Agonistic Encounters Research Articles

‘The bone is mine’: affective and referential aspects of dog growls

A number of species are considered to use functionally referential signals such as alarm calls or food-related vocalizations. However, this particular function of communicative interaction has not previously been found in canids. We provide the first experimental indication that domestic dogs, Canis familiaris, rely on context-dependent signals during interspecific agonistic encounters. We recorded several sequences of growls from dogs in three different contexts: during play, guarding a bone from another dog, and reacting to a threatening stranger. We analysed the acoustic structure of the growls and additionally performed playback tests in a seminatural food-guarding situation. We found that play growls differed acoustically from the other two (agonistic) types of growls, mainly in their fundamental frequencies and formant dispersions. Results of the playback experiment showed that food-guarding growls deterred other dogs from taking away a seemingly unattended bone more effectively than growls recorded in the threatening stranger situation. We ruled out an effect of the signaller's body weight on the subjects' responses. These results provide the first evidence of context specificity of agonistic vocalizations in the dog. We discuss the possible aspects of honesty and deception through acoustic modulation of growls.

Enhanced habit-based learning and decreased neurogenesis in the adult hippocampus in a murine model of chronic social stress

Stress can induce preferential engagement of habit learning mediated by the basal ganglia, relative to learning that involves complex spatial associations contributed by the hippocampal formation. We explored in mice the influence that chronic episodes of social stress exert on the selection of cognitive/spatial vs. habit-based learning strategies. Male mice were exposed to repeated episodes of social confrontation and were categorized as dominant, subordinate or undetermined according to quantitative ethologically relevant parameters of aggression. Mice were then trained in a conditional discrimination task in the T-maze in the presence of allocentric cues until five correct choices were made. The T-maze was then turned 180° and mice were categorized as “cue-learners” or “place-learners” on the basis of their first response in the probe test. Mice showed a graded preference for place vs. cue learning strategies depending on their social categorization (control>undetermined>dominant>subordinate), which ranged from 55% in controls to only 10% in subordinate mice. The response of subordinate mice differed significantly from controls. Hippocampal neurogenesis was studied in the different groups of mice. In keeping with the tendency to engage habit learning, 2,5-bromodeoxyuridine (BrdU) incorporation in the DG was reduced in mice that experienced agonistic encounters, and so was the expression of doublecortin, a marker for immature neurons. These observations suggest that chronic social stress impairs neurogenesis in the adult hippocampus, weakens spatial learning and strengthens habit-like responses.

Shelter size influences self-assessment of size in crayfish, Orconectes rusticus: Consequences for agonistic fights

AbstractTheoretical models of animal assessment and decision strategies have assumed animals possess accurate information about themselves. The imperfect nature of self-assessment could cause animals to make inaccurate decisions during agonistic encounters. By manipulating sensory information used in self-assessment in crayfish, Orconectes rusticus, it may be possible to alter either decision-making paradigms or outcomes associated with agonistic encounters. We examined the role of self-assessment in the agonistic behaviour of crayfish by using differing shelter sizes to alter self-assessment of size. Similar-sized crayfish were kept in tanks with small, medium, and large shelters (relative to body size) and subsequently were fought against size-matched, naive opponents in novel chambers to remove resource value as a variable. Crayfish in small shelter treatments initiated and won more fights than crayfish in other shelters. Crayfish in the small shelter treatment had shorter durations of fight when compared to crayfish with the medium and large shelters. These results are consistent with the concept that crayfish placed in smaller shelters may be self-assessing their physical size and, thus, their fighting ability, higher than crayfish in larger shelters. Based on these results, current theoretical models need to include the role of inaccurate self-assessments in agonistic decision making paradigms.

