Sibling Aggression Research Articles

The relative roles of hunger and size asymmetry in sibling aggression between nestling ospreys, <i>Pandion haliaetus</i>

The relative roles of hunger and size asymmetry in sibling aggression between nestling ospreys, <i>Pandion haliaetus</i>

Ontogeny of agonistic behaviour in dispersing blind mole rats (spalax ehrenbergi)

The blind mole rat (Spalax ehrenbergi) is a solitary and aggressive fossorial rodent that inhabits its own individual tunnel system. Mole rats breed mainly in the winter, giving birth to a litter of three to four young on average [Nevo (1961) Mammalia 25:127–143]. Intraspecific interactions between adults occur mostly during the winter, which is the rainy season in Israel; after parturition, interactions occur between the mother and her offspring and between the siblings themselves until dispersal. Until now, the subterranean habitat of mole rats has prevented direct observation in the wild. Thus, there has been no direct evidence for the proximal factors that drive juvenile mole rats out of the maternal tunnel system as they shift from the social life-style of the young to the solitary life-style of the adult. In the present study, female mole rats were caught in the wild with their offspring and brought to the laboratory. The dispersal process of the young from the maternal nest was followed under two housing conditions: (1) restricted housing, simulating the physical conditions under which dispersal is delayed past the usual age, as often occurs during late winter floods in the field in areas of nonporous clay soils, and (2) nonrestricted housing, simulating the physical conditions that allow the young to disperse from their maternal tunnels at any time. In each housing condition, the mothers raised either one or three pups. During pup development, we monitored both maternal and juvenile agonistic variables during intraspecific interactions. Five major behavioural phases were identified during the ontogeny of the mole rat pups. Maternal aggression toward pups increased mainly in the first three phases of pup development, and sibling aggression increased from the third phase until dispersal. Under the restricted housing conditions, there was a delay in both the appearance of aggression in the “family group” as well as in dispersal age compared with the nonrestricted conditions. In the nonrestricted conditions, increased litter size resulted in delay in dispersal age. We suggest that the main proximate factor initiating natal dispersal is increased intolerance and aggression among siblings rather than maternal aggression. Only after the young began dispersing from the maternal tunnels did maternal aggression dramatically increase, thus preventing the offspring from returning. However, following floods, inhibition of aggression may enable the mole rat pups to stay with the family group and to disperse later when the physical conditions permit it. Aggr. Behav. 24:455–470, 1998. © 1998 Wiley-Liss, Inc.

Ontogeny of agonistic behaviour in dispersing blind mole rats (spalax ehrenbergi)

The blind mole rat (Spalax ehrenbergi) is a solitary and aggressive fossorial rodent that inhabits its own individual tunnel system. Mole rats breed mainly in the winter, giving birth to a litter of three to four young on average [Nevo (1961) Mammalia 25:127–143]. Intraspecific interactions between adults occur mostly during the winter, which is the rainy season in Israel; after parturition, interactions occur between the mother and her offspring and between the siblings themselves until dispersal. Until now, the subterranean habitat of mole rats has prevented direct observation in the wild. Thus, there has been no direct evidence for the proximal factors that drive juvenile mole rats out of the maternal tunnel system as they shift from the social life-style of the young to the solitary life-style of the adult. In the present study, female mole rats were caught in the wild with their offspring and brought to the laboratory. The dispersal process of the young from the maternal nest was followed under two housing conditions: (1) restricted housing, simulating the physical conditions under which dispersal is delayed past the usual age, as often occurs during late winter floods in the field in areas of nonporous clay soils, and (2) nonrestricted housing, simulating the physical conditions that allow the young to disperse from their maternal tunnels at any time. In each housing condition, the mothers raised either one or three pups. During pup development, we monitored both maternal and juvenile agonistic variables during intraspecific interactions. Five major behavioural phases were identified during the ontogeny of the mole rat pups. Maternal aggression toward pups increased mainly in the first three phases of pup development, and sibling aggression increased from the third phase until dispersal. Under the restricted housing conditions, there was a delay in both the appearance of aggression in the “family group” as well as in dispersal age compared with the nonrestricted conditions. In the nonrestricted conditions, increased litter size resulted in delay in dispersal age. We suggest that the main proximate factor initiating natal dispersal is increased intolerance and aggression among siblings rather than maternal aggression. Only after the young began dispersing from the maternal tunnels did maternal aggression dramatically increase, thus preventing the offspring from returning. However, following floods, inhibition of aggression may enable the mole rat pups to stay with the family group and to disperse later when the physical conditions permit it. Aggr. Behav. 24:455–470, 1998. © 1998 Wiley-Liss, Inc.

