Hormones and social behavior in the lizard, Anolis carolinensis

The purpose of this experiment was to study the effects of homologous and heterologous gonadal hormones on sexual and aggressive behavior in a reptilian species. Thirty adult male and thirty adult female lizards (Anolis carolinensis) were divided into 10 groups of six each (five groups per sex) and each group was given one of five treatments: either left intact, sham-castrated and injected with the hormone vehicle, castrated and injected with the hormone vehicle, castrated and injected with estradiol benzoate, or castrated and injected with testosterone propionate. After a week of visual isolation and daily hormone injection, animals were tested four times, twice with a stimulus animal of each sex. Females treated with estrogen were receptive, but did not court. Females treated with androgen were receptive and also courted and pursued stimulus females as frequently as males given androgen. No males in any group were receptive, and thus the female appears to be more capable of heterotypical sexual behavior than the male. Castrated males failed to court. Courtship and pursuit of stimulus females was readily stimulated in males with testosterone, and weakly stimulated by estrogen. Intact males were very aggressive, but lower levels of aggression were independent of gonadal hormones, as was subordination (head-nodding). The results for aggression and subordination are interpreted with reference to naturally-occurring Anolis behavior, and the results for sexual behavior are compared with similar experiments with mammals and birds.     

Body size as a factor determining dominance in staged agonistic encounters between male brown anoles (Anolis sagrei)

The results of staged agonistic encounters between males indicate that body size is an important determinant of dominance in male brown anoles (Anolis sagrei). Larger males defended their perch sites more successfully than did smaller males, perched higher than smaller males, and were most often the first male to enter the other male's territory. Larger males also exhibited more challenge displays than did smaller males. Head-nods, a display given by subordinate individuals, were observed only in smaller males.     

Rainfall,activity and social behaviour in the lizard, Anolis aeneus

The effects of drought periods on the behaviour of a tropical lizard (Anolis aeneus) were measured. The number of lizards active at study sites and display frequencies varied inversely with days since rainfall. Animals that were artificially watered during a drought drank and showed an immediate recovery to prior, non-drought display levels. Rainfall records indicate that drought periods of the duration studied here are frequent in the natural habitat.     

Dynamic changes in social dominance and mPOA GnRH expression in male mice following social opportunity

Social competence - the ability of animals to dynamically adjust their social behavior dependent on the current social context – is fundamental to the successful establishment and maintenance of social relationships in group-living species. The social opportunity paradigm, where animals rapidly ascend a social hierarchy following the removal of more dominant individuals, is a well-established approach for studying the neural and neuroendocrine mechanisms underlying socially competent behavior. In the current study, we demonstrate that this paradigm can be successfully adapted for studying socially competent behavior in laboratory mice. Replicating our previous reports, we show that male laboratory mice housed in a semi-natural environment form stable linear social hierarchies. Novel to the current study, we find that subdominant male mice immediately respond to the removal of the alpha male from a hierarchy by initiating a dramatic increase in aggressive behavior towards more subordinate individuals. Consequently, subdominants assume the role of the alpha male. Analysis of brain gene expression in individuals 1 h following social ascent indicates elevated gonadotropin-releasing hormone (GnRH) mRNA levels in the medial preoptic area (mPOA) of the hypothalamus compared to individuals that do not experience a social opportunity. Moreover, hormonal analyses indicate that subdominant individuals have increased circulating plasma testosterone levels compared to subordinate individuals. Our findings demonstrate that male mice are able to dynamically and rapidly adjust both behavior and neuroendocrine function in response to changes in social context. Further, we establish the social opportunity paradigm as an ethologically relevant approach for studying social competence and behavioral plasticity in mammals.     

Linear social dominance hierarchy and corticosterone responses in male mallards and pintails

In winter, dabbling ducks gather in large flocks and males compete not only for food resources but also for mates. Setting up a social hierarchy is one way to reduce the costs of conflicts, but the position in the hierarchy has implications for individuals, for instance their susceptibility to conflict and interference, hence to social stress. We investigated relationships between linear social dominance and corticosterone levels, baseline levels and induced-stress response, in captive male mallards Anas platyrhynchos and pintails Anas acuta during the winter period. We hypothesised that corticosterone responses would reflect the costs associated with social stress. From previous work on dominance and corticosterone in wintering birds, we expected that, where the social hierarchy is linear, there would be (1) no relationship between social ranks and baseline corticosterone levels and (2) a significant positive relationship between dominance ranks and responses to acute stress. Our results demonstrated the existence of a linear hierarchy in both species (h' = 0.95 for mallards and h' = 0.97 for pintails), and we found that pintails had on average more corticosterone than mallards. The relationship between dominance and corticosterone responses followed the predictions, with no differences for baseline levels and an attenuated response to induced-stress for subordinates. We discuss these results in the perspective of the cost-benefits of dominance and wintering strategies of waterbirds.     

The social component of dominance relationships in rhesus monkeys (Macaca mulatta)

An all-male rhesus group was added to a second group, one animal at a time, at weekly intervals, in reverse order of their dominance ranks. Significant positive correlations prevailed for the host group dominance ranks throughout the study, but the dominance ranks of the introduced males were random with respect to their original ranks. A second experiment repeatedly introduced the alpha and beta males of a multi-male heterosexual group into a second group where they were the lowest-ranking animals. On each return they reclaimed alpha and beta positions. When the alpha and beta males of the second group were introduced to the first group, they became the lowest-ranking animals in the group. Reversal of the direction of introduction again demonstrated that relative ranks were dependent on the social context.     

