Reproductive suppression in female Alpine marmots, Marmota marmota

We studied mechanisms responsible for reproductive skew in female Alpine marmots. In each social group, only dominant females produced weaned young although subordinate females had similar body condition, were fertile as indicated by high levels of oestradiol during the mating season, and occasionally started pregnancies. During the period of gestation, dominant females initiated significantly more agonistic interactions against subordinate females, resulting in significantly increased levels of glucocorticoids and decreased levels of progesterone in subordinates. Results suggested that reproductive suppression in female Alpine marmots is mediated by the negative effects of stress (glucocorticoids) on the activity of the hypothalamic-pituitary-gonadal axis. The strength of competition between subordinate and dominant females was affected by their relatedness. Dominant females attacked unrelated subordinate females more, whereas amicable behaviour was observed mainly between dominant females and their daughters. These differences could be explained by differences in indirect fitness: related subordinate females benefited from warming the offspring of the dominant female during hibernation. Reproductive suppression was apparently costly for dominant females, because their reproductive success decreased as the number of adult subordinate females in a group increased. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

The Effects of Food Deprivation in Dyadic Encounters in Apodemus sylvaticus

The nature of dominance in relationships among male Apodemus sylvaticus was investigated using multivariate analysis and by experimental manipulation of the behaviour of dominants. The behaviour of dominant mice was typified by greater frequencies and durations of exploration, approach and pursuit of subordinates in short encounters in a small arena. The direction of encounters normally remained constant over 48 h. When the dominant mouse in a dyad was deprived of food there was a change in its behaviour and an immediate change in the behaviour of the subordinate. Subordinates showed greater interest in starving, though known, dominants, explored more freely and indulged more in pursuit than control subordinate mice i. e. those matched with dominants on an adequate diet. Results suggest that in A. sylvaticus, dominance is associated with the inhibition of the activity of a subordinate and the relationship is maintained by the behaviour of the dominant rather than the deference of the subordinate.     

Winter social behaviour of dark-eyed juncos: Communication,social organization,and ecological implications

This paper reports on winter social behaviour in free-living and captive flocks of dark-eyed juncos (Junco hyemalis). Visual and vocal signals used by flocked juncos are analysed structurally and functionally. An examination is made of the frequency and form of agonistic behaviour in several social and environmental contexts in an attempt to provide insight into the relationship between junco communication and social organization. A system of classification of signals, based upon the meaning of cues to the signaller, is proposed. Evidence is given to suggest that the behaviour of subordinate, rather than of dominant, individuals controls the operation of the social system in junco flocks. Some ecological implications of junco social behaviour are discussed.     

Preparing for battle? Potential intergroup conflict promotes current intragroup affiliation

Groups of human soldiers increase their affiliative behaviour when moving into combat zones. Despite numerous other species also competing as groups, little is known about how potential intergroup conflict might influence current intragroup affiliative behaviour in non-human animals. Here, I show that allopreening (when one individual preens another) increases in groups of cooperatively breeding green woodhoopoes (Phoeniculus purpureus) when they enter areas where conflicts with neighbours are more likely. Self-preening, which is an indicator of stress in other species, did not increase in conflict areas, suggesting that the change in affiliative behaviour is not the simple consequence of greater stress. Instead, because it is the dominant breeding pair that increase their preening of subordinate helpers, it is possible that current affiliative behaviour is being exchanged for agonistic support in any intergroup conflicts that might ensue. These results are important for our understanding of group dynamics, cooperation and the evolution of sociality, but also bring to mind the intriguing possibilities of social contracts and future planning in birds.     

