Individual variation in mate care by alpha males in crested tit winter flocks

Summary Winter flocks of crested tits Parus cristatus, typically consisting of two adults and one or two non-kin 1st-year birds, were observed to split up into subflocks in a way related to ambient temperature. On warm days, when they were foraging in subflocks without 1 st-year birds, alpha males always occupied the most preferred upper foraging sites, as expected from their top dominance rank. On cold days, when foraging in flocks with 1st-year birds, 8 out of 13 alpha males shifted to lower (less preferred) positions below their alpha mates while allowing the latter to forage at the best sites. As enhanced access to preferred microsites on days with high energy stress is believed to increase overwinter survival probability, this shifting behaviour of alpha males can be considered as a form of mate care. Out of 13 alpha males, however, 5 did not shift and always occupied the best foraging sites irrespective of flock composition. As (i) these non-shifting males were in poorer physical condition than shifting males, (ii) they scanned significantly less for predators than either females or shifting males when foraging in the uppermost tree parts, and (iii) four out of five non-shifting males were replaced by immigrants in early spring, absence of mate care during winter may be caused by constraints due to condition. High-quality-territory owners in poor condition at the end of autumn were most vulnerable to replacement by immigrants. Therefore, as four out of five replacements affected high-quality territories, selective intrusion by immigrants is suggested. Correspondence to: L. Lens     

Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)

Summary We studied how age, body size and prior residency affected social dominance in the willow tit (Parus montanus) groups. The contribution of each variable was experimentally tested in unisexual two-bird trials, in which the birds were matched for all variables except the one studied. Large birds were dominant over smaller ones (Fig. 1). The effect of body size was more prominent in males than in females. Age had no influence on dominance. Residents became dominant more often than newcomers (Fig. 2). Adulthood or larger body size did not override the advantage of prior residency (Fig. 2). Therefore, the proximate reason for the age-dependent dominance seen in natural willow tit flocks is most likely the prior residency advantage of the adults. Factors connected with fighting ability (body size and age) seem to be less important than the time of establishment of rank, which may reflect the importance of resource value differences between residents and newcomers in this context. The advantage of residency might make it advantageous to be a member of a flock even as a subordinate, rather than being solitary.     

Mass-dependent take-off ability in wintering great tits (Parus major): comparison of top-ranked adult males and subordinate juvenile females

When birds are attacked by predators, initial take-off is crucial for survival. Theoretical studies have predicted that predation risk in terms of impaired flight ability increases with body weight. However, studies in which attacks were simulated, and within-individual daily changes in body weight were used to test mass-dependent take-off outside migration period, have so far failed to show an effect of mass on velocity. In this field study I compared the mass/velocity relationships of alarmed adult male and juvenile female great tits, Parus major. Fattening strategies differ among members of the dominance-structured basic flocks of wintering great tits, and dominant individuals often carry significantly less amount of fat reserves than subordinates. Since the range of body weight gain/loss is the least among dominant males, it was expected that impaired flight ability is more likely in lower-ranked female great tits. The results show that the birds differed significantly in their daily increase of relative body weight. Average daily weight increase of adult males was 6.2%, while it was 12.2% in juvenile females. Males were faster than females at take-off both at dawn and at dusk. Flight velocity of males did not differ significantly between dawn and dusk, whereas females took off at a significantly lower speed at dusk than at dawn. The results suggest that the larger fat reserves of subordinate females needed to increase their chances of overwinter survival probably place them at increased risk of predation. Electronic Publication     

Functional aspects of the pair bond in winter in Bewick's swans (Cygnus columbianus bewickii)

Summary The benefits to partners of monogamous pairs of maintaining continual spatial proximity in the non-breeding season were studied in Bewick's swans wintering in Norfolk, UK.When separated from their mates, females were less successful in aggressive encounters, were threatened more frequently and spent less time feeding than when close to them. Males also suffered reduced success in encounters and a higher frequency of threats by other flock members, though the effect was less pronounced than for females. This sex difference in effect of separation may be associated with the greater weight of males and the fact that success in encounters is related to weight.Partners appeared to assist each other by joining in aggressive encounters, as well as by inhibiting other birds from threatening their mates. However, the precise manner in which the female assisted the male is still obscure, since the highest intensity aggressive encounters — physical fights — involved only male partners. It is suggested that the male may fight harder in his mate's presence.Proximity of partners varied with situation and between different pairs. Partners maintained greater proximity in dense flocks than in dispersed flocks and showed a tendency to stay closer when feeding on winter wheat than on waste potatoes. Partners where the male was high-ranking spent more time together than those where the male was low-ranking.     

