Interactions among females in polygynous yellow-headed blackbirds

Summary Two alternative hypotheses which attempt to predict how females within harems in polygynous species should behave toward one another were examined in a population of yellow-headed blackbirds (Xanthocephalus xanthocephalus). The hypothesis that females within a harem are co-operative was not supported. Females in large harems did not enjoy higher reproductive success than those in smaller harems and females within a territory did not initiate their nests close in time or space. The alternative hypothesis that females within a harem are competitive was also not supported. Nest density did not have a negative effect on female reproductive success, females within a territory did not initiate their nests far apart in time or space, and resident females were not very aggressive toward intruding female mounts during the settling period. The data support the idea that females settled independently of one another and had no effect on one another's reproductive success.     

Dominance and competition in an herbivorous lizard

Summary A study of social organization in an herbivorous lizard Ctenosaura hemilopha investigated the role of dominance in group member behavior. Attention focused on a very populous (16 ind.) colony to examine causes and effects of crowding with respect to competitive factors. Lizards were colonial, with a top-rank male, one or more adult females, and various subadults and juveniles. Top-rank males defended harems, with colonies female-biased (1:4 in focal colony). Dominance hierarchy was observed, with females more aggressive than males, except for the top-rank male. Group awareness facilitated adaptive responses to threatening dominants and predators. Strong correlations existed among individual size, rank, and aggression. Top-rank male aggressiveness was partly explained by harem defense. Food resource competition, which causes aggression in female insectivorous lizards, did not explain female C. hemilopha behavior. Response to predators, predator fecal pellet analysis, and tail break frequencies implicate crevice escape sites for predator avoidance as a prime controller of social and population structure in these lizards.     

Harem choice and breeding experience of female southern elephant seals influence offspring survival

Female mammals can increase their lifetime fitness through modification of investment potential and by providing better rearing environments with improved breeding experience. We examined the relationships between reproductive fitness and the behavioural decisions that female southern elephant seals (Mirounga leonina) made during the breeding season. We examined whether mother age and breeding experience influenced reproductive success (measured as 1st-year survival probability), and whether there was a change in the choice of harem size with increasing age. Pups produced by young mothers had lower 1st-year survival probability than pups produced by older mothers. A significant increase in mean female mass with age required an analysis of both these effects on offspring survival. There was a significant positive effect of both female age and mass, and the interaction between the two, on 1st-year pup survival. The proportion of young mothers (<5 years old) decreased and the proportion of older mothers (>6 years old) increased with increasing harem size (harems surveyed from 1997 to 2001). Females chose larger harems in which to breed as they aged. Females demonstrated fidelity to breeding areas among successive breeding seasons, with older females displaying greater breeding-site fidelity than younger females. The mean number of previous breeding attempts per female within a harem (breeding experience) increased significantly with increasing harem size. Breeding females returned to breed later in the breeding season as they aged—we hypothesize that young, subordinate females gain a priority advantage by returning earlier. These results lend support to the hypothesis that there are fitness advantages, in terms of offspring survival, that are conferred to females that breed in successively larger harems with age. Potential mechanisms that select for females to improve their breeding conditions include improved mate selection and the avoidance of conspecific harassment in harems.Communicated by F. Trillmich     

Sexual selection favours small and symmetric males in the polygynous greater sac-winged bat Saccopteryx bilineata (Emballonuridae, Chiroptera)

We investigated how morphological traits of territorial males in the polygynous bat Saccopteryx bilineata were related to their reproductive success. Because of the frequency of aerial courtship displays and defence manoeuvres, and the high energetic costs of flight, we expected small and symmetric males to be better able to court females on the wing and to monopolize copulations with females in their harems. We predicted that small and symmetric males would sire more offspring within the colony and a larger portion of the young born within their harem than large or asymmetric males. We measured size and fluctuating asymmetry of 21 territorial males and analysed their reproductive success in 6 offspring cohorts (n=209 juveniles) using 11 microsatellite loci. As predicted, small and symmetric males had, on average, a higher reproductive success in the colony than large and asymmetric males. The percentage of young sired by males within their harem increased as males decreased in size, but was not influenced by fluctuating asymmetry. As fluctuating asymmetry of males correlated with their reproductive success within the colony but not within their harems, we infer that fluctuating asymmetry is probably related to female choice, whereas male size is probably important for harem defence on the wing.Communicated by G. Wilkinson     