Allosuckling allows growing offspring to compensate for insufficient maternal milk in farmed guanacos (Lama guanicoe)

While allonursing, the provision of milk to non-offspring by females, involves a potential cost to their own offspring, allosuckling, the suckling from females other than their own mother may allow offspring to compensate for previous deficiencies in maternal milk. We tested this hypothesis in farmed guanacos. Under the compensation hypothesis we predicted that mothers of calves exhibiting allosuckling should be in poorer physical condition and should exhibit relatively low acceptance rates to filial sucking attempts compared to mothers whose calf did not allosuckle. We also predicted that calves exhibiting frequent allosuckling should show similar or greater rates of gain in body weight, but similar total (or final) weight in the long term than calves that nursed from their mothers exclusively. We examined the potential effects of sex and order of birth dates of calves on allosuckling, and the effect of female success during agonistic encounters with other females on allonursing. Two stable groups of 15 and 14 mother–offspring pairs of farmed guanacos were studied from birth to approximately 3 months of age. Allosuckling events comprised 5.7% of all suckling events. Allonursing was performed by 52% of dams and 62% of calves exhibited allosuckling. We found similar gain rates in body weight and total weight at 60 days of age between allosuckling calves and filial sucking calves, irrespective of whether their mothers allonursed or not (P>0.1). Body weight of mothers whose calf allosuckled was significantly lower than that of mothers whose calves nursed from them exclusively (P=0.02). In addition, the percentage of acceptance of filial suckling bouts was significantly lower for allosuckling calves (P=0.004). There was no correlation between the frequency of allonursing and the success of dams during agonistic encounters (P>0.22). Our findings are consistent with the hypothesis that guanaco calves used allosucking to compensate for previous deficiencies in maternal milk.

Anolis carolinensis male-male agonistic encounters: a three year study of the best predictors for determining dominant/subordinate status

Event Abstract Back to Event Anolis carolinensis male-male agonistic encounters: a three year study of the best predictors for determining dominant/subordinate status M. P. Black1*, Scarlett J. Salem1, C. B. Ezeoke1, M. J. Sabula2 and Walter Wilczynski1 1 Georgia State University, United States 2 University of Georgia, United States Agonistic encounters can play an important role in the allocation of reproductive resources and the consequent fitness of an individual. The green anole lizard has an easily recognizable suite of behavioral displays that allow the determination and establishment of dominance and subordinance. To investigate the predictive value of different displays during the agonistic encounter, male Anolis carolinensis were paired with similar-sized males and observed as they established social status during a ninety-minute session. In real time, observers recorded bob A, B, and C, eyespot presence, dewlap displays, lateral displays, open mouth displays, wrestles, pushups, nips, and body color changes. Anole individuals that became dominant were green for a greater duration during the observation period. Dominant males also had a higher bob A and C count, more dewlap displays, and a longer total duration of dewlap displays. In fights that escalated to open mouth displays, dominants had higher frequencies of open mouth displays. In contrast to previous reports, eyespot latency in this study was not predictive of winning or losing, and subordinates most often displayed aggressively first, usually as bob A displays. The latency differences between this study and others may be due to different geographic sources of anoles, smaller differences in size between competitors, or housing males with females as opposed to isolation before encounters. This suggests that displays that are predictive of outcomes may vary depending on the source populations and encounter environment. Conference: 2010 South East Nerve Net (SENN) and Georgia/South Carolina Neuroscience Consortium (GASCNC) conferences, Atlanta , United States, 5 Mar - 7 Mar, 2010. Presentation Type: Poster Presentation Topic: Posters Citation: Black MP, Salem SJ, Ezeoke CB, Sabula MJ and Wilczynski W (2010). Anolis carolinensis male-male agonistic encounters: a three year study of the best predictors for determining dominant/subordinate status. Front. Neurosci. Conference Abstract: 2010 South East Nerve Net (SENN) and Georgia/South Carolina Neuroscience Consortium (GASCNC) conferences. doi: 10.3389/conf.fnins.2010.04.00074 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Mar 2010; Published Online: 18 Mar 2010. * Correspondence: M. P Black, Georgia State University, Atlanta, United States, ssalem1@student.gsu.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers M. P Black Scarlett J Salem C. B Ezeoke M. J Sabula Walter Wilczynski Google M. P Black Scarlett J Salem C. B Ezeoke M. J Sabula Walter Wilczynski Google Scholar M. P Black Scarlett J Salem C. B Ezeoke M. J Sabula Walter Wilczynski PubMed M. P Black Scarlett J Salem C. B Ezeoke M. J Sabula Walter Wilczynski Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