Perceptions of parenting as predictors of boys' sibling and peer relations.

This study included 71 target boys (8 to 10 years), their siblings, and mothers to examine the relations among mothering, fathering, sibling aggression, and peer outcomes. Siblings whose mothers were known to be more rejecting were observed and reported to be more aggressive with one another than siblings whose mothers were less rejecting. Moreover, boys who experienced more aggressive sibling interactions were more likely to be nominated by their peers as being aggressive and were less accepted by their peers. Although fathering failed to evince a direct influence on sibling aggression, an indirect effect was evidenced in that less accepting fathering related to more rejecting mothering. It appeared that boys' aggressive experiences with their siblings mediated, in part, the association between maternal rejection and their peer aggression and that peer aggression was a mediating link between sibling aggression and boys' acceptance by their peers.

Perceptions of parenting as predictors of boys' sibling and peer relations.

This study included 71 target boys (8 to 10 years), their siblings, and mothers to examine the relations among mothering, fathering, sibling aggression, and peer outcomes. Siblings whose mothers were known to be more rejecting were observed and reported to be more aggressive with one another than siblings whose mothers were less rejecting. Moreover, boys who experienced more aggressive sibling interactions were more likely to be nominated by their peers as being aggressive and were less accepted by their peers. Although fathering failed to evince a direct influence on sibling aggression, an indirect effect was evidenced in that less accepting fathering related to more rejecting mothering. It appeared that boys' aggressive experiences with their siblings mediated, in part, the association between maternal rejection and their peer aggression and that peer aggression was a mediating link between sibling aggression and boys' acceptance by their peers.

Siblicide: The Evolutionary Blackmail

Avian siblicide, whereby some chicks are eliminated from a brood as a direct consequence of sibling aggression, has often been interpreted as the outcome of a parent-offspring conflict over brood size This conflict, which arises from the fact that each offspring values itself more than it values its siblings, results in offspring favoring smaller families, of higher-quality individuals, than would be optimal for their parents to produce. Considered as an evolutionary game, this conflict is complicated by two factors: before deciding to enforce their preferred brood size by eliminating unwanted brood mates, offspring must take into account the readjustments in parental investment that will normally ensue from brood reduction. Likewise, optimal parental investment will depend on offspring reaction to it. In particular, it is shown that the cost that parents must pay to provide enough resources to prevent siblicide will often be smaller than the benefit they obtain from maintaining their preferred brood size. Siblicide can be viewed in these instances as evolutionary blackmail, because its threat is sufficient to make parents increase brood provisioning levels. As a result of the blackmail, sibling aggression may acquire a signaling role in the parent-offspring interplay.

Proximate control of siblicide in cattle egrets: a test of the food-amount hypothesis

Avian sibling aggression plays an important role in controlling food distribution among nest mates. The food-amount hypothesis predicts that the amount of food obtained by siblings acts as a proximate cue controlling their level of aggression. The importance of this cue to cattle egrets (Bubulcus ibis) in Oklahoma was evaluated by studying parental feeding rates, food distribution among nestlings, and nestling aggression in three-chick broods [A-(oldest), B-, and C-chicks]. We compared nests in which siblicide occurred to nests where all three chicks survived (nonsiblicide nests). Relative to nonsiblicide nests, siblicide nests received decreasing amounts of food as the day of siblicide approached. Similarly, aggression levels of all three chicks in siblicide nests increased significantly compared with nonsiblicide nests, with most of the aggressive behavior involving B- and C-chicks. As a result, on a per-day basis, food amount and aggression exhibited a strong, negative correlation, thus providing support for the food-amount hypothesis. Though the total amount of food delivered to siblicide nests declined on the days prior to siblicide, the amount of food received by A-chicks in these nests did not. Relative to nonsiblicide nests, A-chicks in siblicide nests received an increased share (percent of volume) of the food delivered at the expense of C-chicks, with the percent remaining the same for B-chicks. The percentage of food B-chicks received the day after siblicide increased significantly from that received the day before siblicide, with no change occurring in either the amount or percentage of food received by A-chicks; clearly, B-chicks were the beneficiary of their aggressive actions that resulted in the death of C-chicks. We provide evidence supporting the hypothesis that aggression is proximately linked to food amount, possibly by a relatively simple cue such as hunger.