Female aggressive response and hormonal correlates—an intrusion experiment in a free-living passerine

The optimal mating system is rarely the same for males and females—whereas males usually benefit from attracting additional females to the territory, this could incur costs for the resident female. Females should therefore prevent prospecting females from settling on the territory. We studied the male and female behavioral and hormonal responses to simulated female territorial intrusions in free-living bluethroats during the pre-laying period. In the study population, polygyny occurs with potential fitness costs for the resident female. We recorded different aspects of aggressive behavior before and after presentation of a live female decoy and playback of female song. These behaviors were compared with a set of intrusions using a male decoy. At the end of a trial, the birds were captured, and blood samples were analyzed for androstenedione, testosterone, estradiol and corticosterone. During the pre-intrusion period, none of the females were observed. Females generally responded strongly to the female decoy by increased flight rate, vocalizations, and by conspicuous perching. Nearly half of the males displayed to the female decoy but never while the resident female was present. We suggest that resident female aggression in bluethroats prevents courtship by her mate and signals her mating status to the female intruder. Female aggression could therefore prevent additional females to settle on the territory and shape the mating system. Females that responded with song had higher levels of estradiol. These findings suggest that estradiol may support aggression in breeding female birds.     

A cost worth paying: energetically expensive interactions with males protect females from intrasexual aggression

When approached by males, females of the Amarillo fish (Girardinichthys multiradiatus) perform a behaviour called vibration or they are aggressively challenged. We quantified vibration and assessed whether it compromises the rate of feeding attempts in dyads kept in outdoor enclosures. Male approaches resulted in female vibration and in a reduced feeding rate. Vibration was not evoked by female–female aggression, which was frequent and always ended in the subordinate fleeing from the dominant female. Using a closed respirometer we found that vibration is costly; oxygen consumption of females was greater in the presence of a male (which evoked vibration) than in the presence of a non-familiar female (when no vibration occurred). By recording interactions of females confined in aquaria in the presence and in the absence of males, we confirmed that escaping is the only available response to deal with female aggression. Females kept without males participated in frequent aggressive (even lethal) interactions that did not abate while the subordinate female was in sight of the dominant, and which caused premature births and injuries. Yet in the alternative treatment aggression ceased when a male approached, prompting vibration in both females. Thus, in the Amarillo, in as much as it evokes energetically costly female vibrations, male courtship is an expression of sexual conflict. However, in the absence of males, frequent female aggression potentially annuls the benefits of not vibrating. We propose that a complete appraisal of the consequences of sexual conflict must include an assessment of the costs imposed by intra-sexual interactions.     

Kinship and social tolerance among female and juvenile wild house mice: kin bias but not kin discrimination

Kin-biased social tolerance among house mice has been interpreted in terms of kin discrimination. However, several lines of evidence suggest it may instead be an incidental artifact of group member discrimination. This leads to very different predictions about the social consequences of relatedness within and between social groups. Social interactions between wild-stock adult female and juvenile house mice (Mus domesticus) established in neighbouring territorial groups within enclosures reveal relatedness to dominant males within groups as the major factor determining social tolerance of juveniles by females. Relatedness to the female herself had no significant independent effect on responses indicating tolerance. Females were generally more aggressive toward neighbouring-group juveniles (all unrelated to females) compared with those from their own group (all related to females), but were most aggressive toward neighbouring juveniles sired by the neighbouring dominant male. They were also more aggressive toward their own-group juveniles that had been sired by the neighbouring dominant but only when encountered in the neighbouring territory and with a greater bias against female juveniles. Females were least aggressive toward own-group juveniles sired by their own-group dominant male. The sire-bias in tolerance among females is similar to that reported among the dominant males themselves in an earlier study. As a result of the combined sire-bias in tolerance by adult males and females, juveniles sired by their own-group dominant males become less likely to intrude into a neighbouring territory with time. Overall, the results suggest that differences in social tolerance reflect discrimination on the basis of social group membership rather than relatedness between interactants and thus provide strong experimental evidence in support of incidental kin bias rather than kin discrimination.     

