The effect of helpers on artificially increased brood size in sociable weavers (Philetairus socius)

In cooperatively breeding birds, the presence of helpers is expected to increase the reproductive success of the breeding pair. However, some studies fail to find this effect. A positive effect of helpers may be restricted to cases in which a breeding pair has a poor likelihood of raising the entire brood on its own, as would be the case under stressful environmental conditions or with enlarged brood sizes. We conducted brood size manipulations in a cooperative breeder, the sociable weaver, Philetairus socius, to investigate the relationship between the difficulty of raising nestlings and the effort and impact of helpers. Overall, sociable weavers did not work harder to raise the enlarged broods. However, the presence of helpers significantly increased the feeding rates at enlarged nests, but not controls. This was insufficient to prevent generalised brood reduction in enlarged broods, whether attended by pairs alone or with helpers. Nonetheless, the presence of helpers was associated with decreased nestling mortality and an increase in the numbers of young fledged. Our results suggest that groups are better able to respond to the needs of enlarged broods than pairs alone and that the presence of helpers has a beneficial effect on overall reproductive success.Communicated by J. Dickinson     

Kin associations and direct vs indirect fitness benefits in colonial cooperatively breeding sociable weavers Philetairus socius

Indirect fitness benefits are believed to be an important force behind the evolution of cooperative breeding. However, helpers may associate with their relatives as a result of delayed dispersal, hence, kin associations might be a consequence of demographic viscosity rather than active choice. In addition, recent studies showed that helpers may have access to reproduction therefore direct benefits might also play an important role. Here, we investigate the possible roles of direct genetic benefits and kin associations on helping behavior in the sociable weaver Philetairus socius, a colonial and cooperatively breeding passerine. We used a microsatellite-based genotyping method to describe the genetic structure within nests and colonies. Within a colony, we found considerable genetic structure between males but not females. Sociable weaver colonies have several nests that are simultaneously active, giving individuals a choice of associating with a range of first-order kin to unrelated individuals. Helpers were significantly more related to the young in the helped nests than in other nests of the colony, suggesting an active choice for associating with kin. The helpers were generally offspring or first-order relatives of one (50%) or both (43%) breeders, although more infrequently, seemingly unrelated individuals also helped (7%). We found no supporting evidence of extrapair parentage and hence no direct genetic gains from helping in our population. This strong reproductive skew is contrary to theoretical models predicting conflicts over reproduction in stepfamilies. We discuss whether female decisions and/or other direct benefits of remaining in kin associations or helping might explain the high skew observed.     

Multiple paternity in the cooperatively breeding fish <Emphasis Type="Italic">Neolamprologus pulcher</Emphasis>

In cooperative breeders, mature males may compete for fertilizations. In this study, we measured the degree of multiple paternity in a natural population of a cooperatively breeding fish. Neolamprologus pulcher (Perciformes: Cichlidae) is a highly social cichlid endemic to Lake Tanganyika. We used highly variable microsatellite loci to survey 12 groups with an average number of 10.6 brood care helpers per group and a total of 43 offspring (mean 3.6 per brood). In 11 of 12 groups, all young were assigned to the dominant female. The dominant male sired all offspring in three groups, part of the offspring in four groups, and in five groups, he had no paternity at all. In total, 44.2% of young were not fathered by the current male territory owner. Multiple paternity was found in 5 of 12 broods (41.7 %), with 8 of 35 young (22.9 %) being sired by males other than the respective territory owners. This is an exceptionally high rate of extra-pair paternity among cooperatively breeding vertebrates. Neither helpers present in these territories during collection nor neighbouring males were unequivocally assigned to have sired these extra-pair young. However, behavioural observations suggest that male helpers may have produced these young before being expelled from the territory in response to this reproductive parasitism. We discuss these results in the light of reproductive skew theory, cooperative breeding in vertebrates and alternative reproductive tactics in fish.     