Do female black-capped chickadees prefer high-ranking males as extra-pair partners?

Previous studies have shown that some female black-capped chickadees (Poecile atricapillus) solicit copulations from males that rank higher in winter flocks than their social mates, and extra-pair paternity in nests occurs commonly enough to be considered a potential female mating tactic. This study uses blood samples collected in 1992–1995 from 58 families of black-capped chickadees to test whether females with extra-pair offspring have chosen extra-pair sires higher in social rank than their mates. Paternity was assessed with multilocus DNA fingerprinting in 1992–1994 nests and with microsatellite and single-locus minisatellite DNA typing in 1995 nests. Seventeen of 58 nests (29.3%) contained young genetically mismatched with their social father. In 11 of 15 cases where the identity of the extra-pair male was known, the extra-pair male was dominant to the social father. Using data from 29 nests located in 1994 and 1995 for which we had the most data on relative ranks of males, high-ranking males had greater realized reproductive success than low-ranking males as a result of extra-pair fertilizations. There was no significant difference between the number of nests containing extra-pair young of females mated to low-ranked versus high-ranked males. Two nests in 1995 contained young either genetically mismatched with both social parents (intraspecific brood parasitism) or, in one nest, genetically mismatched with the social mother but not the social father (quasi-parasitism). The implications of female strategies acquiring genetic benefits through extra-pair copulations are discussed. Received: 7 July 1997 / Accepted after revision: 14 March 1998     

Singing and the establishment and maintenance of dominance hierarchies in captive brown-headed cowbirds

Summary The importance of singing in the establishment and maintenance of dominance rank was investigated in captive flocks of brown-headed cowbirds (Molothrus ater). Each flock consisted of 3 males and 1 female. In one experiment the two largest males in each of 4 flocks were temporarily devocalized by puncturing the interclavicular air sac just before group formation. The third male in each group was sham-operated. There was a significant tendency for the non-devocalized birds to become the dominant member of their group. Only sham-operated males sang immediately after the operation. In a second experiment, dominant males from flocks with established hierarchies were temporarily devocalized. None of these males lost their dominant positions, despite their inability to sing. These results suggest that song is important in the establishment of dominance rank; however, once established, social hierarchies may be maintained by non-vocal means, such as social inertia or individual recognition.     

Reproductive suppression and inbreeding avoidance in wild populations of co-operatively breeding meerkats (Suricata suricatta)

Meerkats live in co-operatively breeding familial groups in which reproduction is monopolised by a dominant pair of breeders. Offspring of the breeders are behaviourally subordinate, and typically remain in their natal group as sexually mature, non-breeding helpers. In this study, we investigated the proximate factors limiting subordinate reproduction. Evidence for reproductive suppression by dominants was investigated by comparing life history, behaviour and hormonal profiles of dominants and subordinates. Baseline levels of plasma luteinising hormone (LH) were significantly higher in dominant than in subordinate females. However, following an exogenous injection of gonadotrophin-releasing hormone (GnRH), both categories had comparable concentrations of circulating LH. There were no significant differences in pre- or post-GnRH challenge LH levels in dominant or subordinate males. Reproduction in both dominant and subordinate females rarely occurred in the absence of unrelated males. Given that groups typically comprise parents and offspring, lack of suitable mates emerged as the primary constraint on subordinate reproduction. When this constraint was removed, subordinates typically bred but at a lower rate than dominants. This difference in reproduction may be attributed to intrasexual competition manifested through direct interference by dominant females through subordinate evictions, infanticide and the abandoning of subordinate litters. We argue that differences in reproductive regulation within mammalian co-operative breeding systems may be explained by differences in the mating strategy (inbreeding versus outbreeding) and the probability that subordinates in obligate outbreeding species will encounter unrelated opposite-sex partners. Received: 19 April 2000 / Accepted: 17 July 2000     

Rank-dependent hoarding effort in willow tits (<Emphasis Type="Italic">Parus montanus</Emphasis>): a test of theoretical predictions

Many birds and mammals store energy as hoarded food supplies. A supply of stored food is beneficial during periods when food is scarce, but building up and managing such a supply also entails costs. The optimal number of caches will be reached when the net benefit is at its maximum. If dominants can steal more stored food from subordinates than the other way around, the optimum will differ between these categories. A previous theoretical model of hoarding in groups with dominant and subordinate members produced three testable predictions: (1) hoarders should store more food as anticipated future conditions get worse; (2) subordinate flock members should store more food than dominants; and (3) dominants should increase hoarding relatively more than subordinates as conditions get worse. Here we present a field experiment on willow tits (Parus montanus) designed to test these predictions. We found support for all three. Hoarding increased as conditions got worse, subordinates stored at a higher rate than dominants, and dominants increased their hoarding effort relatively more than subordinates as conditions worsened. These results support the incorporation of information on dominance and food availability into models predicting food storage behaviour.Communciated by J. Dickinson     