The influence of reproductive state on the agonistic interactions between male and female crayfish (Orconectes rusticus)

[Research analyzing social dynamics has shown that systems can be structured by individuals of different age, size, sex and reproductive state within a population. Extensive research on male–male interactions in crayfish has illustrated the mechanisms that underlie the agonism and social relationships that develop between conspecifics. Despite the wealth of knowledge on male–male interactions, there is a lack of knowledge on the role that reproductive state plays in modulating aggression between the sexes in crayfish. The purpose of this study was to investigate aggression between crayfish of different sexes and reproductive forms. Form I and II male crayfish were paired against glair and non-glair female crayfish in agonistic bouts. Fight outcomes, intensities and mating events were calculated from the paired interactions. Form I male crayfish won significantly more bouts with females than form II males. In addition, reproductive females won more bouts than non-reproductive males. Thus, reproductive state of the individual has a significant influence over the outcome and level of aggression within agonistic encounters. This finding may indicate that the individuals are approaching interactions with motivational states (reproductive needs versus resource acquisition)., Research analyzing social dynamics has shown that systems can be structured by individuals of different age, size, sex and reproductive state within a population. Extensive research on male–male interactions in crayfish has illustrated the mechanisms that underlie the agonism and social relationships that develop between conspecifics. Despite the wealth of knowledge on male–male interactions, there is a lack of knowledge on the role that reproductive state plays in modulating aggression between the sexes in crayfish. The purpose of this study was to investigate aggression between crayfish of different sexes and reproductive forms. Form I and II male crayfish were paired against glair and non-glair female crayfish in agonistic bouts. Fight outcomes, intensities and mating events were calculated from the paired interactions. Form I male crayfish won significantly more bouts with females than form II males. In addition, reproductive females won more bouts than non-reproductive males. Thus, reproductive state of the individual has a significant influence over the outcome and level of aggression within agonistic encounters. This finding may indicate that the individuals are approaching interactions with motivational states (reproductive needs versus resource acquisition).]

Vasotocin actions on electric behavior: interspecific, seasonal, and social context-dependent differences

Social behavior diversity is correlated with distinctively distributed patterns of a conserved brain network, which depend on the action of neuroendocrine messengers that integrate extrinsic and intrinsic cues. Arginine vasotocin (AVT) is a key integrator underlying differences in behavior across vertebrate taxa. Weakly electric fish use their electric organ discharges (EODs) as social behavioral displays. We examined the effect of AVT on EOD rate in two species of Gymnotiformes with different social strategies: Gymnotus omarorum, territorial and highly aggressive, and Brachyhypopomus gauderio, gregarious and aggressive only between breeding males. AVT induced a long-lasting and progressive increase of EOD rate in isolated B. gauderio, partially blocked by the V1a AVT receptor antagonist (Manning compound, MC), and had no effects in G. omarorum. AVT also induced a long-lasting increase in the firing rate (prevented by MC) of the isolated medullary pacemaker nucleus (PN) of B. gauderio when tested in an in vitro preparation, indicating that the PN is the direct effector of AVT actions. AVT is involved in the seasonal, social context-dependent nocturnal increase of EOD rate that has been recently described in B. gauderio to play a role in mate selection. AVT produced the additional nocturnal increase of EOD rate in non-breeding males, whereas MC blocked it in breeding males. Also, AVT induced a larger EOD rate increase in reproductive dyads than in agonistic encounters. We demonstrated interspecific, seasonal, and context-dependent actions of AVT on the PN that contribute to the understanding of the mechanisms the brain uses to shape sociality.