The effects of helpers in cooperatively breeding purple gallinules

Summary The effects of helpers in a population of cooperatively breeding purple gallinules (Porphyrula martinica) were examined. All young birds past the age of 2 months helped feed and protect subsequent broods of chicks and participated in territorial defense. Most helpers remained on their natal territories for approximately 1 year. The number of helpers varied both among and within breeding groups. Clutch production and chick survival were related positively to the number of helpers in the group. The increase in chick survival was independent of several measures of territory quality. Helpers possibly aided chick survival by provicing extra food for the chicks and decreasing predation risk. Helpers were necessary in order for a breeding pair to keep a territory long enough to produce more than one clutch of eggs. A change in the number of helpers (increase or decrease) often was followed by a similar change in territory size. These results suggest that purple gallnule helpers can increase the reproductive success of the breeding group and may be vital for the continued maintenance of a breeding territory.     

Social system and reproduction of helpers in a cooperatively breeding cichlid fish (Julidochromis ornatus) in Lake Tanganyika: field observations and parentage analyses

It is suggested that some fish of the genus Julidochromis, substrate-brooding Tanganyikan cichlids with biparental care, breed cooperatively with helpers. We studied the social system of J. ornatus in the wild and analysed genetic parentage using microsatellites. Within the studied population three patterns of social system were identified: monogamous pairs (61%, 44 of 72 groups), pairs with helpers (29%, 21), and polygamous harems with helpers (controlled by either a large female or large male owner; 10%, 7). In cooperatively breeding groups, the number of helpers at each nest ranged from 1 to 6 (median 1), and male helpers were more numerous than female helpers. In both sexes, the body size was different among individuals of different social status (harem owner > breeder > helper). Helpers and harem owners of both sexes exhibited brood defence although its frequency was low. The molecular analysis revealed that (1) the helpers were mostly unrelated to dominant breeders, (2) many helpers of both sexes contributed genetically to the next generations, (3) male helpers had high siring success (41% of young in total), and (4) large young unrelated to group members were detected at 30% of observed nests, which may be due to breeder (or helper) replacements and immigration of young. We discuss the implications of these results for understanding the complex social system of this species, especially the low reproductive skew in comparison with other cooperatively breeding cichlids.     

Sociality and cooperative breeding of red-cockaded woodpeckers,Picoides borealis

Summary Thirty groups of red-cockaded woodpeckers (Picoides borealis) were studied from 1976–1982 to (1) determine the demographic structure of groups, (2) identify the role helpers play in reproductive activities, and (3) investigate the selective pressures promoting sociality and helping behavior. Groups had only 1 mated pair and 0–2 helpers. Approximately half of all groups had helpers and a given group had helpers some years but not others. Helpers, with rare exception, were males 1 or 2 years old and progeny of 1 or both members of the breeding pair. As a result of higher nestling survival, groups with helpers fledged significantly more young per year than unassisted pairs ( , SD=0.97,n=43 vs. , SD=1.01,n=50). Nesting success was also associated with size and quality of nesting period home range, but evidence suggested that the increased number of young fledged resulted directly from the action of helpers. There was a significant positive correlation between reproductive success and experience of breeding females among unassisted pairs but a significant negative correlation among pairs with helpers. In groups with experienced females, helpers were assisting both their mothers and fathers and, therefore, were related to the offspring on the average by 0.50. In groups with inexperienced females, helpers were assisting their fathers and unrelated females and were related to the offspring by 0.25. The red-cockaded woodpecker's unique habit of excavating nest and roost cavities in living pines and the extended period of time required for excavation may be an important ecological constraint that promotes the retention of helpers. Because helpers are related to the offspring they help rear, kin selection and gains in indirect fitness may provide a partial explanation of why red-cockaded woodpecker helpers help. However, the negative correlation between the efficacy of helping behavior and the helpers' relatedness to the offspring they help rear implies that helpers are least effective in producing offspring which would represent greatest gains to indirect fitness. This raises questions about the relative importance of kin selection and indirect fitness in the evolution of helping behavior among red-cockaded woodpeckers.     

