Juvenile Female Aggression in Cooperatively Breeding Pied Babblers: Causes and Contexts

Sex biases in adult aggression have been well studied and commonly arise when resources which affect survival or lifetime reproductive success are less abundant or more valuable for individuals of one sex. Despite the prevalence of sex biases in adult aggression, evidence for sex biases in juvenile aggression is scant. Here, we present evidence for female‐biased juvenile aggression in cooperatively breeding pied babblers (Turdoides bicolor). Unlike most cases of non‐lethal sibling aggression, juvenile aggression in pied babblers does not seem to be determined by food availability. Instead, we found that juvenile aggression was related to adult dispersal patterns. This study shows that females that were more aggressive as juveniles attempted dispersal earlier than less aggressive females. Potential explanations for the association between juvenile aggression and adult dispersal patterns are discussed.     

Sex-Specific Aggression and Antipredator Behaviour in Young Brown Trout

Sex differences in adult behaviour are often interpreted as consequences of sexual selection and/or different reproductive roles in males and females. Sex-specific juvenile behaviour, however, has received less attention. Adult brown trout males are more aggressive than females during spawning and juvenile aggression may be genetically correlated with adult aggression in fish. We therefore tested the prediction that immature brown trout males are more aggressive and bolder than immature females. Because previous work has suggested that precocious maturation increases dominance in salmonids, we included precocious males in the study to test the prediction that early sexual maturation increase male aggression and boldness. Aggression and dominance relations were estimated in dyadic contests, whereas boldness was measured as a response to simulated predation risk using a model heron. Independent of maturity state, males initiated more than twice as many agonistic interactions as females in intersexual contests. However, males were not significantly more likely to win these contests than females. The response to a first predator attack did not differ between sex categories, but males reacted less to a second predator attack than females. Sexual maturity did not affect the antipredator response in males. Since there is no evidence from field studies that stream-living immature male and female salmonids differ in growth rate, it appears unlikely that the sex differences demonstrated are behavioural consequences of sex-specific investment in growth. It seems more likely that sex-specific behaviour arises as a correlated response to sexually selected gene actions promoting differential behaviour in adult males and females during reproduction. Alternatively, sex differences may develop gradually during juvenile life, because a gradual developmental program should be less costly than a sudden behavioural change at the onset of sexual maturity.     

Female competition in chimpanzees

Female chimpanzees exhibit exceptionally slow rates of reproduction and raise their offspring without direct paternal care. Therefore, their reproductive success depends critically on long-term access to high-quality food resources over a long lifespan. Chimpanzee communities contain multiple adult males, multiple adult females and their offspring. Because males are philopatric and jointly defend the community range while most females transfer to new communities before breeding, adult females are typically surrounded by unrelated competitors. Communities are fission–fusion societies in which individuals spend time alone or in fluid subgroups, whose size depends mostly on the abundance and distribution of food. To varying extents in different populations, females avoid direct competition by foraging alone or in small groups in distinct, but overlapping core areas within the community range to which they show high fidelity. Although rates of aggression are low, females compete for space and access to food. High rank correlates with high reproductive success, and high-ranking females win direct contests for food and gain preferential access to resource-rich sites. Females are aggressive to immigrant females and even kill the newborn infants of community members. The intensity of such aggression correlates with population density. These patterns are compared to those in other species, including humans.     

Sex differences in the drivers of reproductive skew in a cooperative breeder

Many cooperatively breeding societies are characterized by high reproductive skew, such that some socially dominant individuals breed, while socially subordinate individuals provide help. Inbreeding avoidance serves as a source of reproductive skew in many high‐skew societies, but few empirical studies have examined sources of skew operating alongside inbreeding avoidance or compared individual attempts to reproduce (reproductive competition) with individual reproductive success. Here, we use long‐term genetic and observational data to examine factors affecting reproductive skew in the high‐skew cooperatively breeding southern pied babbler (Turdoides bicolor). When subordinates can breed, skew remains high, suggesting factors additional to inbreeding avoidance drive skew. Subordinate females are more likely to compete to breed when older or when ecological constraints on dispersal are high, but heavy subordinate females are more likely to successfully breed. Subordinate males are more likely to compete when they are older, during high ecological constraints, or when they are related to the dominant male, but only the presence of within‐group unrelated subordinate females predicts subordinate male breeding success. Reproductive skew is not driven by reproductive effort, but by forces such as intrinsic physical limitations and intrasexual conflict (for females) or female mate choice, male mate‐guarding and potentially reproductive restraint (for males). Ecological conditions or “outside options” affect the occurrence of reproductive conflict, supporting predictions of recent synthetic skew models. Inbreeding avoidance together with competition for access to reproduction may generate high skew in animal societies, and disparate processes may be operating to maintain male vs. female reproductive skew in the same species.     