Behavior and pro-inflammatory cytokine variations among submissive and dominant mice engaged in aggressive encounters: moderation by corticosterone reactivity.

Psychosocial stressors contribute to the pathophysiology of affective disorders and variations of cytokine functioning have been implicated in this process. The present investigation demonstrated, in mice, the impact of stressful aggressive encounters on activity levels, plasma corticosterone and cytokine concentrations, and on cytokine mRNA expression within the prefrontal cortex (PFC) and hippocampus. As glucocorticoids have been tied to cytokine variations, mice were subdivided into low or high corticosterone responders, defined in terms of circulating hormone levels 75 min post-confrontation. Interestingly, stressor-induced effects among low and high responders varied as a function of whether mice were submissive or dominant during the aggressive bout. Agonistic encounters elicited subsequent hyperactivity, particularly among low corticosterone responders and among dominant mice. Plasma levels of corticosterone and interleukin (IL)-6 concomitantly increased after aggressive encounters and varied with dominance status and with the low versus high corticosterone response. Among the low responders corticosterone and IL-6 increases were modest and only apparent among submissive mice, whereas among high responders these elevations were more pronounced and comparable in submissive and dominant mice. Aggressive episodes also increased IL-1β and IL-6 mRNA brain expression. The IL-1β rise was greater in the PFC and hippocampus of submissive mice that were low responders. Among high responders IL-1β and IL-6 increased in both groups, although in the PFC this effect was specific to dominant mice. The data are discussed in terms of their relevance to the impact of aggressive encounters on affective behaviors, and to the role that cytokines might play in this regard.

Intrasexual competition facilitates the evolution of alternative mating strategies in a colour polymorphic fish

BackgroundIntense competition for access to females can lead to males exploiting different components of sexual selection, and result in the evolution of alternative mating strategies (AMSs). Males of Poecilia parae, a colour polymorphic fish, exhibit five distinct phenotypes: drab-coloured (immaculata), striped (parae), structural-coloured (blue) and carotenoid-based red and yellow morphs. Previous work indicates that immaculata males employ a sneaker strategy, whereas the red and yellow morphs exploit female preferences for carotenoid-based colours. Mating strategies favouring the maintenance of the other morphs remain to be determined. Here, we report the role of agonistic male-male interactions in influencing female mating preferences and male mating success, and in facilitating the evolution of AMSs.ResultsOur study reveals variation in aggressiveness among P. parae morphs during indirect and direct interactions with sexually receptive females. Two morphs, parae and yellow, use aggression to enhance their mating success (i.e., number of copulations) by 1) directly monopolizing access to females, and 2) modifying female preferences after winning agonistic encounters. Conversely, we found that the success of the drab-coloured immaculata morph, which specializes in a sneak copulation strategy, relies in its ability to circumvent both male aggression and female choice when facing all but yellow males.ConclusionsStrong directional selection is expected to deplete genetic variation, yet many species show striking genetically-based polymorphisms. Most studies evoke frequency dependent selection to explain the persistence of such variation. Consistent with a growing body of evidence, our findings suggest that a complex form of balancing selection may alternatively explain the evolution and maintenance of AMSs in a colour polymorphic fish. In particular, this study demonstrates that intrasexual competition results in phenotypically distinct males exhibiting clear differences in their levels of aggression to exclude potential sexual rivals. By being dominant, the more aggressive males are able to circumvent female mating preferences for attractive males, whereas another male type incorporates subordinate behaviours that allow them to circumvent male aggression and female mating preferences. Together, these and previous results indicate that exploiting different aspects of social interactions may allow males to evolve distinct mating strategies and thus the long term maintenance of polymorphisms within populations.