Brood Reduction and Siblicide in Black-Billed Magpies (Pica pica)

In many avian species, brood reduction is considered to be adaptive and may be attributed either to sibling competition (passive starvation, active sibling aggression) or parental effects (manipulation of hatching spread, active infanticide). However, nonadaptive factors such as environmental effects, may contribute substantially to nestling mortality. I determined mechanisms of brood reduction and survival probabilities of nestling Black-billed Magpies (Pica pica) in relation to intraclutch egg-size variation, brood size, nestling age and size, and weather. Median brood survival time was weakly, but positively, correlated with intraclutch egg-mass variation. Starvation accounted for most nestling mortality within the first 9 days posthatching, although 12 dead and moribund nestlings were found badly bruised around the head. Siblicide and sibling cannibalism were observed in two broods and impli- cated in the deaths of nestlings in three other broods. Siblicidal events occurred when nestlings were between 15 and 20 days old. Expected median survival times and probability of survival to fledging of nestlings was not linearly related to brood size at hatching, being highest for broods of five, lowest for broods of three, and intermediate for broods of four and six. Dead nestlings were smaller and lighter than surviving siblings at any age; however, asymptotic body mass and linear measurements were negatively correlated with brood size at fledging. The probability of mortality was affected by prevailing weather conditions; rain and low temperatures doubled the estimated risk of death for young nestlings, independent of brood size. These data suggest that factors influencing the occurrence and maintenance of brood-reduction strategies in a population may be more complex than previously thought. Brood-reduction mechanisms are affected by the interaction of biotic and abiotic factors, and may vary in response to factors outside of either parental or offspring control. Received 9 September 1994, accepted 25 April 1995.

The function of hatching asynchrony in the blue-footed booby

The blue-footed booby (Sula nebouxii) commonly hatches two eggs 4 days apart; then the senior (first-hatched) chick aggressively dominates the other and sometimes kills it. Two hypotheses explaining the function of the hatching interval were tested by creating broods with synchronous hatching: the facultative brood reduction hypothesis of Lack (1954) and the sibling rivalry reduction hypothesis of Hahn (1981). The results contradicted most predictions of both hypotheses: synchronous broods formed an aggressive hierarchy similarly to asynchronous broods (controls), and subordinate chicks grew poorly (Fig. 1) and died frequently, similarly to junior chicks in control broods. However, compared with synchronous broods, asynchronous broods showed less aggression (Fig. 2), diminished food allocation to subordinate chicks (Fig. 3) and less total food consumption (30% fewer feeds at age 0–10 days). These behavioral comparisons took into account the different ages of chicks in different treatments. The results suggest that natural asynchrony makes brood reduction more efficient and decreases the costs of sibling aggression to parents, in terms of their future survival or fecundity, as proposed by Mock and Ploger (1987). Further, in exaggeratedly asynchronous broods (8-day hatch interval) junior chicks suffered more aggression (Fig. 4) and grew more slowly than junior chicks in control broods. This result supports the hypothesis of optimal hatch asynchrony of Mock and Ploger (1987).