Trends of reproductive hormones in male rats during psychosocial stress: role of glucocorticoid metabolism in behavioral dominance

Stress in socially subordinate male rats, associated with aggressive attacks by dominant males, was studied in a group-housing context called the visible burrow system (VBS). It has been established that subordinate males have reduced serum testosterone (T) and higher corticosterone (CORT) relative to dominant and singly housed control males. The relationship of the decreased circulating T levels in subordinate males to changes in serum LH concentrations has not been evaluated previously. Since decreases in LH during stress may cause reductions in Leydig cell steroidogenic activity, the present study defined the temporal profiles of serum LH, T, and CORT in dominant and subordinate males on Days 4, 7, and 14 of a 14-day housing period in the VBS. The same parameters were followed in serum samples from single-housed control males. Leydig cells express glucocorticoid receptors and may also be targeted for direct inhibition of steroidogenesis by glucocorticoid. We hypothesize that Leydig cells are protected from inhibition by CORT at basal concentrations through oxidative inactivation of glucocorticoid by 11beta-hydroxysteroid dehydrogenase (11betaHSD). However, Leydig cell steroidogenesis is inhibited when 11betaHSD metabolizing capacity is exceeded. Therefore, 11betaHSD enzyme activity levels were measured in Leydig cells of VBS-housed males at the same time points. Significant increases in LH and T relative to control were observed in the dominant animals on Day 4, which were associated with the overt establishment of behavioral dominance as evidenced by victorious agonistic encounters. Serum LH and T were lower in subordinate males on Day 7, but T alone was lower on Day 14, suggesting that lowered LH secretion in subordinates may gradually be reversed by declines in androgen-negative feedback. Serum CORT levels were higher in subordinate males compared to control at all three time points. In contrast, oxidative 11betaHSD activity in Leydig cells of dominant males was higher relative to control and unchanged in subordinates. These results suggest the following: 1) failure of Leydig cells of subordinate males to compensate for increased glucocorticoid action during stress, by increasing 11betaHSD oxidative activity, potentiates stress-mediated reductions in T secretion; and 2) an inhibition of the reproductive axis in subordinate males at the level of the pituitary.     

The effects of dominance on leadership and energetic gain: a dynamic game between pairs of social foragers

Although social behaviour can bring many benefits to an individual, there are also costs that may be incurred whenever the members of a social group interact. The formation of dominance hierarchies could offer a means of reducing some of the costs of social interaction, but individuals within the hierarchy may end up paying differing costs dependent upon their position within the hierarchy. These differing interaction costs may therefore influence the behaviour of the group, as subordinate individuals may experience very different benefits and costs to dominants when the group is conducting a given behaviour. Here, a state-dependent dynamic game is described which considers a pair of social foragers where there is a set dominance relationship within the pair. The model considers the case where the subordinate member of the pair pays an interference cost when it and the dominant individual conduct specific pairs of behaviours together. The model demonstrates that if the subordinate individual pays these energetic costs when it interacts with the dominant individual, this has effects upon the behaviour of both subordinate and the dominant individuals. Including interaction costs increases the amount of foraging behaviour both individuals conduct, with the behaviour of the pair being driven by the subordinate individual. The subordinate will tend to be the lighter individual for longer periods of time when interaction costs are imposed. This supports earlier suggestions that lighter individuals should act as the decision-maker within the pair, giving leadership-like behaviours that are based upon energetic state. Pre-existing properties of individuals such as their dominance will be less important for determining which individual makes the decisions for the pair. This suggests that, even with strict behavioural hierarchies, identifying which individual is the dominant one is not sufficient for identifying which one is the leader.     

Plasma cortisol concentrations before and after social stress in rainbow trout and brown trout

Two related experiments examined the relationship between plasma cortisol concentrations and the development of social hierarchies in fish. In the first, rainbow trout, Oncorhynchus mykiss, and brown trout, Salmo trutta, were observed for dominance interactions when confined within single-species pairs for 4, 48, or 168 h. Subordinate members of a pair exhibited significantly higher cortisol concentrations than dominant and single fish, but the pattern of cortisol elevation differed between the two species, being quicker to rise and increasing to a higher level in rainbow trout. Cortisol concentrations were correlated with behavioural measurements; the more subordinate the behaviour exhibited by a fish, the higher its cortisol concentration. Social stress was a chronic stressor, and no acclimation to social status occurred during the week. In the second experiment, measurements of plasma cortisol were made before pairing of rainbow trout and then after 48 h of confinement in pairs. Subordinate fish demonstrated significantly higher concentrations of plasma cortisol both before and after social stress. It therefore appears that in addition to cortisol being elevated during periods of social stress, an association may exist between initial cortisol levels and the likelihood of a fish becoming subordinate.     