Cooperatively breeding superb fairy-wrens show no facultative manipulation of offspring sex ratio despite plausible benefits

We report a long-term study of offspring sex ratios in the cooperatively breeding superb fairy-wren Malurus cyaneus. Detailed study of this species had revealed a suite of potentially strong selection pressures on the sex ratio. First, females gain substantial fitness benefits from the presence of helpers; so females without male helpers would benefit from any strategy that increased the probability of recruiting help, such as overproduction of sons (local resource enhancement hypothesis), but large numbers of helper males compete among themselves, favouring the production of daughters (local resource competition). Second, daughters fledged early in the season have far greater chances of recruitment to the breeding population than late-fledged daughters, so mothers would benefit from production of daughters early in the breeding season (early bird hypothesis). Third, extra-group mate choice imposes strong sexual selection on males, suggesting that females mating with attractive sires could benefit from investing in sons (sexual selection hypothesis). However, the predictions from these and other sex ratio hypotheses were rejected. The only convincing evidence for manipulation of the sex ratio was a slight bias towards sons (11 sons to 10 daughters) that occurred regardless of context. This result does not support current theory.     

Effects of paternal care on reproductive success in the polygynous spotless starling Sturnus unicolor

For males of socially polygynous avian species like the spotless starling, there may exist a trade-off between investing in paternal care and controlling several nests. To determine how the intensity of paternal care affects reproductive success per brood sired or expressed as the total number of young raised in all nests controlled by the same male, it is necessary to manipulate paternal care. Testosterone (T) has been shown to depress the tendency for males to care for their young, and induces them to acquire more mates. The effects of paternal care on reproductive success were studied by treating certain male starlings with exogenous T and others with the antiandrogen cyproterone acetate (CA), and comparing the parental behavior of T- and CA-males throughout the breeding season with that of controls. CA-males fed their chicks more during the first week after hatching than T-males, with controls feeding at intermediate rates, both on a per nest basis and as total effort for all nests controlled by the same male. Paternal feeding rates during the first week of chick life had a significant positive effect on the number of fledged young. The hormone treatment significantly affected the number of chicks raised per nest, CA-males having a higher breeding success per nest than T-males, and controls showing intermediate levels of success. There was no significant effect of treatment on total reproductive success attained by males throughout the season. In the polygonous spotless starling, the intensity of paternal care of young affects reproductive success per nest positively but not on a seasonal basis. Received: 6 February 1999 / Received in revised form: 30 June 1999 / Accepted: 11 July 1999     

Helpers and egg investment in the cooperatively breeding acorn woodpecker: testing the concealed helper effects hypothesis

In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the “concealed helper effects” hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall’s acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.     

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     

Correlates of group size in a cooperatively breeding cichlid fish (Neolamprologus pulcher)

Neolamprologus pulcher, a cooperatively breeding cichlid fish from Lake Tanganyika, lives in permanent social groups comprising one breeding pair and helpers of both sexes. Variation in group size (1-14 helpers) provides an opportunity to investigate factors that affect how many helpers remain in a group and in turn how group size affects reproductive success. This field study showed that larger groups live in larger territories with more shelter. Group size was more strongly correlated with territory quality than with breeder size. Experimental enhancement of territory quality did not affect group size but group size decreased when territory quality was reduced. Breeders living in a large group benefit because such individuals feed more often and have lower workloads and greater reproductive success. Helpers in larger groups also fed more frequently but did not have lower workloads. This is one of the first experimental studies to examine the factors influencing group size in cooperative breeders.     

