Medial amygdala ERα expression influences monogamous behaviour of male prairie voles in the field

Formation of long-term pair-bonds is a complex process, involving multiple neural circuits and is context- and experience-dependent. While laboratory studies using prairie voles have identified the involvement of several neural mechanisms, efforts to translate these findings into predictable field outcomes have been inconsistent at best. Here we test the hypothesis that inhibition of oestrogen receptor alpha (ERα) in the medial amygdala of male prairie voles would significantly increase the expression of social monogamy in the field. Prairie vole populations of equal sex ratio were established in outdoor enclosures with males bred for high levels of ERα expression and low levels of prosocial behaviour associated with social monogamy. Medial amygdala ERα expression was knocked down in half the males per population. Knockdown males displayed a greater degree of social monogamy in five of the eight behavioural indices assessed. This study demonstrates the robust nature of ERα in playing a critical role in the expression of male social monogamy in a field setting.

The role of personality traits in pair bond formation: pairing is influenced by the trait of exploration

In species with long-term pair bonds, such as zebra finches, evaluating the quality of potential mates is critically important. Courtship is an opportunity to evaluate information from dynamic behavioural cues. Personality traits, as stable individual differences in behaviour, could predict the quality of a potential mate. How might personality traits influence mate choice? We examined the influence of several personality traits, including exploration, aggression, and social preference, on pair formation in zebra finches. We provided birds with a variety of potential mates and allowed them to select a pair partner. Our semi-naturalistic mate choice paradigm allowed birds to observe social information over an extended period, simulating the challenges of social evaluation that birds encounter in the wild. We found that pairing is influenced by personality, with birds selecting mates similar to them in exploration. The partner’s exploration score relative to their own was more important than the absolute exploration score.

Mother-in-law daughter-in-law conflict: An evolutionary perspective, ethnographic review, and report of empirical data from the United States

Relationships with genetic relatives have been extensively studied in the evolutionary social sciences, but affinal relationships have received much less attention, and virtually no work has examined both cooperation and conflict among affines from an evolutionary perspective. Yet humans have extensive interactions with the kin of their mates, i.e., in-laws or affines, as humans form long-term pair bonds with mates, with both sexes investing heavily in resulting offspring, thus leading to many opportunities for interacting with extended kinship networks. To begin to address the gap in scholarship on affinal bonds, and particularly on affinal conflict, we conducted an ethnographic review and collected empirical data on (cooperation and) conflict among affines. Here we present (1) a sample 37 of ethnographies showing cross-cultural evidence of conflict in affinal relationships. We also report (2) empirical evidence of self-reported (cooperative and) conflictual aspects in affinal relationships in a Western sample. U.S. men and women both reported more conflict with mothers-in-law than with mothers, and mothers reported more conflict with their daughters-in-law than with their daughters. We discuss the implications of this work and directions for future research.

Observation of female-male mounting in the carrion crow

We report an observation of a female carrion crow, Corvus corone corone, mounting her long-term, pair-bonded, male partner. The report highlights the importance of more systematic quantitative studies of rare socio-sexual behaviours, which could provide important insights into the evolution of non-conceptive socio-sexual behaviours.

Why do females sing?—pair communication and other song functions in eastern bluebirds

Female bird song has been underappreciated and understudied, especially in temperate species. Birdsong was originally thought to be a trait used primarily by male songbirds for mate attraction and male/male contest. However, ornithologists have long known that females sing in many tropical songbirds, often for similar functions to male song. Yet, studies of female song in temperate regions remain scarce. Increasing our understanding of the function of female song in temperate species is a powerful step towards discerning the selective pressures that maintain elaborate female signals. In the last few decades, studies of temperate species have highlighted five major functional categories of female song. Using a modeling framework, based on all known functions of song in other species, we tested the function of female song in eastern bluebirds. The modeling framework allowed us to test the effect of multiple complex behaviors simultaneously to predict female song function. Additionally, modeling mitigated issues of multiple testing across the five different functional categories. We found that female song in eastern bluebirds is primarily used in pair communication. Specifically, females sing to strengthen and maintain long-term pair bonds. Strengthening pair-bonds may be advantageous for eastern bluebirds as pairs that remain together between nesting attempts and between years have higher reproductive success. We demonstrate a clear link between the function of female song in pair communication and the likely selective force of long-term pair bonds acting on eastern bluebird reproductive success. Additionally, our study highlights a major function of female song in a temperate species.