The assessment of female reproductive state during courtship and scramble competition in the fiddler crab, Uca paradussumieri

Male fiddler crabs, Uca paradussumieri, mate underground during a 4- to 7-day period each full and new moon. As soon as the tide recedes, males enter the burrows of females that will ovulate the following day ('pre-ovigerous' females). Males copulate with and guard these females until they ovulate. When interrupted by an intruding male, the first male to reach the female is usually able to defend her and successfully mate with her. In fiddler crabs, females mate multiply and there is last male sperm precedence. Before each semi-lunar mating period, male U. paradussumieri were more likely to court females with whom they would later mate than other nearby females with whom they did not mate. This suggests that males collect information on female reproductive state prior to the females becoming ovigerous. In this species, aggression was common between males that courted the same female. When previously courted females were approached by other males, the initial courter attempted to forcefully disrupt the courtship. This behavior may allow males the exclusive use of information on female reproductive condition. It also suggests a type of scramble competition between males over females. Furthermore, it indicates that males are able to locate receptive females prior to their becoming ovigerous. The shorter guarding period observed in this species, as compared with other fiddler crabs, is caused by females rejecting longer guarding periods. Male ability to assess female reproductive status may therefore be advantageous because it increases male mating success within a scramble type of competitive polygyny.     

Female resistance and duration of mate-guarding in three aquatic peracarids (Crustacea)

Both theoretical and empirical studies have treated mate-guarding in aquatic Crustacea purely as a male decision problem. However, male and female interests are rarely identical, as implied by observations of female resistance against guarding attempts. We tested experimentally the occurrence of sexual conflict over guarding duration in three crustacean species: Idotea baltica, Asellus aquaticus (Isopoda), and Gammarus zaddachi (Amphipoda). Specifically, we manipulated, by osmotic stress or a neuromuscular blocking agent, the female's ability to resist guarding attempts. Female manipulation, by both methods, roughly doubled precopula duration in I. baltica (Figs. 1 and 2) showing that female resistance effectively diminishes guarding duration. However, in A. aquaticus and G. zaddachi female manipulation had no effect on guarding duration, which also was longer than in I. baltica (Fig. 2). This implies either that male and female interests are equal or that the conflict is resolved according to the male interest in these species. The lack of female resistance in such species allows long precopulatory guarding. In I. baltica we also manipulated, by osmotic stress and by clipping nails, male ability to hold the female. These treatments had no effect on guarding duration (Figs. 1 and 2). Male size tended to correlate positively with guarding duration in control groups, but not in female manipulation groups (Fig. 3). Thus, conflict is mainly resolved according to the female interest in I. baltica. Results in this species also suggest that female resistance selects for large male size. Consequently, mechanisms of sexual selection may differ considerably between species with otherwise comparable mating patterns.     

Vocalizations by Alaskan moose: female incitation of male aggression

Evidence of female fomentation of male–male aggression as a mechanism of mate choice is rare, especially in mammals. Female choice of mates in polygynous species may be masked by intense male competition or by males attempting to restrict female choice. We studied protest moans of female Alaskan moose Alces alces gigas in interior Alaska, USA, from 1987 to 1990, to determine if moans incited male–male aggression. Alaskan moose exhibit a mating system in which one dominant male (the harem master) herds, defends, courts, and attempts to mate with females in his harem. Protest moans were given by females only in response to courtship. We hypothesized that if protest moans were related to females reducing harassment and exercising mate choice, females should give protest moans more frequently when courted by small males and less often when courted by large males, and that rates of male–male aggression would be elevated following protest moans. Harems were composed of one large male, with a mean of 4.4 females (median = 3 females); 10% of 132 harems included ≥10 females. The temporal pattern of protest moans from late August through November was associated with, but tended to lag behind, mating behavior. The rate of protest moans given by females decreased with increasing size of males courting them. Male–male aggression was significantly less during periods without protest moans than during periods in which protest moans occurred. These results indicate that female moose gave protest moans to reduce harassment by smaller males, and assure a mating opportunity with the most dominant male. Such a subtle mechanism of indirect mate choice by females may occur in other vertebrates in which choice is limited by a mating system in which male–male combat and male dominance over females reduces opportunities for female choice. The importance of female choice may be undervalued in studies of sexual selection in mammals.     