Lateralization of Aggression during Reproduction in Male Siamese Fighting Fish

A preference for the left‐eye use during aggressive interactions has been widely reported in the literature, even though in some cases the direction of lateralization varies among individuals within populations. Laterality of aggression in male Siamese fighting fish has been described in a number of studies, yet very little is known about lateralization of aggression during reproduction and/or parental care in fish. Here, we aimed to investigate the relationship between the different reproductive phases and lateralization in eye use during aggressive interactions in males of Siamese fighting fish. Lateralization in eye use is influenced during the early reproductive state, before and after the bubble nest construction stages. We found that nest‐holding males preferentially used the right eye before and after bubble nest construction independent of the sex of the intruder. During the later reproductive phases, aggressiveness increased whereby the direction of lateralization rather than the degree was influenced supporting the hypothesis that reproductive state influences behavioral consistency in Siamese fighting fish.     

Queen–worker ratio affects reproductive skew in a socially polymorphic ant

The partitioning of reproduction among individuals in communally breeding animals varies greatly among species, from the monopolization of reproduction (high reproductive skew) to similar contribution to the offspring in others (low skew). Reproductive skew models explain how relatedness or ecological constraints affect the magnitude of reproductive skew. They typically assume that individuals are capable of flexibly reacting to social and environmental changes. Most models predict a decrease of skew when benefits of staying in the group are reduced. In the ant Leptothorax acervorum, queens in colonies from marginal habitats form dominance hierarchies and only the top‐ranking queen lays eggs (“functional monogyny”). In contrast, queens in colonies from extended coniferous forests throughout the Palaearctic rarely interact aggressively and all lay eggs (“polygyny”). An experimental increase of queen:worker ratios in colonies from low‐skew populations elicits queen–queen aggression similar to that in functionally monogynous populations. Here, we show that this manipulation also results in increased reproductive inequalities among queens. Queens from natural overwintering colonies differed in the number of developing oocytes in their ovaries. These differences were greatly augmented in queens from colonies with increased queen:worker ratios relative to colonies with a low queen:worker ratio. As assumed by models of reproductive skew, L. acervorum colonies thus appear to be capable of flexibly adjusting reproductive skew to social conditions, yet in the opposite way than predicted by most models.     

Contest Duration and Dynamics are Affected by Body Size in a Potentially Subsocial Crayfish (Crustacea: Decapoda)

Fighting is a costly behavior, consuming both time and energy. As a result, the benefits of acquiring resources must outweigh these costs. Resource value will thus influence willingness to invest in a contest through its objective (the intrinsic properties of the resource) and subjective value (context/state dependent). In burrowing crayfish, subjective resource value may vary with life stage: adults, subadults, and juveniles differ in their ability to obtain resources. As juveniles cannot dig their own burrows, we hypothesize that earlier life stages will exhibit lower aggression than later life stages. To test this, we evaluated contests between paired individuals according to their cephalothorax length (CL), encompassing different life stages of Parastacus brasiliensis. To quantify aggression levels, we recorded contest duration, the frequency of low and highly aggressive behaviors, the time to escalate to highly aggressive behaviors, the probability of initiating contests with highly aggressive behaviors, and the latency to initiate a contest. We examined the relationship between these dependent variables and CL (independent variable) using GLMs to test how aggressive behaviors develop. Contest duration increased with pair mean, winner's and loser's CL. Frequency of low aggressive behaviors increased with CL, whereas highly aggressive behaviors, latency, and time to reach highly aggressive behaviors were unrelated to CL. Smaller individuals had a higher probability of initiating contests with highly aggressive behaviors. Self‐assessment explains the contest dynamics of P. brasiliensis, with smaller individuals giving up sooner, probably due to lower energy and time budgets.     