Female choice and the quality of parental care in the great tit Parus major

Summary Previous work on great tits suggested that female mate choice was based on the characteristics of the male rather than the quality of his territory. The aim of this paper was to see whether females were deriving any direct benefits as a result of such choice by comparing male plumage colouration (the size of the central black breast-stripe) with the quality of parental care provided by both members of the pair. Males with large stripes were more likely to defend their broods because their nest attentiveness was higher increasing the chances of predator detection. Males with large stripes produced heavier fledglings primarily as a result of faster growth early in the nestling period. Previous work on great tits has shown that heavier fledglings have a higher survival probability than lighter ones. The results of this study can more easily be explained by differences in parental quality between males rather than as a result of variation in female or territory quality. Therefore, female great tits could increase their reproductive success by pairing with males with a large stripe because such males appear to be better quality parents. This suggests that female choice in the great tit may concern, at least in part, the quality of male parental care.     

Associations among members of a black-capped chickadee flock

Summary One color-banded black-capped chickadee flock was studied intensively at a winter feeder in Wisconsin. Flock structure was examined by measuring association among flock members in three ways: spatial proximity, temporal association, and association of individuals during flock movements. Additional information was obtained on aggressive interactions at and near the feeder. By midwinter, the strongest attachments were between males and females who subsequently mated; pair attachments strengthened as the breeding season approached. Lower ranked individuals in the dominance hierarchy showed weaker associations with other flock members in early spring than did more dominant individuals. The strong attachment to the mate may have implications for altruistic behavior within the winter flock.     

Do female bank voles (Clethrionomys glareolus) mate multiply to improve on previous mates?

Contrary to classical sexual selection theories, females of many taxa mate with multiple males during one reproductive cycle. In this study, we conducted an experiment on the “trade-up hypothesis”, which proposes that females remate if a subsequently encountered male is potentially superior to previous mates to maximize the genetic quality of their offspring. We presented bank vole females (Clethrionomys glareolus) sequentially with two males of known dominance rank in different orders, i.e., either first subordinate and second dominant, first dominant and second subordinate, or two males that were equal in dominance (high ranking) and observed their mating behavior. We found that 92% of the females mated multiply and did not base their remating decision on male social status. Therefore, polyandry cannot be explained by the “trade-up hypothesis” based on dominance rank in this species. However, we found that dominant males sired significantly more offspring than subordinate males. This varied according to mating order: dominant males sired more offspring when they were second than when they were first. Moreover, litter sizes were significantly smaller when the dominant male was first (smallest relative success of dominant males) compared to litter sizes when mating order was reversed or both males equal in status. Our results suggest that even though multimale mating includes males that are of poorer quality and thus potentially decreases the fitness of offspring, most of a female’s offspring are sired by dominant males. Whether this is due to cryptic female choice, sperm competition, or a combination of both, remains to be tested.     

Costs of switching social groups for dominant and subordinate dark-eyed juncos (Junco hyemalis)

I quantified the costs of switching from a familiar to an unfamiliar flock for captive dark-eyed juncos (Junco h. hyemalis) by measuring several physiological and behavioral variables before and after flock switching. Birds that were initially dominant dropped in status in unfamiliar flocks, and experienced increased metabolic rates, while subordinate birds appeared to undergo less physiological change when switching flocks. This difference occurred despite a lack of any rank-related differences in the effects of joining a new flock on rates of aggression, weight change, access to food, or plasma corticosterone levels. These results suggest that for dominant, but not subordinate, individuals there is a measurable metabolic cost to joining a new social group, even in the absence of adverse factors such as food limitation. Dominant individuals may be less likely than subordinates to leave familiar flocks because of their higher metabolic costs when joining a new social group.     

