How much multiple paternity should we expect? A study of birds and contrast with mammals

Parentage analyses via molecular markers have revealed multiple paternity within the broods of polytocous species, reshaping our understanding of animal behavior, ecology, and evolution. In a meta-analysis of multiple paternity in bird and mammal species, we conducted a literature search and found 138 bird and 64 mammal populations with microsatellite DNA paternity results. Bird populations averaged 19.5% multiple paternity and mammals more than twice that level (46.1%). We used a Bayesian approach to construct a null model for how multiple paternity should behave at random among species, under the assumption that all mated males have equal likelihood of siring success, given mean brood size and mean number of sires. We compared the differences between the null model and the actual probabilities of multiple paternity. While a few bird populations fell close to the null model, most did not, averaging 34.0-percentage points below null model predictions; mammals had an average probability of multiple paternity 13.6-percentage points below the null model. Differences between bird and mammal species were also subjected to comparative phylogenetic analyses that generally confirmed our analyses that did not adjust for estimated historical relationships. Birds exhibited extremely low probabilities of multiple paternity, not only compared to mammals but also relative to other major animal taxa. The generally low probability of multiple paternity in birds might be produced by a variety of factors, including behaviors that reflect sexual selection (extreme mate guarding or unifocal female choice) and sperm competition (e.g., precedence effects favoring fertilization by early or late matings).

Female behavior drives the formation of distinct social structures in C57BL/6J versus wild-derived outbred mice in field enclosures

BACKGROUND

Social behavior and social organization have major influences on individual health and fitness. Yet, biomedical research focuses on studying a few genotypes under impoverished social conditions. Understanding how lab conditions have modified social organizations of model organisms, such as lab mice, relative to natural populations is a missing link between socioecology and biomedical science.

RESULTS

Using a common garden design, we describe the formation of social structure in the well-studied laboratory mouse strain, C57BL/6J, in replicated mixed-sex populations over 10-day trials compared to control trials with wild-derived outbred house mice in outdoor field enclosures. We focus on three key features of mouse social systems: (i) territory establishment in males, (ii) female social relationships, and (iii) the social networks formed by the populations. Male territorial behaviors were similar but muted in C57 compared to wild-derived mice. Female C57 sharply differed from wild-derived females, showing little social bias toward cage mates and exploring substantially more of the enclosures compared to all other groups. Female behavior consistently generated denser social networks in C57 than in wild-derived mice.

CONCLUSIONS

C57 and wild-derived mice individually vary in their social and spatial behaviors which scale to shape overall social organization. The repeatable societies formed under field conditions highlights opportunities to experimentally study the interplay between society and individual biology using model organisms.

Social evolution in mammals

[Figure: see text].

Parental investment in the Columbian ground squirrel: empirical tests of sex allocation models

Parental allocation of resources into male or female offspring and differences in the balance of offspring sexes in natural populations are central research topics in evolutionary ecology. Fisher (Fisher, R. A. 1930. The genetical theory of natural selection, Clarendon Press, Oxford, UK) identified frequency-dependent selection as the mechanism responsible for an equal investment in the sexes of offspring at the end of parental care. Three main theories have been proposed for explaining departures from Fisherian sex ratios in light of variation in environmental (social) and individual (maternal condition) characteristics. The Trivers-Willard model (Trivers, R., and D. Willard. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90-92) of male-biased sex allocation by mothers in the best body condition is based on the competitive ability of male offspring for future access to mates and thus superior reproduction. The local resource competition model is based on competitive interactions in matrilines, as occur in many mammal species, where producing sons reduces future intrasexual competition with daughters. A final model invokes advantages of maintaining matrilines for philopatric females, despite any increased competition among females. We used 29 yr of pedigree and demographic data to evaluate these hypotheses in the Colombian ground squirrel (Urocitellus columbianus), a semisocial species characterized by strong female philopatry. Overall, male offspring were heavier than female offspring at birth and at weaning, suggesting a higher production cost. With more local kin present, mothers in the best condition biased their offspring sex ratio in favor of males, and mothers in poor condition biased offspring sex ratio in favor of females. Without co-breeding close kin, the pattern was reversed, with mothers in the best condition producing more daughters, and mothers in poor condition producing more sons. Our results do not provide strong support for any of the single-factor models of allocation to the sexes of offspring, but rather suggest combined influences of relative maternal condition and matriline dominance on offspring sex ratio.