Mate guarding in the swallowHirundo rustica

Summary The importance of mate guarding by males in the monogamous swallowHirundo rustica was studied by temporarily detaining the males. Mate guarding reduced the frequency of extra-pair copulations and of sexual chases involving female mates. Males participated in sexual chases more frequently if they had a non-fertile female. Neighbouring males of ‘widowed’ females increased their own mate guarding presumably in response to the experimentally increased rate of sexual chases. Neighbouring males with a fertile female increased their mate guarding more than did males with a non-fertile female. Addition of eggs to swallow nests in the post-fledging period of the first brood induced mate guarding by male nest owners. These males also copulated more frequently with their mates than did control males. Neighbouring male swallows responded to the increased mate guarding by showing sexual interest in the guarded females. removal of eggs from swallow nests during the laying period, leaving only one egg in the nest, resulted in reduced nest attendance by females. Male mates responded by increasing their mate guarding intensity as compared to controls, and neighbouring males showed an increased sexual interest in these females.     

Aggressive brood defense by females in the stomatopod Gonodactylus bredini

Summary Defense behavior by brooding Gonodactylus bredini females was studied in staged fights against conspecific challengers. Females brooding eggs were more successful at defending their dwelling cavities than were nonreproductive females. Females caring for larvae were no more successful at cavity defense than were controls.Brooding females do not defend cavities by using more aggressive acts or by engaging in longer fights than nonreproductive females. The increase in defense success seems to be due to a greater willingness to continue a contest after being struck by a challenger. Also, brooding females make more frequent use of threats, in particular the Meral Spread display, than controls.Females defend cavities as vigorously as if they still had eggs 4 or 5 days after their eggs have been removed. Within the first 24 h after oviposition, a female's success at defending a cavity is greatly reduced compared to that of a female just 1 day later.     

Plasticity in spawning visits of female damselfish, Stegastes nigricans : effect of distance to mates

Both sexes of the damselfish Stegastes nigricans hold individual territories in which they feed on filamentous algae. At dawn, females visit males' territories to spawn, and the males guard the eggs until hatching. We examined how females' spawning behavior varied according to the distances to their mates. Females usually mated with a single male per morning. The distance to the territory of a mate (0.7–12.8 m) did not affect a female's total spawning time per morning, but affected the number of her spawning visits with that male. Females made many repeated spawning visits when spawning with males at short distances, while they spawned the entire clutch in one visit when spawning with males at long distances. This plasticity in female behavior appears to be related to two costs during the spawning visits: (1) intrusions by other fish to feed on algae in the female's territory during her absence, which may cause the female to return repeatedly to her territory for defense, and (2) attacks on the female by other territorial fish, which increased with the distance to their mates' territories. To minimize the sum of both costs, females should change the number of spawning visits depending on the distance to the males' territories. Received: 30 September 1996 / Accepted after revision: 17 March 1997     

Aggression and submission reflect reproductive conflict between females in cooperatively breeding meerkats Suricata suricatta

In many cooperatively breeding species, dominant females suppress reproduction in subordinates. Although it is commonly assumed that aggression from dominant females plays a role in reproductive suppression, little is known about the distribution of aggressive interactions. Here, we investigate the distribution of aggressive and submissive interactions among female meerkats (Suricata suricatta). In this species, dominant females produce more than 80% of the litters, but older subordinates occasionally breed. Dominant females commonly kill the pups of subordinates and usually evict older female subordinates from the group 1–3 weeks before the birth of the dominant female's litter. The aggression frequency of the dominant female toward subordinates and the submission frequency that each subordinate female showed to the dominant female increased as the age of the subordinate female increased and as the birth of the dominant female's pups approached. Moreover, as birth approached, both of these behaviors intensified more quickly between the dominant female and older subordinates than between the dominant female and younger subordinates. The aggression frequency of the dominant female toward each subordinate female predicted whether that subordinate female was evicted from the group; the submission frequency by each subordinate female predicted the timing of their eviction during the pregnancy period of the dominant female. These results support the idea that conflict between dominant and subordinate females increases with the age of subordinates and, since older subordinate females are most likely to reproduce, suggest that dominant females may less easily control reproductive attempts by older subordinate females.     