Repeated Recent Aggressive Encounters Do Not Affect Behavioral Consistency in Male Siamese Fighting Fish

Consistent individual differences in behavior suggest that individuals respond in a predictable and repeatable manner in a specific situation while differing from other individuals. Male Siamese fighting fish exhibit consistent individual differences in decision‐making strategies when they encounter a receptive female and a rival male simultaneously. However, whether these differences are altered by recent experience is unknown. We examined the influence of repeated aggressive encounters on behavioral consistency and decision‐making. Males were presented with paired female–male dummies prior to any aggressive experiences to obtain a baseline measure. Next, males either won or lost three consecutive contests against rivals and then received the paired female–male dummies after each of these encounters. Overall levels of highly aggressive behaviors were affected by contest outcome, while levels of female‐directed were not. Not surprisingly, winning a fight led to an increase in male‐directed bites, an overtly aggressive behavior that only occurs after fights have escalated. Fighting a male before encountering the dummies caused males to perform more tail beats to the dummy male, perhaps as a result of increased motivation. Males exhibited similar levels of repeatability and used the same strategies when faced with conflicting stimuli regardless of fighting experience. Thus, while winning or losing a fight impacts overall aggression, it does not influence behavioral consistency. This study demonstrates that consistent individual differences and decision‐making strategies may be resistant to recent aggressive experiences, even over a period of days.     

Reproductive Consequences of Aggression in a Territorial Songbird

Territorial aggression can influence males’ ability to obtain high‐quality resources and access to mates; however, in many species, the reproductive consequences of variation in aggression are unknown. In this study, we investigated how individual variation in aggressive behavior relates to reproductive success in socially monogamous, genetically polygynous song sparrows (Melospiza melodia). Prior research in this species shows that male song sparrows differ in their willingness to engage in agonistic interactions with territorial intruders and that individual variation in aggression appears to have functional significance. Aggressive males have been shown to obtain territories where females produce larger clutch sizes, suggesting that individuals who display high levels of territorial aggression are defending high‐quality territories or females. Further, aggressive males are considered a greater threat to territory‐holding males than less aggressive males. In this study, we ask whether individual differences in aggression are linked to differences in extra‐pair reproductive success, annual reproductive success, and offspring quality. We did not uncover a relationship between aggression and annual reproductive success or patterns of extra‐pair paternity. However, we found that the nestlings of aggressive males grew at a faster rate than the nestlings of less aggressive males. Future studies should attempt to identify mechanisms to explain the relationship between offspring growth rate and male aggression and investigate whether faster offspring growth rates translate to greater survival and recruitment of offspring.     

Persistent directional selection on body size and a resolution to the paradox of stasis

Directional selection on size is common but often fails to result in microevolution in the wild. Similarly, macroevolutionary rates in size are low relative to the observed strength of selection in nature. We show that many estimates of selection on size have been measured on juveniles, not adults. Further, parents influence juvenile size by adjusting investment per offspring. In light of these observations, we help resolve this paradox by suggesting that the observed upward selection on size is balanced by selection against investment per offspring, resulting in little or no net selection gradient on size. We find that trade‐offs between fecundity and juvenile size are common, consistent with the notion of selection against investment per offspring. We also find that median directional selection on size is positive for juveniles but no net directional selection exists for adult size. This is expected because parent–offspring conflict exists over size, and juvenile size is more strongly affected by investment per offspring than adult size. These findings provide qualitative support for the hypothesis that upward selection on size is balanced by selection against investment per offspring, where parent–offspring conflict over size is embodied in the opposing signs of the two selection gradients.     

Condition,not eyespan,predicts contest outcome in female stalk‐eyed flies,Teleopsis dalmanni

In contests among males, body condition is often the key determinant of a successful outcome, with fighting ability signaled by so‐called armaments, that is, exaggerated, condition‐dependent traits. However, it is not known whether condition and exaggerated traits function in the same way in females. Here, we manipulated adult condition by varying larval nutrition in the stalk‐eyed fly, Teleopsis dalmanni, a species in which eyespan is exaggerated in both sexes, and we measured the outcome of contests between females of similar or different body condition and relative eyespan. We found that females in higher condition, with both larger bodies and eyespan, won a higher proportion of encounters when competing against rivals of lower condition. However, when females were of equal condition, neither eyespan nor body length had an effect on the outcome of a contest. An analysis of previously published data revealed a similar pattern in males: individuals with large relative eyespan did not win significantly more encounters when competing with individuals of a similar body size. Contrary to expectations, and to previous findings in males, there was no clear effect of differences in body size or eyespan affecting contest duration in females. Taken together, our findings suggest that although eyespan can provide an honest indicator of condition, large eyespans provide no additional benefit to either sex in intrasexual aggressive encounters; body size is instead the most important factor.     