3H]SCH 23390 binding in various brain regions of C57BL/6J mice with repeated experience of victory or social defeat in agonistic interactions

The binding of [3H]SCH 23390 has been studied in various brain regions of male mice with the experience of repeated victory (winners) or defeat (losers) gained over 10 (T10) and 20 (T20) days of daily agonistic confrontations. In the frontal cortex, B(max) of [3H]SCH 23390 binding sites was found to be increased in T10 losers and decreased in T20 losers when compared to the control mice. In the striatum, T10 and T20 winners had reduced values of [3H]SCH 23390 binding sites than the ones in the control mice. The K(d) was increased in the frontal cortex of T10 losers and T10 winners as well as in the amygdala of T20 losers. Reduced K(d) values were found in the striatum of all experimental groups as well as in the amygdala of T20 winners. Thus, both specific changes relating to social behavior patterns and non-specific ones in [3H]SCH 23390 binding were found in the brain regions of mice after 10 and 20 days of intermale confrontations.

Transgressive aggression in Sceloporus hybrids confers fitness through advantages in male agonistic encounters

1. We investigated agonistic behaviour and associated characteristics of Sceloporus woodi (Florida scrub lizard), Sceloporus undulatus (Eastern fence lizard) and their hybrids using staged territorial encounters. 2. These Sceloporus hybrids exhibit transgressive aggression and transgressive head-girth relative to the parental species and the transgressive aggression was specifically associated with an advantage in agonistic encounters. Our results suggest a hybrid advantage in natural habitats when defending and invading territories against either parental species. 3. We further analysed general advantages in agonistic encounters across the entire three-group system to elucidate characteristics that may be advantageous under specific circumstances. Individuals with larger body size (SVL) and greater aggression had an overall advantage in agonistic encounters; however, smaller individuals could win when slightly more aggressive and fatter, and less aggressive individuals could win when slightly larger, especially with greater head-girth. 4. The extreme hybrid phenotypes likely occurred through transgressive segregation, which has been implicated as a process through which homoploid, hybrid speciation can occur. Some form of ecological divergence is necessary, however, to impede parental gene flow. Our data suggest that ecological divergence could manifest in territorial species through transgressive aggression.

Agonistic encounters and cellular angst: social interactions induce heat shock proteins in juvenile salmonid fish

Juvenile salmonid fish readily form dominance hierarchies when faced with limited resources. While these social interactions may result in profound behavioural and physiological stress, it is unknown if this social stress is evident at the level of the cellular stress response--specifically, the induction of stress or heat shock proteins (Hsps). Thus, the goal of our study was to determine if Hsps are induced during hierarchy formation in juvenile rainbow trout (Oncorhynchus mykiss). To this end, we measured levels of three Hsps, Hsp70, Hsc (heat shock cognate)70 and Hsp90 in the white muscle, liver and brain of trout that had been interacting for 36 h, 72 h or 6 days. Our data indicate that Hsps are induced in both dominant and subordinate fish in a time- and tissue-specific manner. In further mechanistic experiments on fasted and cortisol-treated fish, we demonstrated that high plasma cortisol does not affect Hsp induction in trout white muscle or liver, but both conditions may be part of the mechanism for Hsp induction with social stress in the brain. We conclude that the behavioural and physiological stress experienced by juvenile rainbow trout in dominance hierarchies can be extended to the induction of Hsps.

Signal residuals and hermit crab displays: flaunt it if you have it!

In animal contests selection should favour information gathering regarding the likely costs and benefits of continued conflict, and displays may provide a means for contestants to gain information about the fighting ability or aggressive intent of competitors. However, there is debate over the reliability of such displays and low levels of deception may occur within otherwise honest signalling systems. Hermit crabs use displays involving the chelipeds during agonistic encounters. We examined how variation in chelae size in relation to body size, a determinant of fighting ability, affects their use in displays and the process and outcome of contests over gastropod shells. In accordance with deceptive use of an otherwise honest signal, we found that contestants with large chelipeds for their body size spent more time performing the cheliped presentation display. Moreover, cheliped residuals and displays influenced the escalation level of encounters. There was a positive association between cheliped displays and the occurrence of ‘grappling’, but a negative association between displays and the occurrence of shell fights, suggesting that displays may signal aggressive intent and a reluctance to back off or accept the more passive defender role in a fight. Furthermore, the smaller of the two contestants in shell fights had larger cheliped residuals compared to those smaller contestants not involved in shell fights, which is consistent with disrupted opponent assessment. This study adds to mounting evidence that when acting as a signaller, individuals for whom the display exaggerates competitive ability attempt to manipulate opponents, using the display more often.