Female-enhanced male competition determines the first mate and principal sire in the spider Linyphia litigiosa (Linyphiidae)

Summary Two to five days before sexual maturation, female sierra dome spiders (Linyphia litigiosa: Linyphiidae) undergo a transformation in their behavior toward males that visit their webs. During this latter part of their penultimate instar, females change from consistently positioning themselves far away from males to actively maintaining close proximity, reactions I call avoidant and associative behavior, respectively. Consistent associative behavior ceases after the female's first mating and thus is limited to soon-to-mature penultimate females. When a mate-seeking male fords an associative female, he attempts to guard her until she matures; this is often a multi-day affair. In contrast, males guard immature avoidant and mature mated females for only a single day. This dichotomy in male guarding times can be understood by the fact that associative behavior signals that the female will soon develop peak reproductive value. Upon completion of their final molt, 98% of females immediately mate with the current guarding male. Secondary suitors are not as likely to achieve mating. Moreover, first mates father 1.8 times more offspring, on average, than secondary mates. Whenever they meet on any female's web, males fight until one of the contestants withdraws. Fights typically are intensive, occasionally deadly, and often result in usurpation of the web by the newly arriving male. Larger males win more fights, but other qualities (e.g., vigor and persistence) appear to be important when contestants differ by less than 10–20% in body weight. Prolonged (i.e., multi-day) guarding of associative females enhances the intrasexual selection process by ensuring that every male that arrives at the web finds it already guarded. Therefore every male that finds the web becomes a participant in a series of male-male conflicts and web usurpations which span the period between the resident female's commencement of associative behavior and her sexual maturation. Since unforced male departures from the webs of associative females are rare, victors are retained on the web until they themselves lose a fight. This facilitates a steady increase in the fighting ability of sequential guards throughout the associative period, up until female maturation and mating. On my study site, first mates represented the final winners in a combative sorting process based on a minimum average of 2 fights; they were heavier and larger than secondary mates and randomly sampled males. The combination of (1) associative behavior by nearly mature females, (2) high mating propensity of newly mature females, and (3) first male sperm priority, constitutes a system whereby females enhance male-male competition and boost the expected fighting prowess of the principal sire of their progeny. Since males appear to make no material contribution toward progeny, the female's behavior probably functions to improve the genetic constitution of the offspring. In addition, the timing of associative behavior may limit prolonged guarding by food-stealing males to a period (1) encompassing the female's pre-molt fast and (2) before the heavy yolking of eggs, thereby ameliorating the nutritional costs of intrasexual selection.     

Mate quality,not aggressive spillover,explains sexual cannibalism in a size-dimorphic spider

Sexual cannibalism particularly before mating is costly for the male victim but also for the female aggressor if she risks remaining unmated. The aggressive spillover hypothesis explains the persistence of this behavior as a maladaptive side effect of positive selection on aggressiveness in a foraging context. The hypothesis predicts that the occurrence of sexual cannibalism is explained by female aggressiveness but is not related to male phenotype or behavioral type. An alternative hypothesis invokes sexual selection and makes the opposite prediction namely that sexual cannibalism is an expression of female choice and should hence mainly target males of low quality. We tested the above hypotheses on a sexually dimorphic nephilid spider Nephilengys livida, known for male monopolization of females via genital damage, female genital plugging, and mate guarding, by staging mating trials during which we recorded mating behaviors and occurrences of pre- and postcopulatory cannibalism. We did not restrict assessment of aggressiveness to the mating and foraging context but also included aggression against same sex conspecifics. To assess female personalities, i.e., consistent individual differences in behavior including aggressiveness, we repeatedly tested them for intra-sex aggression, voracity towards prey, locomotory activity, and boldness. Females exhibited consistent differences in intra-sex aggressiveness, latency to attack prey, and boldness. Aggressive females had shorter latencies to attack prey and were more active than non-aggressive ones. In contrast to the predictions of the aggressive spillover hypothesis, females that were aggressive towards prey and towards other females were not more likely to attack a male than non-aggressive females. In support of the mate choice hypothesis, less aggressive males were more likely attacked and cannibalized than more aggressive ones. This hints at sexual selection for aggressiveness in males and raises the question of mechanisms that maintain variation in male aggressiveness.     