Maternal characteristics and the production and recruitment of sons in the eastern kingbird (<Emphasis Type="BoldItalic">Tyrannus tyrannus</Emphasis>)

Sex allocation theory predicts that if variance in reproductive success differs between the sexes, females who are able to produce high-quality young should bias offspring sex ratio towards the sex with the higher potential reproductive success. We tested the hypothesis that high-quality (i.e., heavy) female eastern kingbirds (Tyrannus tyrannus) that bred early in the breeding season would produce male-biased clutches. A significant opportunity for sexual selection also exists in this socially monogamous but cryptically polygamous species, and we predicted that successful extra-pair (EP) sires would be associated with an excess of male offspring. Although population brood sex ratio did not differ from parity, it increased significantly with female body mass and declined with female breeding date, but was independent of the morphology and display (song) behavior (correlates of reproductive success) of social males and EP sires. Male offspring were significantly heavier than female offspring at fledging. Moreover, the probability that male offspring were resighted in subsequent years declined with breeding date, and was greater in replacement clutches, but lower when clutch size was large. Probability of resighting female offspring varied annually, but was independent of all other variables. Given that variance in reproductive success of male kingbirds is much greater than that of females, and that male offspring are more expensive to produce and have a higher probability of recruitment if fledged early in the season, our results support predictions of sex allocation theory: high-quality (heavy) females breeding when conditions were optimal for male recruitment produced an excess of sons.     

Cooperatively breeding carrion crows adjust offspring sex ratio according to group composition

In cooperatively breeding species, parents should bias offspring sex ratio towards the philopatric sex to obtain new helpers (helper repayment hypothesis). However, philopatric offspring might increase within-group competition for resources (local resource competition hypothesis), diluting the benefits of helper acquisition. Furthermore, benefits of offspring sex bias on parents’ fitness may depend on different costs of production and/or different breeding opportunities of sons and daughters. Because of these counteracting factors, strategies of offspring sex allocation in cooperative species are often difficult to investigate. In carrion crows Corvus corone corone in northern Spain, sons are more philopatric and more helpful at the nest than daughters, which disperse earlier and have higher chances to find a breeding vacancy. Consistent with the helper repayment hypothesis, we found that crows fledged more sons in groups short of subordinate males than in groups with sufficient helper contingent, where daughters were preferred. Crow females also proved able to bias primary sex ratio, allocating offspring sex along the hatching sequence in a way that provided the highest fledging probability to sons in the first breeding attempt and to daughters in the following ones. The higher cost of producing male offspring may explain this pattern, with breeding females shifting to the cheapest sex (female) as a response to the costs generated by previous reproductive attempts. Our results suggest complex adjustments of offspring sex ratio that allowed crows to maximize the value of daughters and sons.     

Do helpers increase reproductive success?

Summary Although several different hypotheses have been proposed to explain the evolution of helping behavior, most are based on the assumption that helping enhances the reproductive success of recipient breeders. I tested this assumption by removal experiments in the cooperatively breeding Florida scrub jay (Aphelocoma c. coerulescens). This species lives in permanently territorial social units containing a single breeding pair and none to six nonbreeders, which are usually offspring of the breeding pair and which usually act as helpers by feeding the nestlings and fledglings produced by the breeding pair. Although experimental removals of non-breeders in 1987–1988 had no significant effect on breeder survival, egg production, or hatching success, experimental groups suffered higher rates of predation on nestlings (1987) and lower rates of fledgling survival (both years) than did unmanipulated controls. As a result, experimental groups produced an average of only 0.56 independent juveniles, compared to 1.62 young for controls. Analysis of the factors contributing to nestling and fledgling mortality indicates that helping behavior per se (i.e., the aid that nonbreeders provide to dependent young), not the mere presence of nonbreeders, was responsible for the greater reproductive success observed in control groups. Because survival rates of allofeeders (i.e., those nonbreeders that provisioned dependent young) were virtually identical to those of non-allofeeders, the costs of helping behavior in this species appear to be small. Furthermore, nonbreeders are more likely to provision dependent young within their social unit when those young are closely related. I therefore conclude that nonbreeders increase their indirect fitness by serving as helpers and that helping behavior in the Florida scrub jay is a trait that has current selective utility. It remains debatable, however, whether helping in this species is an adaptation that has been shaped by the process of natural selection.     