The Role of Parents in Sibilicidal Brood Reduction of Two Booby Species

ABSTRACr.--Parental regulation of the probability and timing of avian brood reduction is expected to exist and evolve because natural selection should favor parents that match brood size to food availability most efficiently. Since food availability varies among species, interspecific variation in this regulation is expected. Comparative study of species with different brood reduction systems is one way to test the hypothesis that these mechanisms evolve as adaptations. Previous work on obligately siblicidal Masked Boobies (Sula dactylatra) and facultatively siblicidal Blue-footed Boobies (S. nebouxii) has shown that their different hatching asynchronies cannot fully explain the qualitative difference in their brood reduction systems. In this study, I report interspecific differences in nest shape that appear to contribute to early siblicide in Masked Boobies, but that suppress early siblicide in Blue-footed Boobies. Bluefooted Booby nest shape is closely regulated by parents. Differences in egg sizes of nest mates do not appear to contribute to the difference in social systems. Received 19 January 1995, accepted 2 July 1995. FATAL SIBLING AGGRESSION, known as often reduces the brood size of a number of bird species (Stinson 1979, Mock et al. 1990). Lack's (1954) brood-reduction hypothesis is the basis for the common evolutionary interpretation of siblicide, and of brood reduction (Ricklefs 1965) in general. When food supply for nestlings cannot be predicted reliably at the time of laying, selection is expected to favor a strategy of hatching as many nestlings as can be raised in the best conditions, and then of eliminating nestlings after hatching, if necessary, to bring demand into line with supply (see also Temme and Charnov 1987, Forbes and Ydenberg 1992). Siblicide provides parents an optimizing tool with a notable advantage: individual nestlings are best able to gauge their own physiological condition and can use that information, in conjunction with a competitive hierarchy, to maximize efficiency of brood reduction (Mock et al. 1987a).

Reproductive phenology and breeding success of the monk parakeet (Myiopsitta monachus monachus)in Argentina

The breeding biology of the Monk Parakeet (Myiopsitta monachus monachus) was studied in Punta Blanca, central Argentina (34° 56’ S, 57° 39’ W). Observations on 9 communal nests (13 breeding chambers) were made dur‐ing 1988 and 1989. The median date of the first egg for each year was Hand 31 October (the median for 2 years was 16 October). Clutch size ranged from 5 to 12 eggs, averaging 6.9. Incubation took 23.6 days and hatching was asynchronous. Mean hatching success for clutches was 52.2%. Only 17% of the nestlings fledged. Only 8.9% of the total eggs laid resulted in fledglings. Predation by Opossums (Didelphis) and Black Rats (Rattus) was the main cause of mortality at nests (82%), but 17.9% of the nestling mortality was due to intraspecifics attacks, including sibling aggression.

Avian Brood Reduction and Parent-Offspring "Conflict"

Avian brood reduction is widely viewed as a potential source of parent-offspring conflict. Yet empirical evidence belies the existence of conflict; for example, parents create the initial competitive asymmetries that facilitate brood reduction and, in fratricidal species, rarely intervene to stop sibling aggression. Here I examine parent-offspring conflict over brood reduction using a simple game model. Parents establish an initial brood size and level of effort that optimizes their lifetime reproductive success. Offspring can respond by reducing brood size, but if they do so, parents are free to readjust their level of effort. Brood reduction is an evolutionarily stable strategy (ESS) only when offspring fitness is higher following parental readjustment of effort. Model analysis revealed that both conflict and cooperation over brood size are possible ESS(s). Two parental policies of resource allocation among contemporary siblings were examined: equal, in which food is distributed evenly, and despotic, in which food is distributed according to a sibling dominance hierarchy. Parental fitness was diminished slightly by despotic allocation, but the risk of parent-offspring conflict was also diminished. When conflict did occur, the fitness costs and benefits for parents and offspring, respectively, were modest, particularly for despotic allocation. Where parents add "insurance" offspring to the brood, parents and elder offspring may agree over the elimination of redundant offspring, even though the proximate cause of infanticide is sibling rivalry.