Social monitoring via close calls in meerkats

Social monitoring of the actions of group members is thought to be a key development associated with group living. Humans constantly monitor the behaviour of others and respond to them in a flexible way depending on past interactions and the current social context. While other primates have also been reported to change their behaviour towards other group members flexibly based on the current state of their relationship, empirical evidence is typically linked to contextually specific events such as aggressive or reproductive interactions. In the cooperatively breeding meerkat (Suricata suricatta), we investigated whether subordinate females use frequently emitted, non-agonistic close calls to monitor the location of the dominant female and whether they subsequently adjust their response based on recent social interactions during conflict and non-conflict periods. Subjects discriminated between the close calls of the dominant female and control playbacks, responding by approaching the loudspeaker and displaying submissive behaviour only if they were currently threatened by eviction. Our results suggest that meerkats assess the risk for aggressive interactions with close associates depending on social circumstances, and respond accordingly. We argue that social monitoring based on non-agonistic cues is probably a common mechanism in group-living species that allows the adjustment of behaviour depending on variation in relationships.     

Rearing other peoples' young; Brood-mixing in the shelduck Tadorna tadorna

The mixing of ducklings from different broods, in several species of waterfowl, involves care of offspring other than the parents' own. In the eider, Somateria mollissima, this behaviour appears to be of benefit, in terms of increased adult and duckling survival, to both donor and recipient parents, but in the shelduck, Tadorna tadorna, brood-mixing appears not to be advantageous. Dominant males tended to lose ducklings to more subordinate ones but there was no significant difference between donor and recipient parents in the number of their own offspring reared to fledging. The apparent lack of any advantage in brood-mixing suggests that the behaviour might be a consequence of high brood density and that in the shelduck brook defence is adapted to low density.     

The gall of subordination: changes in gall bladder function associated with social stress

Diverse physiological and behavioural mechanisms allow animals to effectively deal with stressors, but chronic activation of the stress axis can have severe consequences. We explored the effects of chronic social stress on agonistic behaviour and gall bladder function, a critical but widely neglected component of stress-induced gastrointestinal dysfunction. Prolonged cohabitation with dominant individuals elicited behavioural modifications and dramatically increased bile retention in subordinate convict cichlid fish (Archocentrus nigrofasciatum). The key predictor of gall bladder hypertrophy was social subordination rather than status-related differences in food intake or body size. Stress-induced inhibition of gall bladder emptying could affect energy assimilation such that subordinate animals would not be able to effectively convert energy-rich food into mass gain. These results parallel changes in gall bladder function preceding cholesterol gallstone formation in humans and other mammals. Thus, social stress may be an important diagnostic criterion in understanding pathologies associated with gall bladder dysfunction.     

Water relations in three species of theConvolvulaceae in the Indian arid zone

The relationship of water deficit, actual water content, area, water loss and stomatal behaviour in leaves of different insertion levels of three members of Indian arid zoneConvolvulaceae—Merremia aegyptia, M. dissecta andIpomoea pes-caprae were measured simultaneously in the month of May, 1974, when field conditions were extremely adverse for growth. The two former species loose far more water than the latter. The values measured varied considerably with the leaf insertion levels and they reversed inI. pes-caprae when compared withMerremia species.     

Optimal foraging by firemouth cichlids, Cichlasoma meeki, in a social context

Pairs of juvenile firemouth cichlids, Cichlasoma meeki, were allowed to feed on Tubifex worms for 5 min. In the experiment, the worms were distributed so as to simulate a patchy resource with two levels of average abundance. The dominant member of each ichlid pair exhibited feeding behaviour consistent with expectations from optimal foraging theory, i.e. it fed primarily in the most profitable patch initially. When food was relatively abundant, the subordinate fish fed predominantly in the second most profitable patch. In this way it minimized the costs associated with agonistic activity. This study illustrates the usefulness of a multivariate approach in the study of foraging behaviour.     

Aggression within social groups of rhesus monkeys (Macaca mulatta): Effect of contact between groups

Social contact between two groups of rhesus monkeys within tunnel-connected enclosures affected the frequency of aggressive interaction within each group. Contact decreased aggression within the subordinate group but increased it within the dominant group. When the dominance relationship between the two groups was experimentally reversed the effect of contact on intragroup aggression correspondingly reversed.     