The impact of paternity on male–infant association in a primate with low paternity certainty

In multimale groups where females mate promiscuously, male–infant associations have rarely been studied. However, recent studies have shown that males selectively support their offspring during agonistic conflicts with other juveniles and that father's presence accelerates offspring maturation. Furthermore, it was shown that males invest in unrelated infants to enhance future mating success with the infant's mother. Hence, infant care might provide fitness gain for males. Here, we investigate male–infant associations in rhesus macaques (Macaca mulatta), a primate with low paternity certainty as females mate with multiple partners and males ensure paternity less efficiently through mate‐guarding. We combined behavioural data with genetic paternity analyses of one cohort of the semi‐free‐ranging population of Cayo Santiago (Puerto Rico) and recorded affiliative and aggressive interactions between focal subjects and adult males from birth to sexual maturation (0–4 years) of focal subjects. Our results revealed that 9.6% of all interactions of focal subjects involved an adult male and 94% of all male–infant interactions were affiliative, indicating the rareness of male–infant aggression. Second and most interestingly, sires were more likely to affiliate with their offspring than nonsires with unrelated infants. This preference was independent of mother's proximity and emphasized during early infancy. Male–infant affiliation rose with infant age and was pronounced between adult males and male rather than female focal subjects. Overall, our results suggest that male–infant affiliation is also an important component in structuring primate societies and affiliation directed towards own offspring presumably represent low‐cost paternal care.     

Male Siamese fighting fish use gill flaring as the first display towards territorial intruders

The Siamese fighting fish (Betta splendens) is well known as an aggressive fish with unique spawning and parental care behavior. During reproduction, male fish construct a bubble nest, court females, protect the brood, and defend the territory through aggressive displays. Aggression in male Siamese fighting fish has long been the subject of investigation; however, the kinematics of aggression during contests have been largely overlooked. Here we investigated how nest-holding, male Siamese fighting fish use two different types of displays, gill flaring and fin spreading, towards intruders during various reproductive phases; before (BB) and after bubble nest building, and after spawning (AS), and hatching (AH). Males were more aggressive towards male than female intruders and the level of aggression changed significantly between reproductive phases. Gill flaring, the more energetically costly display, was the dominant initial display towards male and female intruders in BB, AS, AH phases. However, defending males switched to fin spreading after prolonged exposure to intruders. The results suggest that Siamese fighting fish use gill flaring as an acute response in order to defend their territory; this response may be replaced by fin spreading as a chronic response, probably to reduce the energetic costs during the contest.     

Male–male aggression peaks at intermediate relatedness in a social spider mite

Theory predicts that when individuals live in groups or colonies, male–male aggression peaks at intermediate levels of local average relatedness. Assuming that aggression is costly and directed toward nonrelatives and that competition for reproduction acts within the colony, benefits of aggressive behavior are maximized in colonies with a mix of related and unrelated competitors because aggression hurts nonkin often, thereby favoring reproduction of kin. This leads to a dome‐shaped relation between male–male aggression and average relatedness. This prediction has been tested with bacteria in the laboratory, but not with organisms in the field. We study how male–male aggression varies with relatedness in the social spider mite Stigmaeopsis miscanthi. We sampled 25 populations across a wide geographic range between Taiwan and Japan, representing a gradient of high to low within‐population relatedness. For each population the weaponry of males was measured as the length of the first pair of legs, and male–male aggression was tested by placing pairs of nonsibling males together and scoring the frequency of male death over a given period. As these two morphological and behavioral variables correlate strongly, they both reflect the intensity of male–male conflict. Our data on the social spider mite show that male–male aggression as well as weapon size strongly peak at intermediate, average relatedness, thereby confirming theoretical predictions.     