Development of new food-sharing relationships among nonkin vampire bats

In an individualized animal society, social bonds can foster cooperation and enhance survival and reproduction. Cooperative bonds often exist among kin, but nonkin can also develop high-investment cooperative bonds that share similarities with human friendship. How do such bonds form? One theory suggests that strangers should ‘test the waters’ of a new relationship by making small initial cooperative investments and gradually escalating them with good partners. This ‘raising-the-stakes’ strategy is demonstrated by human strangers in short-term economic games, but it remains unclear whether it applies to helping in a natural long-term social bond. Here we show evidence that unfamiliar vampire bats (Desmodus rotundus) selectively escalate low-cost investments in allogrooming before developing higher-cost food-sharing relationships. We introduced females from geographically distant sites in pairs or groups and observed that bats established new reciprocal grooming relationships, and that increasing grooming rates predicted the occurrence of first food donations, at which point grooming rates no longer increased. New food-sharing relationships emerged reciprocally in 14% of female pairs, typically over 10-15 months, and developed faster when strangers lacked alternative familiar partners. A gradual grooming-to-sharing transition among past strangers suggests that ‘raising the stakes’ might be more evident when tracking multiple cooperative behaviours as new relationships form, rather than measuring a single behavior in an established relationship. ‘Raising the stakes’ could play a similar underappreciated role across a broader spectrum of social decisions with long-term consequences, such as joining a new social group or forming a long-term pair-bond.Significance statementVampire bats form long-term cooperative social bonds that involve reciprocal food sharing. How do two unrelated bats go from being strangers to having a high-investment food-sharing relationship? We introduced unfamiliar bats and found evidence that low-cost grooming paves the way for higher-cost food donations. Food sharing emerged in a reciprocal fashion and it emerged faster when two strangers did not have access to their original groupmates. The bats that formed new food-sharing relationships had a history of escalating reciprocal grooming up until the food sharing began. Our finding that unfamiliar nonkin vampire bats appear to gradually and selectively transition from low-cost to high-cost cooperative behaviors is the first evidence that nonhuman individuals ‘raise the stakes’ when forming new cooperative relationships.

Plasmid transfer is biased towards close kin in bacteria from natural populations

Plasmids play a key role in microbial ecology and evolution, yet the determinants of plasmid transfer rates are poorly understood. Here we investigate the importance of genetic similarity between naturally co-occurringEscherichia coliisolates in the transfer of two plasmids (narrow-host-range R1 and broad-host-range RP4). We uncovered extensive variability, spanning over five orders of magnitude, in the ability of isolates to donate and receive plasmids. Overall, transfer was strongly biased towards clone-mates, but not correlated to genetic distance between donors and recipients. Transfer was limited by the presence of a functional restriction-modification system in recipients, thus bias towards kin might be explained by sharing of identical restriction systems. Such conjugation within lineages sets the stage for longer-term pair-wise coevolutionary interactions between plasmids and bacterial hosts.

Monogamous Mating System in the Hill Mynah Gracula Religiosa: The Role of Female–Female Aggression

This study aimed to discover whether the Hill Mynah Gracula religiosa forms pairs for a single breeding season or engages in long-term pair bonding, and to investigate the role of female aggression in maintaining pair bonds. Experiments were conducted on captive birds in Thailand between 2006 and 2013, and pair behaviours such as allopreening and aggression were observed four times every day. In the first experiment, when each breeding pair was preparing to rear a new clutch, pairs were placed in a new aviary with unfamiliar males and females with which they could potentially form relationships. Results indicated that breeding pairs stayed with the same mates over four continuous breeding seasons (2006–2009) without changing partners or taking additional mates. In a second experiment, the role of female aggression in maintaining long-term pair bonding was examined. Each breeding pair, when preparing to rear a new clutch, was placed in a new aviary with three unfamiliar females. The results showed that female breeders displayed vigorous aggression towards the other three females, forcing them to retreat from the pair; this behaviour was sustained over four continuous breeding seasons (2010–2013). Thus, it is suggested that the Hill Mynah engages in long-term pair bonding and that the aggression of the breeding female is an essential component in the maintenance of pair-bonds.