Emotion-related acoustic communication in bats.

Some features of emotional prosody in human speech may be rooted in mechanisms common to mammals. The role of vocal communication in social interactions was studied in bats, a highly vocal group evolutionarily remote from primates. The present paper focuses on communication during agonistic encounters in the Indian False Vampire bat. Three call types with distinct time-frequency contours occurred; aggression calls, whistles, and response calls. In a first experiment, agonistic approach situations were analyzed to assess the extent to which these call types reflected the specific part of the caller in the interaction and the intensity of the agonistic display. A frame-by-frame video analysis followed by a sound analysis revealed that call type indicated the part of the respective caller while interaction intensity was encoded in similar parameter changes across call types. The systematic change in vocal parameters with affect intensity corresponded to prosodic changes in human speech. A playback experiment based on a habituation-dishabituation paradigm investigated how the bats categorized the vocalizations emitted during third-party agonistic interactions. The bats were able to discriminate between call types; however, they did not necessarily form categories corresponding to call type in reciprocal experiments, an evidence for a context-dependent evaluation of social calls.

Gender differences in aggressive behaviour in convict cichlids

Males and females of many species engage in agonistic encounters. However, differing selection pressures on each sex are predicted to result in sex differences in aggressive behaviour during contests. Comparing male and female intrasexual contests can yield intriguing differences, shedding light on the forces shaping the use of particular aggressive tactics. We investigated whether fundamental gender-related differences in aggression, not explained by current parental role, are present in convict cichlids, Amatitlania nigrofasciata. Intrasexual agonistic encounters between isolated males and between isolated females not previously paired to a breeding partner were staged. Using this approach we first tested for behavioural differences between the sexes. Second, using a novel startle technique aimed at probing motivation to fight, we tested for gender-related differences in aggressive motivation. Third, we examined whether size, rather than gender, plays a role in determining the tactics used during contests. In addressing these aims we found: (1) females used more frontal display and biting, and spent more time in close proximity to their opponent, whereas males used more lateral display and tail beating than females during agonistic encounters; (2) there was no difference in the response of male or female convict cichlids to a startling stimulus aimed at probing motivation to fight; and (3) the addition of focal weight and length as possible covariates had no significant effect on the analyses. Possible causal and functional reasons for these gender-related differences in fight tactics are discussed.

Cranial mechanics and functional interpretation of the horned carnivorous dinosaur Carnotaurus sastrei

ABSTRACT Three-dimensional finite element analyses were performed on the cranium of the horned theropod Carnotaurus sastrei to assess how it would have performed mechanically during biting and frontal butting. This technique proved to be an effective tool to provide a better understanding of the cranial functional morphology of C. sastrei. The analyses indicated that the jaw-closing musculature of C. sastrei would have played a key role in diminishing the stress level on the cranium during biting, particularly for the braincase. Force analyses show that C. sastrei would have had a comparatively weak (and faster than Allosaurus fragillis) muscle-driven bite. The finite element analyses also provide quantitative evidence to suggest that the cranium of C. sastrei could have withstood high velocity impacts to its teeth resulting from a hypothetical hatchet-like biting mode, but could not have endured severe, rapid frontal blows during agonistic encounters.