Why are larger convict cichlid (Cichlasoma nigrofasciatum) fry sometimes adopted into broods of smaller fry?

Females of Elasmucha grisea defend their eggs and small nymphs against invertebrate predators. Females sometimes guard their clutches side by side on the same birch leaf. We studied benefits of this joint guarding both in the field and in the laboratory. We found that adjacent females had significantly larger clutches than solitary females. In the laboratory, we studied the effectiveness of joint versus single defence against ant (Formica uralensis) predators. We established female pairs from initially singly guarding females by cutting off pieces of leaves with egg clutches and pasting them beside another female guarding her clutch. In the control group the females with their clutches were similarly cut off but these clutches were placed on another leaf without any female. The birch twigs where females guarded their clutches were placed in cages in close proximity to laboratory ant nests. In the experimental treatment, two females guarded their clutches together and at the same nest there was another birch twig without a female. In the control treatment two twigs with one female on each were placed close to another ant nest. Two females defended their clutches significantly more successfully, losing fewer eggs than did the single females. This primitive form of female sociality in parent bugs resembles colonial nesting in birds, where communal defence is also important. However, to our knowledge this is the first experiment where the benefit of joint guarding has been tested directly by manipulating the size of the breeding group rather than by measuring the risk of predation in groups of different size.     

Female settlement in marsh wrens: is it affected by other females?

Summary The influence of resident females on the settling patterns of subsequent females was examined for marsh wrens (Cistothorus palustris). Proportionally more females responded to a playback of a female intruder during the pre-laying stage than during the laying and incubation stages. The strength of the response was also greatest during pre-laying and laying stages (Table 1). Female wrens did not settle randomly with respect to the presence of other females, but rather settled with bachelor males more often than predicted by chance (Table 3). Female settlement within territories was more asynchronous than settlement between territories. An attempt to fit these data to two models of female settlement suggested that prospecting females were not displaced by residents, but rather avoided settling with mated males (Table 3). Females within territories were also more dispersed than predicted by chance, a pattern that may be enforced through aggression on the part of the resident female. Staggered settlement reduces the overlap in nestling stages of harem mates and so may be a strategy to reduce competition for resources at this critical stage.     

Histology of the gonads and observations on the social behavior of the Caribbean angelfish Holacanthus tricolor

Male and female specimens of Holacanthus tricolor were collected in May 1981 and September 1982 off the Island of St. Croix. U.S. Virgin Islands, and their gonads were examined histologically. All testes examined showed vasa deferentia which were characteristic of secondary males, suggesting protogynous sex change. A transitional individual supported this interpretation. Ovarian histology and size-specific egg counts revealed seasonal differences in spawning frequency and clutch size. Females appeared to spawn every day in May. The timing of final maturation and hydration of the oocytes indicated that spawning occurred in the late afternoon or early evening. Not all females spawned every day in September, and clutch sizes were smaller than those of May. Behavioral observations of H. tricolor in the field revealed that males defended harems of two to four smaller females. Within each harem, females of different sizes had overlapping home ranges, while the home ranges of females of similar size did not overlap. Besides size dimorphism between the sexes, H. tricolor exhibited a previously unreported sexual dichromatism.     

Two models for the evolution of polygyny

Summary Two models of conditions for the evolution of polygyny are treated axiomatically. Both models assume a social system based on female mate choice in situations in which a female is better off if she mates polygynously with an already mated male on a superior territory than if she selects a bachelor on an inferior territory. One model, the competitive female choice model, assumes that the females of a harem compete for the limited resources of the harem and thus that their fitness decreases as co-wives are added. The cooperative female choice model assumes that, within limits, a female's fitness is improved by the addition of co-wives to her mate's harem, as a result of cooperative interactions within the group. For each model, a sufficient set of independent assumptions is provided. Implications of the models are indicated and methods for testing them are discussed.