Seasonal decline in reproductive performance varies with colony size in the fairy martin, <Emphasis Type="Italic">Petrochelidon ariel</Emphasis>

Reproductive success within populations often varies with the timing of breeding, typically declining over the season. This variation is usually attributed to seasonal changes in resource availability and/or differences in the quality or experience of breeders. In colonial species, the timing of breeding may be of particular importance because the costs and benefits of colonial breeding are likely to vary over the season and also with colony size. In this study, we examine the relationship between timing of breeding and reproductive performance (clutch size and nest success) both within and between variable sized colonies (n = 18) of fairy martins, Petrochelidon ariel. In four of these colonies, we also experimentally delayed laying in selected nests to disentangle the effects of laying date and individual quality/experience on reproductive success. Within colonies, later laying birds produced smaller clutches, but only in larger colonies. The general seasonal decline in nest success was also more pronounced in larger colonies. Late laying birds were generally smaller than earlier laying birds, but morphological differences were also related to colony size, suggesting optimal colony size also varies with phenotype. Experimentally delayed clutches were larger than concurrently produced non-delayed clutches, but only in larger colonies. Similarly, delayed clutches were more likely to produce fledglings, particularly later in the season and in larger colonies. We suggest that the reduced performance of late breeding pairs in larger colonies resulted primarily from inexperienced/low quality birds preferring to settle in larger colonies, possibly exacerbated by an increase in the costs of coloniality (e.g., resource depletion and ectoparasite infestations) with date and colony size. These findings highlight the importance of phenotype-related differences in settlement decisions and reproductive performance to an improved understanding of colonial breeding and variation in colony size.     

Positive correlation between helpers at nest and nestling immune response in a cooperative breeding bird

In cooperatively breeding species, helpers may contribute to the success of the brood by increasing the number of independent offspring, but also, they may affect offspring condition and, hence, their survival and recruitment into the breeding population. This second type of benefits is rarely included in theoretical models or assessed in field studies. Immune response is a good proxy of individual quality and fitness, and there is good evidence that the performance of the immune system of chicks during the nestling phase is related to their chances of survival and future reproduction. However, no study has so far explored whether helpers at the nest might contribute to enhance immune functioning of nestlings in species with a cooperative breeding system. Here we investigate this issue in the azure-winged magpie (Cyanopica cyanus) and found for the first time a positive correlation between the number of helpers at nest and the cell-mediated immune response of nestlings. This effect was not explained by a general improvement of body condition of chicks because it was independent of individual variation in body mass or tarsus length. Our results suggest that helping can have subtle effects on the quality of offspring that may influence their survival and future reproduction.     

Sex and status in a cooperative breeding fish: behavior and androgens

Among taxonomically widespread cooperatively breeding vertebrates, those with non-breeding helpers-at-the-nest provide an excellent opportunity to understand the proximate mechanisms underlying care and allocare. In this study, we examined androgen levels in relation to care behavior in a cooperatively breeding cichlid fish, Neolamprologus pulcher, from Lake Tanganyika. We concentrated on androgens, as these hormones have been linked to the defense behavior, and the defense of young is a common form of parental care in fishes. N. pulcher dominant female breeders performed the most care and also displayed the highest levels of plasma testosterone (T) compared with other individuals within the social group. We also found that dominant male breeders provided a similar amount of care as did the subordinate helpers, but breeding males had the highest levels of 11-ketotestosterone (11KT), an important androgen in fish. Breeders had higher levels of both androgens (T and 11KT) compared to helpers. There was a weak but significant positive correlation between T levels and the frequency of care regardless of sex and status. Our results suggest that androgens may promote defense of young and are in contrast to the commonly reported trade-off between androgen and parental care.     