Ranging behavior and the potential for territoriality in pair-living titi monkeys (Plecturocebus discolor)

Patterns of ranging behavior and space use are key for evaluating current ideas about the evolution and maintenance of pair-living and sexual monogamy as they provide insights into the dispersion of females, the potential for territoriality, and whether males are limited to defending an area that can support only one female and her offspring. We examined ranging behavior and space use to evaluate the potential for territoriality in five groups of red titi monkeys (Plecturocebus discolor) during a 10-year study in Ecuadorian Amazonia. Mean home range size, calculated using a time-sensitive local convex hull estimation procedure, was 4.0 ± 1.4 ha. Annual home ranges of neighboring groups overlapped, on average, 0%-7%. Mean daily path length was 670 ± 194 m, resulting in defendability indices of 2.2-3.6 across groups. Groups visited, on average, 4 of 12 sections of their home range border area per day, but that was not more often than would be expected by chance, and intergroup encounters were infrequent. We did not find evidence of active monitoring for intruders in border areas, in that groups did not travel either faster or slower when at the border than when in central areas of their range. The absence of overt monitoring might be compensated for by engaging in loud calls, which the study groups did throughout their home ranges; these calls may serve as an advertisement of occupancy and a deterrent to intruding conspecifics. Our finding that red titis have a high potential for territoriality is consistent with several of the main hypotheses proposed to explain pair-living in mammals.

Evolutionary transitions toward pair living in nonhuman primates as stepping stones toward more complex societies

Nonhuman primate societies vary tremendously in size and composition, but how and why evolutionary transitions among different states occurred remains highly controversial. In particular, how many times pair living evolved and the social states of the ancestors of pair- and group-living species remains contentious. We examined evolutionary transitions in primate social evolution by using new, independent categorizations of sociality and different phylogenetic hypotheses with a vastly expanded dataset. Using Bayesian phylogenetic comparative methods, we consistently found the strongest support for a model that invokes frequent transitions between solitary ancestors and pair-living descendants, with the latter giving rise to group-living species. This result was robust to systematic variation in social classification, sample size, and phylogeny. Our analyses therefore indicate that pair living was a stepping stone in the evolution of structurally more complex primate societies, a result that bolsters the role of kin selection in social evolution.

High Frequency of Multiple Paternity in Eastern Red Bats, Lasiurus borealis, Based on Microsatellite Analysis

Most species of bats give birth to only 1 pup each year, although Eastern red bats (Lasiurus borealis) can produce up to 5 pups per litter. Offspring in a single litter have been documented to be at different stages of development, suggesting that multiple paternity occurs. We tested the null hypothesis of genetic monogamy in red bats using 6 autosomal microsatellites and 1 X-linked microsatellite from 31 parent/offspring groups for a total of 128 bats. We sampled both pregnant females and mothers with pups that were obtained from bats submitted to departments of health in Oklahoma and Texas for rabies testing. Multiple paternity was assessed using a maximum-likelihood approach, hypothesis testing, and X-linked locus exclusion. The mean polymorphic information content of our markers was high (0.8819) and combined non-exclusion probability was low (0.00027). Results from the maximum-likelihood approach showed that 22 out of 31 (71%) parent/offspring groups consisted of half siblings, hypothesis testing rejected full sibship in 61% of parent/offspring groups, and X-linked locus exclusion suggested multiple paternity in at least 12 parent/offspring groups, rejecting our hypothesis of genetic monogamy. This frequency of multiple paternity is the highest reported thus far for any bat species. High levels of multiple paternity have the potential to impact interpretations of genetic estimates of effective population size in this species. Further, multiple paternity might be an adaptive strategy to allow for increased genetic variation and large litter size, which would be beneficial to a species threatened by population declines from wind turbines.