Ontogenetic and gender-related variation in the isotopic niche within and between three species of fur seals (genus <Emphasis Type="Italic">Arctocephalus</Emphasis>)

Differences in aggressiveness when competing for environmental resources are the main factor leading to social hierarchy in group living fish. Social status acquired is related to changes in physiological parameters, as metabolic rate. Habitat variation can interfere with aggressive behaviour and promote changes in physiological parameters associated with social status. The primary goal of our study was to investigate how differences in habitat complexity affect the relationship between resting metabolic rate (RMR) and social status in the Amazonian dwarf cichlid Apistogramma agassizii. We compared agonistic interactions between pairs of males in aquaria with different habitat enrichment levels, manipulated by adding shelters. RMR was measured before and after hierarchy establishment. Habitat enrichment promotes changes in aggressive behaviour and influences differences in metabolic rate between dominant and subordinate fish. We observed an increase in biting by dominant fish at high enrichment habitat, which could be related to the increase in territory value. We observed an increase in metabolic rate in dominant fish after hierarchy establishment. However, it occurs only in enriched habitats. We concluded that habitat structure interfere with behavioural characteristics in social hierarchies, as aggressiveness, and changes in aggressive interactions affect metabolic rate in different social ranks in the dwarf cichlid Apistogramma agassizii.     

Reproductive skew in female common marmosets: what can proximate mechanisms tell us about ultimate causes?

Common marmosets are cooperatively breeding monkeys that exhibit high reproductive skew: most subordinate females fail to reproduce, while others attempt to breed but produce very few surviving infants. An extensive dataset on the mechanisms limiting reproduction in laboratory-housed and free living subordinate females provides unique insights into the causes of reproductive skew. Non-breeding adult females undergo suppression of ovulation and inhibition of sexual behaviour; however, they receive little or no aggression or mating interference by dominants and do not exhibit behavioural or physiological signs of stress. Breeding subordinate females receive comparable amounts of aggression to non-breeding females but are able to conceive, gestate and lactate normally. In groups containing two breeding females,however, both dominant and subordinate breeders kill one another's infants. These findings suggest that preconception reproductive suppression is not imposed on subordinate females by dominants, at a proximate level, but is instead self-imposed by most subordinates, consistent with restraint models of reproductive skew. In contrast to restraint models, however, this self-suppression probably evolved not in response to the threat of eviction by dominant females but in response to the threat of infanticide. Thus,reproductive skew in this species appears to be generated predominantly by subordinate self-restraint, in a proximate sense, but ultimately by dominant control over subordinates' reproductive attempts.     

Do capuchin monkeys, Cebus apella, know what conspecifics do and do not see?

Capuchin monkeys were tested in five experiments in which two individuals competed over food. When given a choice between retrieving a piece of food that was visible or hidden from the dominant, subordinate animals preferred to retrieve hidden food. This preference is consistent with the hypotheses that either (1) the subordinate knew what the dominant could and could not see or (2) the subordinate was monitoring the behaviour of the dominant and avoiding the piece of food that it approached. To test between these alternatives, we released subordinates with a slight head start forcing them to make their choice (between a piece of food hidden or visible to the dominant) before the dominant entered the area. Unlike chimpanzees, Pan troglodytes, subordinates that were given a head start did not preferentially approach hidden pieces of food first. Therefore, our experiments provide little support for the hypothesis that capuchin monkeys are sensitive to what another individual does or does not see. We compare our results with those obtained with chimpanzees in the same paradigm and discuss the evolution of primate social cognition. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Behavioural tactics of breeders in cooperative meerkats