Cooperation in breeding by nonreproductive wrens: kinship,reciprocity, and demography

Summary Stripe-backed wrens (Campylorhynchus nuchalis) often live as adults in large groups on permanent, communally defended territories. Nonbreeding adults cooperate in rearing the young of a single breeding pair; this aid substantially increases the reproductive success of the breeders. In a 6-year study in Venezuela of a completely colorbanded population of 25–30 groups, most adults participated in breeding only as helpers and priority to breeding status was strictly age-determined. Detailed behavioral observations at breeding nests with nestlings showed that, in a sample of 100, helpers nearly always contributed as much to the care of young as breeders. Further, aid-giving does not vary systematically with relatedness of ycung to helpers or with probability of future reciprocation by young. Young being raised are most often at least half siblings of helpers, but seldom return aid to adults that helped raise them. Even adopted helpers collaborate fully. Patterns of demography and dispersal show slow turnover of breeders, delayed reproduction, and a viscous population structure.Application of Hamilton's condition for selection for aid-giving reveals that most individuals in this population can maximize inclusive fitness in the first 2 years by helping instead of breeding. Variation in helping effort and in age of first breeding is related to variation in natal group size and competition resulting from variable demographic neighborhoods in different years or in different parts of the population. Because reciprocation in the form of specific alliance formation among nonreproductives is uncommon, nonspecific reciprocity between cohorts and kin selection account well for the observed pattern of age-dependence in first breeding. Nondiscriminating helping in this population is associated with stable monogamous pair boncs, stable territory boundaries and group membership, strict seniortiy for breeding position, high viscosity and consistent effectiveness of aid. Under these circumstances, very simple behavioral rules amounting to nearly automatic helping seem sufficient to confer critical inclusive fitness gain on helpers.     

Cooperative breeding increases reproductive success in the social spider <Emphasis Type="Italic">Stegodyphus dumicola</Emphasis> (Araneae,Eresidae)

Sociality in some birds, mammals, and social insects was suggested to have evolved through the lengthening and extension of parental care behaviors to nondirect descendents. In these systems, group members care for young cooperatively and, thus, increase the reproductive success of the breeders and fitness of the young. Parental care behaviors, such as regurgitation feeding and matriphagy (consumption of the mother), occur in several subsocial and social spiders. However, it is not known whether females in a colony cooperate in caring for the young of other females and whether such cooperative care improves reproductive success. To answer this question, we created experimental colonies of the social spider Stegodyphus dumicola (Araneae, Eresidae), allowing only one female in a group to produce young, simulating reproductive skew occurring in nests in nature. In this paper, we show for the first time that females of S. dumicola cooperate in providing regurgitated food for young of other females and are even eaten by those young. Young raised by a group of females were larger and had greater survival than young raised only by their mother. Thus, fitness benefits from raising broods cooperatively may have favored the evolution of sociality in spiders.     

Do stress hormones suppress helper reproduction in the cooperatively breeding red-cockaded woodpecker (<Emphasis Type="Italic">Picoides borealis</Emphasis>)?

In the cooperatively breeding red-cockaded woodpecker (Picoides borealis), male helpers are subordinate to male breeders and do not mate with females, even when unrelated to the breeding female within their group or through extra-group matings, yet exhibit reproductive hormone profiles similar to those of breeders. We investigated whether reproduction might be suppressed in helper males via high levels of the stress hormone corticosterone. We also examined effects of group size and season on corticosterone levels by comparing baseline and maximal plasma levels of corticosterone between helper males and breeding males, and among helper males and breeders of both sexes living in groups of different sizes throughout the reproductive cycle. We also measured plasma levels of estrogen, progesterone, and testosterone to examine other potential hormonal differences between helpers and breeders. Male status did not explain variation in any hormones; therefore, our data do not support the hypothesis that helper males are reproductively suppressed via corticosterone or the other hormones investigated. However, the presence of two or more helper males in a group tended to reduce baseline corticosterone in breeding and helper males, but not breeding females, suggesting that helper males reduce parental effort of other male group members. Seasonally, maximal corticosterone peaked during the nestling provisioning phase for breeding and helper males, but not breeder females, suggesting that males show an increased response to stressors posing a potential threat to survival of offspring.