Multiple paternity and number of offspring in mammals

Many cooperative social attributes are being linked to characteristics of mating systems, particularly to the rate of multiple paternity that typifies a population. Under the logic that greater offspring production by females should engender greater competition among males to mate with females, it is predicted that multiple paternity should increase with litter sizes. We tested the predicted positive association of multiple paternity and litter size with a meta-analysis of 59 species of mammals. The probability of multiple paternity and mean litter size were positively correlated, but not significantly (Zr = 0.202). Also, the mean number of sires of litters increased with mean litter size, but not significantly (Zr = 0.235). We developed a combinatorial formula for the influence of number of male mates and litter size on the probability of multiple paternity. We used Bayesian Markov chain Monte Carlo simulations to generate an expectation for the form of the relationship between the probability of multiple paternity and mean litter size. Under the assumption of random samplings of numbers of mates, the expected association of the probability of multiple paternity and mean litter sizes among species was positive, curvilinear and relatively high. However, the empirical probabilities of multiple paternities were much less than expected, suggesting that behavioural factors (such as mating-associated behaviours) or ecological characteristics (such as population density) probably limit the number of male mates for reproductive females. The probability of multiple paternity in a population is an estimate of mating patterns that does not closely reflect the number of sires of individual litters. We suggest use of the estimated probability of mating success for males as an alternative measure of their contribution to the mating system.

Female dispersion and sex ratios interact in the evolution of mating behavior: a computational model

The evolution of mating strategies is not well understood. Several hypotheses have been proposed to explain the variation in mating strategies, with varying levels of support. Specifically, female dispersion, adult sex ratio and mate guarding have been proposed as drivers of the evolution of monogamous strategies. In this study, we used an agent-based model (ABM) to examine how different mating behaviors evolve in a population under different conditions related to these putative drivers, looking to understand the interaction between them. We found an interaction among different factors in the evolution of social monogamy, and their impact is in this order: adult sex ratio (ASR), female dispersion and extra-pair copulation. Thus, when the adult sex ratio is male-biased, monogamous strategies are strongly favored. However, this is only the case if mate guarding is fully efficient, i.e., if there is no extra-pair copulation. On the other hand, in scenarios where the population is female-biased, or mate guarding is not efficient, we find that polygamous strategies are favored but proportionally to the dispersion of females. These results confirm previous findings regarding mate guarding and sex ratios, while also showing how female dispersion enters the dynamics.

To pair or not to pair: Sources of social variability with white-faced saki monkeys (Pithecia pithecia) as a case study

Intraspecific variability in social systems is gaining increased recognition in primatology. Many primate species display variability in pair-living social organizations through incorporating extra adults into the group. While numerous models exist to explain primate pair-living, our tools to assess how and why variation in this trait occurs are currently limited. Here I outline an approach which: (i) utilizes conceptual models to identify the selective forces driving pair-living; (ii) outlines novel possible causes for variability in social organization; and (iii) conducts a holistic species-level analysis of social behavior to determine the factors contributing to variation in pair-living. A case study on white-faced sakis (Pithecia pithecia) is used to exemplify this approach. This species lives in either male-female pairs or groups incorporating "extra" adult males and/or females. Various conceptual models of pair-living suggest that high same-sex aggression toward extra-group individuals is a key component of the white-faced saki social system. Variable pair-living in white-faced sakis likely represents alternative strategies to achieve competency in this competition, in which animals experience conflicting selection pressures between achieving successful group defense and maintaining sole reproductive access to mates. Additionally, independent decisions by individuals may generate social variation by preventing other animals from adopting a social organization that maximizes fitness. White-faced saki inter-individual relationships and demographic patterns also lend conciliatory support to this conclusion. By utilizing both model-level and species-level approaches, with a consideration for potential sources of variation, researchers can gain insight into the factors generating variation in pair-living social organizations.