The influence of male dominance on social interactions in black ducks and mallards

The influence of male dominance on male-female social interactions was examined in black ducks, Anas rubripes, and mallards, A. platyrhynchos, that were raised from hatch in four different groups (male and female mallards; male and female black ducks; male mallards and female black ducks; male black ducks and female mallards). The mate preference of females, independent of the influence of male dominance, was determined by exposing females to four caged, isolated males, one from each different group. All females preferred the type of male they had been raised with since hatch. Females were then exposed to the same four males, now free-swimming, to determine how male dominance influenced the initial preference. Under these conditions, each female associated primarily with the most dominant of the four males, regardless of her initial mate preference. Mallards were the most dominant males in all tests. These results demonstrate that male dominance influences social interactions and is one factor contributing to hybridization between these two species.     

Female response to filmed displays of Anolis nebulosus (Sauria,Iguanidae)

A film loop apparatus was devised to test a possible social function of the display of Anolis nebulosus. Films of a displaying male were projected onto translucent screens at both ends of an enclosure. One film loop portrayed the display typical of the lizard's population, while the other loop projected an altered version of the basic display. Female A. nebulosus were introduced singly into the centre of the enclosure. They gave a graded response to the film loops; an increasing proportion of females approached the ‘normal’ displaying image as the film loops of altered displays contained a greater number of changes. The various experimental techniques previously used to study the function of lizard displays are reviewed.     

Social influences on female choice in green anole lizards (Anolis carolinensis)

We conducted an experiment on female Anolis carolinensis lizards to investigate whether social factors influenced their selection of an end-chamber in a test arena. We tested (1) whether characteristics of males previously seen in the end-chambers would influence female choice and (2) whether the presence of other females simultaneously choosing would influence choice. In experiment one, females observed a large and a small male in the end-chambers prior to choosing. Females were tested individually and in pairs. When tested individually, females preferred the end-chamber previously inhabited by the larger male. When females were tested in pairs, however, in each case one female chose the large male's end-chamber and the other female failed to make a choice. In experiment two, we conducted the same paired-choice test, but prior to the test we evaluated the dominance relationships between the pair of females. In the majority of cases, the more dominant female was the one to enter the large male's end-chamber. Results indicate that females are influenced by the presence and characteristics of males, but that female competition also plays a role in choice.     

Changes in the dominance rank and reproductive behaviour of male bonnet macaques (Macaca radiata)

The dominance rank of male bonnet macaques influenced their associations and sexual interactions with cycling and conceiving females. Only high-ranking males formed exclusive associations with high-ranking females, and high-ranking males copulated with these females more often than did other males. Changes in male dominance rank were directly related to changes in male mating behaviour, as males who rose in rank spent increasing amounts of time in exclusive proximity to conceiving high-ranking females, and males whose ranks declined spent decreasing amounts of time in proximity to such females. The year of the study in which dominance relationships were most stable was also the year in which the top-ranking male most successfully monopolized access to conceiving high-ranking females.     

Sexual conflict and the evolution of female mate choice and male social dominance

Conflicts between the sexes over control of reproduction are thought to lead to a cost of sexual selection through the evolution of male traits that manipulate female reproductive physiology and behaviour, and female traits that resist this manipulation. Although studies have begun to document negative fitness effects of sexual conflict, studies showing the expected association between sexual conflict and the specific behavioural mechanisms of sexual selection are lacking. Here we experimentally manipulated the opportunity for sexual conflict in the cockroach. Nauphoeta cinerea and showed that, for this species, odour cues in the social environment influence the behavioural strategies and fitness of males and females during sexual selection. Females provided with the opportunity for discriminating between males but not necessarily mating with preferred males produced fewer male offspring than females mated at random. The number of female offspring produced was not affected, nor was the viability of the offspring. Experimental modification of the composition of the males' pheromone showed that the fecundity effects were caused by exposure to the pheromone component that makes males attractive to females but also makes males less likely to be dominant. Female mate choice therefore carries a demographic cost but functions to avoid male manipulation and aggression. Male-male competition appears to function to circumvent mate choice rather than directly manipulating females, as the mate choice can be cryptic. The dynamic struggle between the sexes for control of mating opportunities and outcomes in N. cinerea therefore reveals a unique role for sexual conflict in the evolution of the behavioural components of sexual selection.     

Urine marking and social dominance in male house mice (Mus musculus domesticus)

House mice use urine marking for a variety of forms of social communication. Urine marking varies with dominance status; socially dominant male house mice urine mark more than those that are socially subordinate. Experiment I was designed to confirm this previous finding. Experiment II was designed to test whether urine marking, measured prior to testing males for aggression, could be used to predict social dominance. Mice were tested for urine marking in 20 cmx40 cm rectangular cages with filter paper below the wire mesh bottom of the cage. In Experiment I, groups of four males were tested in a round robin design to assess social dominance and were then placed individually in urine marking cages. Social dominance was a significant predictor of the number of 1 cm squares that contained urine marks, both with regard to interior squares and for perimeter squares in the test cage. In Experiment II, groups of four males were first tested individually in urine marking cages and then used for round robin aggressive encounters to assess social dominance. The number of interior squares with urine marks, and, to a lesser extent, the number of perimeter squares with urine marks, were both significant predictors of aggression scores and social dominance status. Being able to judge social dominance without having the mice encounter each other could be a valuable tool for future work; confounding effects on such parameters as hormone levels could be avoided while obtaining an estimate of male social dominance status.