low inferred PAEs) and lowest in polygynandrous and polygynous species (high inferred PAEs). 2D:4D ratios also varied with the form of polygyny/polygynandry, potentially with regard to the need for competitive over cooperative behaviors and the intensity of female reproductive competition. Species characterized by female dominance had lower 2D:4D ratios than species characterized by male dominance or codominance. There were no significant relationships between 2D:4D ratio and either degree of frugivory or group size. Relationships between 2D:4D ratios and the directional consistency index and 2D:4D ratios and rates of female–female agonism were also nonsignificant although sample sizes for both of these variables were small. Female social relationships are a manifestation of complex competitive and cooperative behaviors and the results suggest that PAEs may act as a proximate mechanism underlying the expression of certain aspects of behavior in female primates in ways that are adaptive to their social system.

Phenotypic evolution in marmoset and tamarin monkeys (Cebidae, Callitrichinae) and a revised genus-level classification

Marmosets and tamarins (Cebidae, Callitrichinae) constitute the most species-rich subfamily of New World monkeys and one of the most diverse phenotypically. Despite the profusion of molecular phylogenies of the group, the evolution of phenotypic characters under the rapidly-emerging consensual phylogeny of the subfamily has been little studied, resulting in taxonomic proposals that have limited support from other datasets. We examined the evolution of 18 phenotypic traits (5 continuous and 13 discrete), including pelage, skull, dentition, postcrania, life-history and vocalization variables in a robust molecular phylogeny of marmoset and tamarin monkeys, quantifying their phylogenetic signal and correlations among some of the traits. At the family level, our resulting topology supports owl monkeys (Aotinae) as sister group of Callitrichinae. The topology of the callitrichine tree was congruent with previous studies except for the position of the midas group of Saguinus tamarins, which placement as sister of the bicolor group did not receive significant statistical support in both Maximum Parsimony and Bayesian Inference analyses. Our results showed that the highest value of phylogenetic signal among continuous traits was displayed by the long call character and the lowest was exhibited in the home range, intermediate values were found in characters related to osteology and skull size. Among discrete traits, pelage and osteology had similar phylogenetic signal. Based on genetic, osteological, pelage and vocalization data, we present an updated genus-level taxonomy of Callitrichinae, which recognizes six genera in the subfamily: Callimico, Callithrix, Cebuella, Mico, Leontopithecus and Saguinus. To reflect their phenotypic distinctiveness and to avoid the use of the informal "species group", we subdivided Saguinus in the subgenera Leontocebus, Saguinus and Tamarinus (revalidated here).

Can females gain extra paternal investment by mating with multiple males? A game theoretic approach

Although females may require only one mating to become inseminated, many female animals engage in costly mating with multiple males. One potential benefit of polyandrous mating is gaining parental investment from multiple males. We developed two game theoretic models to explore this possibility. Our first model showed that male care of multiple females' offspring evolves when male help substantially increases offspring fitness, future mating opportunity is limited, and group size is small. In our second model, we assumed that males invest in the offspring of former mates and evaluated the fitness consequences of female monogamous and polyandrous mating strategies. Females benefit only from limited polyandry, that is, mating with several males. Polyandry is discouraged because females must share male investment with other polyandrous females, and paternal care is likely to experience diminishing returns. Females may enhance their access to male investment by competing with rival females and monopolizing investment, however. The results support the argument that females can gain paternal investment by mating with several males in small social groups (e.g., dunnocks Prunella modularis). The results do not support the argument that females can gain paternal investment from pronounced multiple mating in large social groups, however, as observed in many primate species.

Food transfers to young and mates in wild owl monkeys (Aotus azarai)

Accounts of food sharing within natural populations of mammals have focused on transfers to offspring or transfers of food items that are difficult to obtain (such as meat). Five groups of socially monogamous owl monkeys (Aotus azarai azarai) in Formosa, Argentina were observed during 107 hr to determine the pattern of food sharing under natural conditions. There were a total of 42 social interactions involving food with food being transferred on eight occasions. Adult males transferred food to young more often than did adult females. All types of food that were readily obtained and eaten by all age/sex classes were transferred to young. Adult females also transferred food to their mates. This type of food sharing is very rare among animals and may have social benefits specific to monogamous mammals with paternal care.

Changes in experimental conditions alter anti-predator vigilance and sequence predictability in captive marmosets

Anti-predator vigilance and its sequence predictability in captive adult male black tufted-ear marmosets (Callithrix penicillata) was analyzed under three experimental conditions: (1) four novel (maze) environment habituation trials; (2) six taxidermized (cat) predator confrontation trials in the same maze environment; and (3) four additional maze-only trials, in the absence of the cat stimulus. Significant quantitative and qualitative differences (aerial vs. terrestrial) were observed in scan and glance behaviors within and between the three experimental conditions. Furthermore, inter-scan bout sequence significantly deviated from randomness during the initial maze habituation and predator confrontation trials, as well as during all predator removal trials. This parameter, however, followed a random pattern during the course of the remaining sessions. Therefore, vigilance in marmosets seems to be an important and highly organized component of this species' anti-predation repertoire, inasmuch as it occurs at high rates, alters according to specific environmental cues and has a subtle differential adaptive response after repeated trials.