Social monogamy in wild owl monkeys (Aotus azarae) of Argentina: the potential influences of resource distribution and ranging patterns

Using published and new data from a population of monogamous owl monkeys in the Argentinean Chaco, I examine the hypothesis that social monogamy is a default social system imposed upon males because the spatial and/or temporal distribution of resources and females makes it difficult for a single male to defend access to more than one mate. First, I examine a set of predictions on ranging patterns, use of space, and population density. This first section is followed by a second one considering predictions related to the abundance and distribution of food. Finally, I conclude with a section attempting to link the ranging and ecological data to demographic and life-history parameters as proxies for reproductive success. In support of the hypothesis, owl monkey species do live at densities (7-64 ind/km(2) ) that are predicted for monogamous species, but groups occupy home ranges and core areas that vary substantially in size, with pronounced overlap of home ranges, but not of core areas. There are strong indications that the availability of food sources in the core areas during the dry season may be of substantial importance for regulating social monogamy in owl monkeys. Finally, none of the proxies for the success of groups were strongly related to the size of the home range or core area. The results I present do not support conclusively any single explanation for the evolution of social monogamy in owl monkeys, but they help us to better understand how it may function. Moreover, the absence of conclusive answers linking ranging, ecology, and reproductive success with the evolution of social monogamy in primates, offer renewed motivation for continuing to explore the evolution of monogamy in owl monkeys.

The evolution of social monogamy in mammals

The evolution of social monogamy has intrigued biologists for over a century. Here, we show that the ancestral condition for all mammalian groups is of solitary individuals and that social monogamy is derived almost exclusively from this social system. The evolution of social monogamy does not appear to have been associated with a high risk of male infanticide, and paternal care is a consequence rather than a cause of social monogamy. Social monogamy has evolved in nonhuman mammals where breeding females are intolerant of each other and female density is low, suggesting that it represents a mating strategy that has developed where males are unable to defend access to multiple females.

Social mating system and sex-biased dispersal in mammals and birds: a phylogenetic analysis

The hypothesis that patterns of sex-biased dispersal are related to social mating system in mammals and birds has gained widespread acceptance over the past 30 years. However, two major complications have obscured the relationship between these two behaviors: 1) dispersal frequency and dispersal distance, which measure different aspects of the dispersal process, have often been confounded, and 2) the relationship between mating system and sex-biased dispersal in these vertebrate groups has not been examined using modern phylogenetic comparative methods. Here, we present a phylogenetic analysis of the relationship between mating system and sex-biased dispersal in mammals and birds. Results indicate that the evolution of female-biased dispersal in mammals may be more likely on monogamous branches of the phylogeny, and that females may disperse farther than males in socially monogamous mammalian species. However, we found no support for a relationship between social mating system and sex-biased dispersal in birds when the effects of phylogeny are taken into consideration. We caution that although there are larger-scale behavioral differences in mating system and sex-biased dispersal between mammals and birds, mating system and sex-biased dispersal are far from perfectly associated within these taxa.

Determinants of Pair-Living in Red-Tailed Sportive Lemurs (Lepilemur ruficaudatus)

Pair-living and a monogamous mating strategy are rare and theoretically unexpected among mammals. Nevertheless, about 10% of primate species exhibit such a social system, which is difficult to explain in the absence of paternal care. In this study, we investigated the two major hypotheses proposed to explain the evolution of monogamy in mammals, the female defence hypothesis (FDH) and the resource defence hypothesis (RDH), in red-tailed sportive lemurs (Lepilemur ruficaudatus), a nocturnal primate from Madagascar. We analysed behavioural data from eight male-female pairs collected during a 24-mo field study to illuminate the determinants of pair-living in this species. Male and female L. ruficaudatus were found to live in dispersed pairs, which are characterised by low cohesion and low encounter rates within a common home range. Social interactions between pair partners were mainly agonistic and characterised by a complete absence of affiliative interactions - body contact was only observed during mating. During the short annual mating season, males exhibited elevated levels of aggression towards mates, as well as extensive mate guarding and increased locomotor activity. In addition, males were exclusively responsible for the maintenance of proximity between pair partners during this period, and they defended their territories against neighbouring males but not against females. Together, these results point towards the importance of female defence in explaining pair-living in L. ruficaudatus. We discuss the spatial and temporal distribution of receptive females in relation to the female defence strategies of males and suggest possible costs that prevent male red-tailed sportive lemurs from defending more than one female.