Intra‐community coalitionary lethal attack of an adult male southern muriqui (Brachyteles arachnoides)

We report on the first evidence of intra‐community coalitionary lethal aggression in muriquis (Brachyteles). The event occurred in southern muriquis (Brachyteles arachnoides) during a long‐term study (>15 years) of two social groups inhabiting mostly pristine Atlantic forest habitat in the Parque Estadual Carlos Botelho, southern São Paulo State, Brazil. The attack took place deep in the core area of the Group Caetê home range. Tense agonistic behaviors and vocalizations preceded the lethal coalitionary attack, and the tension increased over a 36–48 hr period. One adult female and two unidentified individuals also took part in a coalition led by six adult males. The members of the coalition collectively approached, embraced, immobilized and repeatedly bit the entire body of an adult male, resulting in severe bleeding injuries and the victim's death in less than 1 hr after the attack commenced. Combined ecological, behavioral and spatial data related to the event indicate that this was an intra‐community attack and suggest social tensions related to mating competition as the proximate trigger of the coalitionary killing. The attack resembled those reported for chimpanzees, with clear numeric superiority and a low risk of injury to aggressors, resulting in the death of a lone conspecific victim. This observation (n=1) is suggestive of a capacity for escalated aggression in muriquis and reinforces arguments for the potential adaptive significance of intra‐community aggression in male philopatric societies, as reported for spider monkeys and chimpanzees. These characteristics challenge the view of the muriquis as a peaceful primate and support the general hypothesis that imbalances of power contribute to intra‐specific killing in primates, such as chimpanzees and humans. Am. J. Primatol. 71:860–867, 2009. © 2009 Wiley‐Liss, Inc.     

Variation in Helper Type Affects Group Stability and Reproductive Decisions in a Cooperative Breeder

Recent studies have shown that differences in life history may lead to consistent inter‐individual variation in behavioural traits, so‐called behavioural syndromes, animal personalities or temperaments. Consistencies of behaviours and behavioural syndromes have mainly been studied in non‐cooperative species. Insights on the evolution of cooperation could be gained from studying individual differences in life histories and behavioural traits. Kin selection theory predicts that if an individual’s reproductive ability is low, it had to aim at gaining inclusive fitness benefits by helping others. We tested this prediction in the cooperatively breeding cichlid Neolamprologus pulcher, by assessing reproductive parameters of adults that had been tested earlier for aggressiveness and for their propensity to assist breeders when they had been young (‘juveniles’). We found that juvenile aggression levels predicted the acceptance of a subordinate in the group when adult. Males which were aggressive as juveniles were significantly more likely to tolerate a subordinate in the group when compared with males which were peaceful as juveniles, whereas females which were more aggressive as juveniles tended to expel subordinates more often. Females produced significantly smaller clutches when paired to males which had helped more as a juvenile, despite the fact that adult males hardly provided direct brood care. There was no evidence that females with a high propensity to help when young, produced smaller clutches or eggs when adult, but they took longer to lay their first clutch when compared with females with a low propensity to help when young. These results suggest that variation in behavioural types might explain variation in cooperation, the extent of group‐living and reproductive decisions.     

Aggression and growth depression in juvenile Atlantic salmon: the consequences of individual variation in standard metabolic rate

In aquaculture, competitive interactions for food are a major source of growth rate variation, since they result in aggressive individuals acquiring a disproportionate amount of food and growing faster. Consequently, such competition increases the variance and skew of the size distribution of fish. In Atlantic salmon Salmo salar acquaculture, rearing is often initiated with juveniles of uniform size. However, the initial factors allowing fish to out-compete others of the same size is unclear. This study shows that individual differences in standard metabolic rate (SMR) may contribute to differences in aggression between juvenile Atlantic salmon. Fish were segregated into three holding tanks on the basis of differences in relative SMR aggression was highest in the tank containing high SMR fish, and lowest in a low SMR group. However, there were no significant differences in mean growth between the three treatments, although the skew of the size distribution was much less in the low SMR group than in the high SMR group. Therefore, although mean growth was not improved in the low SMR group by excluding high SMR fish, their growth subsequently showed less variability, possibly as a consequence of fewer despotic individuals gaining disproportionate access to food.     