Female rock shrimp<Emphasis Type="Italic"> Rhynchocinetes typus</Emphasis> mate in rapid succession up a male dominance hierarchy

The dynamics of male-male competition for mates and patterns of female choice depend critically on the social environment. We released newly molted sexually receptive females of the rock shrimp Rhynchocinetes typus in the field and recorded their interactions with males. In the dense aggregations in which these shrimp live, most females were encountered and seized by males within 2 min. Usually, females were first seized by subordinate males, and subsequently taken over by the dominant males. Many females (17 out of 23) had multiple mates during the 10-min observation period, and most of them received spermatophores from multiple males. Males used different mating tactics in accordance with their dominance status: subordinate males often used the sneaking tactic, seizing the female and immediately transferring spermatophores. In contrast, all dominant males used the primary mating tactic; they seized and stimulated the female before transferring spermatophores. Results from previous studies had indicated that females may reduce the fertilization chances of subordinate males by delaying spawning and removing spermatophores. We suggest that this capability in combination with the observed rapid mate succession may enable females to exploit male contest behaviors.Communicated by P. Backwell     

Reproductive defence priorities of male willow ptarmigan (Lagopus lagopus): enhancing mate survival or extending paternity options?

Summary Cock willow ptarmigan (Lagopus lagopus) closely guard their mates from predators and conspecific males, and vigorously defend their nests and young. In view of potential costs and benefits of behavioural guarding descisions, I designed a test to examine if, when and how males altered defence priorities. Cock willow ptarmigan were very attentive to their mates throughout the breeding season, unaccompanied hens comprising less than 3% of the sightings of mated females. From the second week of incubation until shortly after hatch, males attempted to guard both mates and offspring. Males guarded mates preferentially over nests until the third week of incubation; they then changed their priority to defence of offspring (nest and/or chicks). The overall defence response initially was strong, but decreased after the first week of incubation. Strength of response increased again in late incubation, and declined as chicks began to fly. Consistency of defence decisions was highest during prenesting and egg laying and lowest during late incubation. Since males defended their mates for a longer period than required for protection of paternity, hypotheses predicting enhanced mate survival and extended paternity options through renesting were examined. Removal of males did not result in reduced survival or increased weight loss of widows during incubation. In light of heavy nest depredation and displacement pressures by conspecifics, mate guarding throughout incubation enhanced male fitness by ensuring paternity in renesting attempts.     

Extra-pair paternity in the monogamous alpine marmot (Marmota marmota): the roles of social setting and female mate choice

Extra-pair paternity (EPP) can be influenced by both social setting and female mate choice. If evidence suggests that females try to obtain extra-pair copulations (EPCs) in order to gain genetic benefits when mated to a homozygous and/or to a related male, females may not be able to choose freely among extra-pair mates (EPMs) as the social mate may constrain female access to EPMs. In this study, we investigated, first, how EPP depended on social setting and specifically on the number of subordinate males in the family group in a highly social and monogamous mammal, the alpine marmot. Second, we investigated how EPP depended on female mate choice for genetic benefits measured as male mate-heterozygosity and within-pair relatedness. Our results reveal, first, that EPP depended on the social setting, increasing with the number of subordinate males. Second, EPPs were related to relatedness between mates. Third, EPMs were found to be more heterozygous than within-pair males. Thus, social setting may constrain female choice by limiting opportunities for EPC. However, after accounting for social confounding factors, female choice for genetic benefits may be a mechanism driving EPP in monogamous species.     

Infant care,infanticide, and female reproductive strategies in polygynous groups of common marmosets (Callithrix jacchus)

Reproduction in marmoset and tamarin groups is typically restricted to a single dominant female, but it is unclear why subordinate females tolerate delayed reproduction. The presence of two breeding females in free-ranging groups of common marmosets (Callithrix jacchus) presented a unique opportunity to examine differences in the reproductive strategies of dominant and subordinate females. Three groups were monitored for 12–18 months at a forest reserve in northeastern Brazil. Data on infant care were collected during two consecutive all-day follows every 10 days until the infants were 2 months old. Carrying patterns for infants born to dominant females were similar to those observed in groups containing a single breeding female. All group members over 5 months of age participated in infant care, and dominant females allowed some group members to carry their infants from the 1 st day of life. In contrast, subordinate females were protective of their offspring and were their sole caretakers for at least a week following birth. One infant born to a subordinate was killed in an attack involving the dominant female. Overall, dominant females gave birth to more infants and had higher infant survival than did subordinate breeding females. subordinate females were successful in rearing young only when the timing of births was such that they did not overlap with the dependency period of infants born to the dominant female. These patterns suggest that subordinate females may face resource competition, especially over access to helpers. In times of severe competition, subordinate females that delay reproduction may be avoiding a wasted reproductive effort.     