In eusocial invertebrates, queens commonly show morphological and behavioural modifications to their role as the principal breeders in their colonies. With the exception of naked mole-rats, Heterocephalus glaber, morphological modification of breeders has yet to be shown in cooperative vertebrates, but the behaviour of dominant individuals may be modified so as to maximize reproductive success. We studied the cooperative behaviour of dominant and subordinate adults in meerkats, Suricata suricatta, and found that the decision rules governing the contributions of dominant breeders differed from those of subordinate helpers. Dominant breeders contributed less than adult helpers to babysitting and pup feeding, but raised their individual contributions to pup care to a greater extent when helper:pup ratios were low. In contrast to subordinates, dominant breeders did not increase their contributions when they foraged successfully. Finally, while subordinates of both sexes assisted in rearing the young when dominants bred, dominant females contributed little when subordinates attempted to breed, and male helpers (but not females) reduced their contributions to the care of pups. Our results suggest that the division of labour between breeders and helpers in meerkats is intermediate between that of facultatively cooperative species, where parents are principally responsible for rearing young, and that of specialized eusocial species, which show a well-defined division of labour between breeders and workers.     

Arbuscular mycorrhizal fungi reduce the differences in competitiveness between dominant and subordinate plant species

In grassland communities, plants can be classified as dominants or subordinates according to their relative abundances, but the factors controlling such distributions remain unclear. Here, we test whether the presence of the arbuscular mycorrhizal (AM) fungus Glomus intraradices affects the competitiveness of two dominant (Taraxacum officinale and Agrostis capillaris) and two subordinate species (Prunella vulgaris and Achillea millefolium). Plants were grown in pots in the presence or absence of the fungus, in monoculture and in mixtures of both species groups with two and four species. In the absence of G. intraradices, dominants were clearly more competitive than subordinates. In inoculated pots, the fungus acted towards the parasitic end of the mutualism–parasitism continuum and had an overall negative effect on the growth of the plant species. However, the negative effects of the AM fungus were more pronounced on dominant species reducing the differences in competitiveness between dominant and subordinate species. The effects of G. intraradices varied with species composition highlighting the importance of plant community to mediate the effects of AM fungi. Dominant species were negatively affected from the AM fungus in mixtures, while subordinates grew identically with and without the fungus. Therefore, our findings predict that the plant dominance hierarchy may flatten out when dominant species are more reduced than subordinate species in an unfavourable AM fungal relationship (parasitism).     

Signal value of stress behaviour

Physiological and psychological stress are accompanied by nonverbal behaviour across a wide range of species. The function of this ‘stress behaviour’ is not well understood but is often assumed to be read by others as a cue to stress. Displaying signs of weakness is, however, difficult to understand from an evolutionary perspective and therefore further investigation into why these behaviours exist is needed. Here, we test whether displacement behaviours (i.e., those known to be associated with stress) are reliable indicators of stress in humans. To do this, we presented raters (N = 133) with videos of individuals (N = 31) undergoing a stress-inducting task. Self-directed displacement behaviours and self-reported stress were both associated with stress ratings given by raters. Therefore, such behaviours can provide reliable information to others and can be considered communicative. Individuals producing more nonverbal stress behaviour were rated as more likeable by raters (perhaps presenting as more honest signallers), indicating a benefit and potential adaptive function of displaying stress. Raters also differed in their accuracy in detecting stress from nonverbal cues. Findings suggest that the accuracy with which individuals were able to detect stress was linked to the number of social connections they reported to have. However, this association was non-linear, with individuals who were most and least accurate reporting the least network connections. This could indicate that the ability to read behaviour is associated with an ability to form and maintain social networks.     

Social anxiety, relationships and self-directed behaviour among wild female olive baboons

Self-directed behaviour (SDB) can be used as a behavioural indicator of stress and anxiety in nonhuman primates (Maestripieri et al. 1992, Animal Behaviour, 44, 967-979). We investigated the effect of nearest neighbours' relative dominance status on the SDB of sexually mature female olive baboons, Papio anubis. When the animal nearest to (within 5 m of) a female was a dominant individual, SDB rates (a combined measure of self-scratching, self-grooming, self-touching, body shaking and yawning) increased by ca. 40% over those observed when the nearest neighbour was a subordinate. The results indicate that (1) SDB can be used as a measure of uncertainty during the social interactions of cercopithecine primates and (2) as there was considerable variation in SDB response according to the nature of the dominant individual, SDB can be used to assess relationship security (i.e. the perceived predictability of a relationship for one partner). Finally, in combination with measures of affiliation rate, SDB may provide insight into relationship value. Copyright 1999 The Association for the Study of Animal Behaviour.