The effects of paternal reproductive tactic and rearing environment on juvenile variation in growth as mediated through aggression and foraging behaviours of Chinook salmon (Oncorhynchus tshawytscha)

In species with indeterminate growth, differential growth rates can lead to animals adopting alternative reproductive tactics such as sneak–guard phenotypes, which is partially predicted by variation in growth during the juvenile life‐history stage. To investigate sources of growth variation, we examined the independent and joint effects of paternal reproductive tactic (G) and rearing environment (E) on juvenile growth in Chinook salmon (Oncorhynchus tshawytscha), hypothesizing G and E effects are partially mediated through differences in behaviour such as aggressive interactions and resulting foraging behaviours. We created maternal half‐sibling families with one‐half of the female's eggs fertilized by the milt of a sneaker “jack” and the other half by a guarder “hooknose”. At the exogenous feeding stage, each split‐clutch family was then divided again and reared in a rationed diet or growth‐promotion diet environment for approximately 6 months, during which growth parameters were measured. Before saltwater transfer at 9 months of age, social interactions were observed in groups of six fish of various competitor origins. We found ration restricts growth rate and juvenile mass, and evidence of genetic effects on growth depensation, where jack‐sired individuals grew less uniformly over time. These growth‐related differences influenced an individual's level of aggression, with individuals raised on a restricted diet and those whose families experienced greatest growth being most aggressive. These individuals were more likely to feed than not and feed most often. Jack‐sired individuals were additionally aggressive in the absence of food, and when raised on a rationed diet outcompeted others to feed most. These results show how individuals may achieve higher growth rates via intrinsic (G) or induced (E) aggressive behavioural phenotypes, and eventually attain the threshold body size necessary during the saltwater phase to precociously sexually mature and adopt alternative reproductive phenotypes.     

Non‐Conceptive Sexual Behavior in Spiders: A Form of Play Associated with Body Condition,Personality Type,and Male Intrasexual Selection

In the socially polymorphic spider Anelosimus studiosus, males mature early in the reproductive season and recruit to the webs of juvenile females and guard them until they mature. During the period before females mature, males and females engage in repeated bouts of non‐conceptive (play) sexual behavior, where the pair courts and engages in mock copulation; both males and females gain performance‐enhancing experience via these encounters. In this study, we examined the factors that underlie individual variation in the tendency to engage in non‐conceptive mating and determine whether it impacts male–male competition for females. We found that docile females, being less resistant to mating in general, are more likely to accept male courtship and non‐conceptive copulation as juveniles. Personality type influenced the exhibition of non‐conceptive sexual behavior in males as well. High body condition males of the aggressive phenotype were more likely to engage in non‐conceptive sexual behavior than males with lower body condition. Body condition did not influence docile males’ propensity to engage in non‐conceptive sexual behavior, but female size did. Docile males engaged in more non‐conceptive sexual displays with larger females. Engaging in non‐conceptive sexual displays negatively impacted male performance in staged male–male contests for access to females. This cost was greatest for males of the aggressive phenotype, which are otherwise favored in male–male contests. Our findings indicate expression of non‐conceptive sexual displays is linked to personality and results in reproductive performance trade‐offs for male A. studiosus.     

Why are small males aggressive?

Aggression is ubiquitous in the animal kingdom, whenever the interests of individuals conflict. In contests between animals, the larger opponent is often victorious. However, counter intuitively, an individual that has little chance of winning (generally smaller individuals) sometimes initiates contests. A number of hypotheses have been put forward to explain this behaviour, including the "desperado effect" according to which, the likely losers initiate aggression due to lack of alternative options. An alternative explanation suggested recently is that likely losers attack due to an error in perception: they mistakenly perceive their chances of winning as being greater than they are. We show that explaining the apparently maladaptive aggression initiated by the likely loser can be explained on purely economic grounds, without requiring either the desperado effect or perception errors. Using a game-theoretical model, we show that if smaller individuals can accurately assess their chance of winning, if this chance is less than, but close to, a half, and if resources are scarce (or the contested resource is of relatively low value), they are predicted to be as aggressive as their larger opponents. In addition, when resources are abundant, and small individuals have some chance of winning, they may be more aggressive than their larger opponents, as it may benefit larger individuals to avoid the costs of fighting and seek alternative uncontested resources.