Brown pelican siblicide and the prey-size hypothesis

We asked whether the brown pelican (Pelecanus occidentalis) practices facultative brood reduction and tested two predictions of Mock's (1985) prey-size hypothesis: (1) if chicks take food directly from the parental mouth, nestmates should compete aggressively; (2) aggression between nestmates should increase during the developmental transition from indirect feeding (parents deposit food on the substrate) to direct feeding (parents pass food from mouth to mouth). Eggs in two-egg and three-egg clutches were laid and hatched 2 days apart. Junior (second- and third-hatched) chicks, fed less than their nestmates, grew more slowly and died more frequently. Of non-hatchling victims 74% were underweight, 55% bore peck wounds and 43% were found outside the nest, implying starvation, aggression and expulsion from the nest. Aggression occurred in all observed sibships, and junior chicks were subordinate in eight of ten cases. A progressive transition from indirect to direct feeding occurred during the 30 days after hatching, but there was no evidence of an increase in pecking during the transition. In conclusion, the brown pelican practices facultative brood reduction, and chick loss is mediated by sibling aggression as predicted by the preysize hypothesis. Pecking during indirect feeds seems to contradict the prey-size hypothesis, but may serve partly to establish a relationship of dominance-subordinance.

Behavioral Ecology of Black-Legged Kittiwakes during Chick Rearing in a Failing Colony

Compared with their Atlantic counterparts, Black-legged Kittiwakes (Rissa tridactyla) in North Pacific colonies are notably unproductive. A large colony on Middleton Island, Alaska, has in most years since 1981 seen complete breeding failure and the population has declined by half. We compared parent-offspring behaviors in this colony during two years that differed in overall breeding success. Potential indicators of food stress included parental attendance at the nest, foraging trip lengths, chick feeding and begging rates, and sibling aggression. Whereas chick feeding and begging rates were strongly correlated with overall breeding performance, patterns of time allocation by adults (nest attendance and foraging trips) were not. Contrasts between years and comparisons with data from other colonies in and outside Alaska point to food shortage as the likely cause of recurrent breeding failure on Middleton.

Managing sibling aggression: Overcorrection as an alternative to time-out

The present study compared the efficacy and treatment acceptability of two interventions for sibling aggression: time-out and overcorrection. Thirty mothers and their aggressive sibling dyads participated in one of three group assignments: time-out, over-correction, and control. Parent training procedures were conducted individually in the clinic. Frequency of sibling aggression, recorded by mothers, served as the dependent variable. The results indicate that both time-out and overcorrection significantly reduced sibling aggression rates as compared to a control group. Treatment effects were maintained one month following participation. Generally, parents rated the acceptability of their respective intervention higher after treatment implementation than before. Treatment acceptability ratings for time-out and overcorrection were not significantly different. It was concluded that overcorrection is a socially valid, viable alternative to time-out for managing sibling aggression in oppositional children.

Limitations of “don't” instructional control

This study examined the relative efficacy of a prohibitive (“don't”) instructional procedure compared with immediate timeout (TO) in reducing physical aggression between siblings. The “don't” instructional procedure included “don't” instructions, praise for compliance (i.e., inhibiting on-going aggression), warnings for noncompliance (i.e., continued fighting), and chair TOs for noncompliance to warnings. An alternating treatment design was employed with three oppositional, aggressive, preschool children and their mothers in the home setting. Sibling aggression data were collected by the mother. The reliability of maternal observations, however, was not independently confirmed. Results suggested that the immediate TO procedure was more effective than the “don't” instructional procedure. Sibling aggression appears to be a misbehavior that is not readily treated with an obedience-provoking routine. Theoretical reasons behind the limitations of “don't” instructional control are discussed.

The Role of Brood Size in Regulating Egret Sibling Aggression

In siblicidal birds, overt aggression is used by nestlings to resolve competition for food, and the nestlings of most species diminish or abate their combat when that competition is relaxed. Previous studies of colonial herons and egrets, however, have indicated that the proximate cue on which the aggression was assumed to be predicated, namely, current food levels, has no effect on sibling fight rates; that is, hunger does not provide such a switch-off cue. We tested whether aggression might be curtailed in cattle egrets by the obvious alternative, attention to overall demand. When one of three chicks was temporarily removed from experi- mental nests, the remaining sibs virtually ceased fighting; when it was returned, hostilities resumed. This effect was not influenced directly by the rank of the removed chick, or by the amount of food available to the others (indeed, parents corrected for the change in brood size by bringing less food). By terminating aggression when brood size drops from three to two, the remaining nestlings stand to save the costs of needless fighting when they have little to gain from continuing attacks. We propose that species probably do best by relying on current food amount for the truncation of sibling fighting whenever that cue allows an accurate assessment of pending competition, with brood size used mainly as an alternative or backup system. Supply-based mechanisms should be more flexible and reversible than those predicated on demand. In species such as egrets, the senior siblings can probably achieve a modicum of reversibility by exercising restraint during attacks. By using nonfatal intimidation of nest mates, seniors probably control the competition while forestalling the irreversible step of prema- ture siblicide.