Molecular cladistic markers in New World monkey phylogeny (Platyrrhini, Primates)

Transpositions of primate-specific Alu elements were applied as molecular cladistic markers in a phylogenetic analysis of South American primates. Seventy-four human and platyrrhine loci containing intronic Alu elements were PCR screened in various New World monkeys and the human outgroup to detect the presence of orthologous retrotransposons informative of New World monkey phylogeny. Six loci revealed size polymorphism in the amplification pattern, indicating a shared derived character state due to the presence of orthologous Alu elements confirmed by subsequent sequencing. Three markers corroborate (1) New World monkey monophyly and one marker supports each of the following callitrichine relationships: (2) Callithrix and Cebuella are more closely related to each other than to any other callitrichine, (3) the callitrichines form a monophyletic clade including Callimico, and (4) the next living relatives to the callitrichines are Cebus, Saimiri, and Aotus.

Grandmother hypothesis and primate life histories

The adaptive significance of midlife menopause in human females has long engaged the attention of evolutionary anthropologists. In spite of extensive debate, the problem has only recently been examined in the context of primate life histories. Here I extend those investigations by comparing life history traits in 16 primate species to test predictions generated from life history theory. In humans, late ages of maturity and higher than expected birth rates are systematically associated with extended postmenopausal longevity. Links among these adjustments on the primate pattern can explain how selection could slow somatic senescence without favoring extension of the fertile span. This conclusion is consistent with the observation that our fertile spans are similar to those of other pongids. The shape of the argument herein demonstrates the utility of life history theory for solving problems of adaptive evolution in female life history traits, with consequences for broader arguments regarding human evolution.

Phylogenetic analysis of twinning in callitrichinae

The callitrichines are known for twinning and for a communal rearing system in which all or most group members help care for the offspring. The origin of twinning has been the subject of much speculation. In this study predictions from earlier hypotheses are tested on the basis of two alternative phylogenetic trees. From this analysis we infer that helping behavior and male care preceded the origin of twinning, and that these traits did not coevolve with, but might have been important prerequisites for twinning in callitrichines. Small body size does not necessarily result in twinning, although it might still have been a prerequisite for its evolution. Gum feeding was an ecological change which evolved along with twinning. If nutrition was a limiting factor in the number of off-spring produced, then the use of a new feeding resource could have been crucial for the origin of twinning in callitrichines. According to one of the two alternative solutions inferred by the total evidence tree, and in accordance with the morphological tree, semi-annual breeding appears in the marmosets together with specialization in gum feeding. The fact that gums are available for these monkeys all year may have facilitated semiannual breeding. We suggest that the exploitation of gums as a feeding resource could have been the decisive factor in the increase of the reproductive rate by twinning and by semi-annual breeding.

Proposed chromosomal phylogeny for the South American primates of the Callitrichidae family (Platyrrhini)

Cytogenetic and cytotaxonomic studies (G, C, sequential G/C, and NOR banding) were performed on 110 specimens representing the four genera of South American primates of the family Callitrichidae: Cebuella (C. pygmaea), Callithrix, groups argentata (C. argentata, C. emiliae, C. chrysoleuca, C. humeralifera, C. mauesi), and jacchus (C. aurita, C. geoffroyi, C. jacchus, C. kuhli, C. penicillata), Leontopithecus (L. chrysomelas, L. rosalia), and Saguinus (S. midas midas, S. m. niger). Mitotic chromosomes are characterized, and the rearrangements distinguishing the karyotypes of the taxa are inferred from arm homologies. The results were then converted into numerical data and submitted to cladistic analysis. The following conclusions were achieved: 1) Five karyotypic classes were observed, which correspond to the five taxa studied. Differences between them are as follows: a) Cebuella (2n = 44, 10 acrocentrics, A + 32 bi-armed autosomes, bi) and the argentata group (2n = 44, 10A + 32bi) are different from each other due to a reciprocal translocation; b) both can be distinguished from the jacchus group (2n = 46, 14A + 30bi) by a centric fusion/fission rearrangement and a paracentric inversion; c) Leontopithecus (2n = 46, 14A + 30bi) and Saguinus (2n = 46, 14A + 30bi) differ from the jacchus group by a reciprocal translocation and three paracentric inversions; and d) Saguinus is different from the others by one paracentric inversion and pericentric inversions in at least four pairs of acrocentric autosomes. 2) The cladistic analysis separates Cebus (used as an outgroup) from the Callitrichidae groups, which forms a clade. Among the Callitrichidae, marmosets (Cebuella and Callithrix) form a sub-clade, Cebuella and the argentata group being more closely related to each other than both are to the jacchus group. Tamarins (Leontopithecus and Saguinus) are also quite close, so that if one was not derived from the other, they with the marmosets share a common ancestor. Among the tamarins, Leontopithecus is karyotypically closest to the marmosets, specifically to the jacchus group. 3) Based on the chromosome information and considering the possible direction of the evolutionary changes (primitivity or phyletic dwarfism hypothesis, previously advanced by other authors), it was possible to propose the ancestral karyotypes and to develop two alternatives for the origin, differentiation and dispersion of the callitrichid. Both proposals are plausible, but when the geographical distribution is considered, the phyletic dwarfism hypothesis seems to be the most probable.