Calling and Courtship Behaviors Initiated by Male-Male Contact Via Agonistic Encounters in the CricketGryllus bimaculatus

We investigated the stridulatory courtship initiated by male-male contact after agonistic encounters and the effect of dominance status on subsequent reproductive behavior in the cricket Gryllus bimaculatus. When two male crickets were kept together in a small area, their dominance status was quickly established through fighting or non-fighting interactions. Approximately 10 min after pairing, most dominant males produced calling and/or courtship songs in the presence of subordinate males. This behavior appeared to be triggered by some contact chemicals on the body surface of the males. Stimulation using the forewing of a neutral male induced courtship in dominant males at a higher level compared with neutral males which were not previously paired with males. These observations suggest that the sexual motivation in dominant males increased because of previous agonistic interactions. In contrast, subordinate males remained silent. Stimulation using the male forewing induced stridulation to a lesser degree in subordinate males than in neutral males, suggesting decreased sexual motivation in subordinate males. Furthermore, only 40% of subordinate males exhibited courtship behavior under triadic conditions (dominant male, subordinate male and female) in contrast with 100% in male-female pairs. This result reveals that subordinate males, being less sexually motivated, are continuously suppressed in their courtship by intermittent attacks by the nearby dominant males. In the other triadic condition where the males were allowed to copulate, 65% of the dominant males copulated, while none of the subordinates did. These results suggest that dominant males have a greater chance to copulate and produce offspring.

Cheek Plumage Uniformity as a Social Status Signal in Great Tits

Some plumage patterns in birds, such as irregularities of feathers, are potential amplifiers because they can reveal damage by conspecifics and therefore social status. We experimentally manipulated the pattern of the white cheeks of male great tits (Parus major) in order to simulate the effects of peckings by others and thus investigate possible changes in dominance during agonistic encounters at artificial feeders exposed to different risks of predation during winter. As predicted, manipulated birds were displaced by controls from safe feeders, and performed a lower number of aggressions and with lower success, although they did not receive more aggressions as expected from the fact that efficient agonistic displays can avoid the occurrence of overt aggressions. Thus, our manipulation changed the social status of birds by converting individuals with manipulated cheeks into subordinates. We propose that cheek colour uniformity acts as an amplifier of the outcomes of aggressions in great tits.

Semen quantity and quality correlate with bank vole males’ social status

Laboratory studies reveal that in several rodent species the females prefer dominant males as mating partners. Here we investigate the correlation between bank vole males' social rank and their sperm quality and quantity. We used agonistic encounters to determine males' social status. Sperm quality was assessed by its motility, viability, maturity, morphology and sperm tail membrane integrity. Relatively more dominant males were heavier than males of lower social status. The males' social position affected the testes, seminal vesicles and coagulation gland development. The weights of these reproductive organs were significantly higher in more dominant males than in more subordinate males. Sperm counts and the values of the other parameters describing sperm quality were higher in high-ranking males than in subordinates. Our results suggest that bank vole females benefit from choosing and mating with high-ranking males by obtaining more and better-quality sperm.

Play behavioural tactics under space reduction: social challenges in bonobos, Pan paniscus

Several studies have suggested that primates vary in their response to spatial reduction, which represents a challenge for captive animals. Under spatial reduction, primates can adopt various strategies to reduce social tension. Such strategies, which are not mutually exclusive, can involve avoidance and coping tactics. Recently, it has been shown that, during short-term crowding, bonobos may use both grooming and nonreproductive sexuality to avoid a potential rise in tension. Because bonobos are extremely playful, and play can be a risky activity, we tested whether play patterns were selectively avoided under space reduction. We collected daily data via focal animal sampling on a group of bonobos housed in the Apenheul Primate Park (Apeldoorn, The Netherlands) over a 12-month period. Among the adults, we found no difference in the contact play levels (play fighting) between the two space conditions (outdoors, 5000 m2, and indoors, 230 m2), whereas indoors, there was a tendency toward an increase in locomotor–rotational play frequency. These findings indicate that bonobos do not adopt an avoidance tactic to reduce the risk involved in space reduction. Nevertheless, dyads showing higher rates of agonistic encounters were less likely to engage in contact play sessions. Even though it is difficult to affirm that space availability significantly induced higher playmate selectivity, we found that play signals significantly increased indoors, thus suggesting the need for a clearer ‘declaration of intentions’ in an unsafe situation.