Fatal Sibling Aggression, Precocial Development, and Androgens in Neonatal Spotted Hyenas

Fatal neonatal sibling aggression is common in predatory birds but has not been previously reported in wild mammals. Spotted hyena females are strongly masculinized, both anatomically and behaviorally, apparently by high levels of androgens during ontogeny. Neonates display elevated androgen levels, precocial motor development, and fully erupted front teeth. Litters are usually twins, and siblings fight violently at birth, apparently leading to the death of one sibling in same-sex litters, whereas in mixed-sex litters both siblings usually survive.

Parental Care, Sibling Relationships and the Development of Aggressive Behaviour in Two Lines of Wild House Mice

Distinct behavioural strategies for dealing with environmental and social challenge are known to exist in the adults of a variety of mammalian species, but little is known about the developmental bases of these strategies. In an attempt to start to fill this gap in knowledge, the present study set out to describe and compare the development of males from two lines of wild house mice which are known to differ in their aggressive behaviour and response to challenging situations. Adult males from one line (Short Attack Latency = SAL line) are more aggressive, show less sensitivity to changes in their environment and behave in a more internally controlled, routine-like way, than do males from the other line (outbred = Control line). The parental care and development of eight litters of four male pups from each line was observed. Paternal care did not differ between the two lines, but SAL pups received higher levels of nursing and general maternal care than did Control pups, and they appeared to be weaned later. Despite these differences, SAL pups grew more slowly than Control pups, suggesting that their high levels of sucking behaviour were a reflection of high milk demand, perhaps due to a low milk supply, rather than high milk intake. SAL pups were also delayed in a number of measures of early behavioural development relative to Control pups. The apparent early retardation in certain measures of SAL pup behavioural development disappeared at the end of the parental care period. From day 32 onwards, SAL pups began to show higher levels of aggression towards each other than did Control pups, and also showed a reduced reactivity to tests involving changes to the home cage environment ; a characteristic of adult SAL males. As expected, SAL males developed into faster attackers than Control males and, in SAL litters, intra-litter variation in attack speed was strongly influenced by preceding experience of sibling aggression. The fastest attackers in each litter were more successful in fights occurring in their litter than their slowest attacking siblings. No such relationship was observed in the Control litters where levels of inter-sibling aggression were low. The results point to the potential significance of maternal care behaviour and pup fighting behaviour in the development of the fast-attacking phenotype. One hypothesis is that inadequate nutrition of young SAL pups, mediated through the mother, promotes increased competition for access to the mother's nipples and predisposes the pups to develop into more active/competitive individuals. Subsequent sibling-sibling aggression appears to influence intra-litter variation in adult attack latency.

BREEDING BIOLOGY OF THE BEARDED VULTURE IN SOUTHERN AFRICA, PART II: THE NESTLING PERIOD

Summary Brown, C. J. 1990. Breeding biology of the Bearded Vulture in southern Africa, Part I: The nestling period. Ostrich 61: 24–32. The nestling period of the Bearded Vulture Gypaetus barbatus in southern Africa was 124–128 days. The hatching interval between the normal two-egg. clutch was usually 3–6 days (range 2–9 days Only one nestling per clutch survived to the third day. Tittle sibling aggression and no infanticide took place, but the older nestling dominated the younger which obtained no food. For the first 40 days the nestling was closely brooded. The nest duties were evenly shared by both parents, but females brooded at night. Food was brought to the nest usually once or twice per day by both parents, and was stored behind the nest. During days 41–90 parental attendance steadily decreased. Dunng this stage the female spent more time in the nesting area (57%) and on the nest (91%) than the male. Towards the end of this stage the nestlin started to feed itself but preferred to be fed by a parent...