Female choice contributes to offspring fitness in the field cricket,Gryllus bimaculatus (De Geer)

Summary Females of the field cricket, Gryllus bimaculatus, are known to mate non-randomly with respect to male size. The reproductive output of females allowed to choose their mates was compared with that of females allocated mates. Females allowed to choose their mates laid a greater proportion of their available eggs, thereby investing more heavily in reproduction. Female choice did not appear to influence hatching success, suggesting that no short term benefits are derived from non-random mating. However, the offspring of females allowed to choose their mates developed more rapidly and began their own reproductive output before those of females who had been allocated mates. Furthermore, the faster developing offspring of females who chose their mates tended to have a consistantly higher survival rate as well as a lower variance in survival (i.e. less risk of mortality) at adult eclosion. Female choice may therefore contribute to offspring fitness and thus to long term reproductive success. Females allocated large males as mates did not equal those of females allowed to choose their mates, in terms of reproductive output and offspring fitness. This suggests that females may choose their mates with regard to a mixture of characters of which male size is only one.     

Kinship and aggression: do house sparrows spare their relatives?

Kin-selection theory predicts that relatedness may reduce the level of aggression among competing group members, leading to indirect fitness benefits for kin-favoring individuals. To test this hypothesis, we investigated whether relatedness affects aggressive behavior during social activities in captive house sparrow (Passer domesticus) flocks. We found that sparrows did not reduce their aggression towards kin, as neither the frequency nor the intensity of fights differed between close kin and unrelated flock-mates. Fighting success was also unrelated to kinship and the presence of relatives in the flock did not influence the birds’ dominance rank. These results suggest that the pay-offs of reduced aggression towards kin may be low in non-breeding flocks of sparrows, e.g. due to competition among relatives as predicted by a recent refinement of kin-selection theory. Our findings indicate that the significance of kin selection may be restricted in some social systems such as winter aggregations of birds. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sexual selection and the function of a melanin-based plumage ornament in polygamous penduline tits <Emphasis Type="Italic">Remiz pendulinus</Emphasis>

Melanin-based ornaments are often involved in signaling aggression and dominance, and their role in sexual selection is increasingly recognized. We investigated the functions of a melanin-based plumage ornament (facial ‘mask’) in male Eurasian penduline tits Remiz pendulinus in the contexts of male–male aggression, mating success, and parental care. The penduline tit is a passerine bird with a unique mating system in which both sexes may mate with several mates in a breeding season, and one (or both) parent deserts the clutch. Our study revealed that mask size of males is more likely an honest signal used by females in their mate choice decisions than a trait involved in male–male competition. First, mask size increased with both age and body condition, indicating that the mask may signal male quality. Second, males with larger masks paired more quickly and had more mates over the breeding season than males with smaller masks. Third, we found no evidence that male mask size signals male–male aggression or dominance during competitive encounters. The increased mating success of large-masked males, however, did not translate into higher reproductive success, as nestling survival decreased with mask size. Therefore, we conclude that there is either no directional selection on male mask size or males with larger masks receive indirect, long-term benefits.     

Sex differences in fitness following a group take-over among Toque macaques: testing models of social evolution

Summary A group of toque macaques took-over the home range of one of its subordinate neighboring groups and fused with it to form a larger cohesive group. In the 7 years before the take-over, the dominant group had consistently won all contests at common feeding sites, yet the fitnesses of the females of these two groups did not differ significantly (Fig. 2A). After the take-over the females of the subjugated group occupied the lowest ranks in the combined dominance hierarchy of the merged groups (Fig. 1) and thereby lost the advantages of an own home range, such as priority of access to food. Consequently, in the merged group, survivorship and reproductive success among the subjugated females were significantly less than among the females of the dominant subgroup (Table 2, 4). The dominant matrilines grew numerically and replaced all of the subjugated females, and all but one of their offspring, within 8 years after the take-over (Fig. 2B). These data support the hypothesis that cooperation among female kin in defending resources against strange females is important in the evolution of female-bonded groups. Before the merger all 5 natal males of the subordinate group had transferred to the dominant group, where they occupied high and mid-level dominance ranks (Fig. 1). These males survived at a significantly greater rate than their subordinate female kin. Thus, the cost of group transfer seems to be greater for females than for males, and this may be one reason that females generally do not emigrate or that groups do not fuse. The data suggested three hypotheses. First, since large body size and other adaptations for fighting, giving males an advantage in male-male competition for mates, are also of advantage in resource competition with males and females, such male characters may also be favored by non-sexual selection, especially where male reproductive strategy involves group transfer. Second, female bonded groups evolved as female defensive coalitions against not only female but also male resource competitors, there having been a mutual influence in the coevolution of large-sized males and female gregariousness. Third, female defensive coalitions against large-sized aggressive males are also advantageous out-side the context of food competition, or, independent of foraging strategy.