Kin discrimination in cooperatively breeding long-tailed tits

Individual vocal recognition across taxa: a review of the literature and a look into the future

Individual vocal recognition (IVR) has been well studied in mammals and birds. These studies have primarily delved into understanding IVR in specific limited contexts (e.g. parent-offspring and mate recognition) where individuals discriminate one individual from all others. However, little research has examined IVR in more socially demanding circumstances, such as when an individual discriminates all individuals in their social or familial group apart. In this review, we describe what IVR is and suggest splitting studies of IVR into two general types based on what questions they answer (IVR-singular, and IVR-multiple). We explain how we currently test for IVR, and many of the benefits and drawbacks of different methods. We address why IVR is so prevalent in the animal kingdom, and the circumstances in which it is often found. Finally, we explain current weaknesses in IVR research including temporality, specificity, and taxonomic bias, and testing paradigms, and provide some solutions to address these weaknesses. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Acorn woodpeckers vocally discriminate current and former group members from nongroup members

In species with long-term social relationships, the ability to recognize individuals after extended separation and the ability to discriminate between former social affiliates that have died and those that have left the group but may return are likely to be beneficial. Few studies, however, have investigated whether animals can make these discriminations. We presented acorn woodpeckers (Melanerpes formicivorus), a group-living, cooperatively breeding bird, with playbacks of current group members, former group members still living nearby, former group members that had died or left the study area, and familiar nongroup members. Subjects responded more quickly to the calls of nongroup members than to the calls of current group members or former group members still living in the study area but did not discriminate between nongroup members and former group members that had died or disappeared. This suggests that acorn woodpeckers can vocally recognize both current group members and former group members that have dispersed to nearby groups and that they either forget former group members that no longer live in the vicinity or classify them differently from former group members that still live nearby. This study suggests an important role for vocal recognition in maintaining valuable relationships with social affiliates postdispersal.

Helping decisions and kin recognition in long-tailed tits: is call similarity used to direct help towards kin?

Most cooperative breeders live in discrete family groups, but in a minority, breeding populations comprise extended social networks of conspecifics that vary in relatedness. Selection for effective kin recognition may be expected for more related individuals in such kin neighbourhoods to maximize indirect fitness. Using a long-term social pedigree, molecular genetics, field observations and acoustic analyses, we examine how vocal similarity affects helping decisions in the long-tailed tit Aegithalos caudatus. Long-tailed tits are cooperative breeders in which help is typically redirected by males that have failed in their own breeding attempts towards the offspring of male relatives living within kin neighbourhoods. We identify a positive correlation between call similarity and kinship, suggesting that vocal cues offer a plausible mechanism for kin discrimination. Furthermore, we show that failed breeders choose to help males with calls more similar to their own. However, although helpers fine-tune their provisioning rates according to how closely related they are to recipients, their effort was not correlated with their vocal similarity to helped breeders. We conclude that although vocalizations are an important part of the recognition system of long-tailed tits, discrimination is likely to be based on prior association and may involve a combination of vocal and non-vocal cues. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

No evidence for prezygotic postcopulatory avoidance of kin despite high inbreeding depression

Offspring resulting from mating among close relatives can suffer from impaired fitness through the expression of recessive alleles with deleterious effects. Postcopulatory sperm selection (a prezygotic mechanism of cryptic female choice) has been suggested to be an effective way to avoid inbreeding. To investigate whether postcopulatory female choice allows the avoidance of fertilization by close kin, we performed artificial inseminations in a promiscuous bird, the houbara bustard (Chlamydotis undulata undulata). Females were inseminated with a mix of sperm from triads of males, each constituted of a male genetically unrelated to the female, a first cousin and a half-sibling. When counting the number of eggs sired by unrelated males, cousins or half-siblings, we found a significant deviation from the expected random distribution, with more eggs sired by unrelated males. However, offspring sired by cousins, and especially by half-siblings suffered from high prehatching mortality, suggesting that the observed paternity skew towards unrelated males might reflect differential offspring mortality rather than sperm selection. In agreement with this hypothesis, within-triad siring success was similar for the three parental relatedness categories, but the relationship between siring and hatching success differed across categories. In clutches with high hatching success, unrelated males had the highest success while in clutches with high failure rate, half-siblings had the highest success. Offspring sired by half-siblings also suffered from reduced growth rate during the first three months and higher posthatching mortality. Hence, despite substantial fitness costs associated with fertilization by close relatives, females do not seem to select sperm of unrelated males.

Male relatedness and familiarity are required to modulate male-induced harm to females in Drosophila

Males compete over mating and fertilization, and often harm females in the process. Inclusive fitness theory predicts that increasing relatedness within groups of males may relax competition and discourage male harm of females as males gain indirect benefits. Recent studies in Drosophila melanogaster are consistent with these predictions, and have found that within-group male relatedness increases female fitness, though others have found no effects. Importantly, these studies did not fully disentangle male genetic relatedness from larval familiarity, so the extent to which modulation of harm to females is explained by male familiarity remains unclear. Here we performed a fully factorial design, isolating the effects of male relatedness and larval familiarity on female harm. While we found no differences in male courtship or aggression, there was a significant interaction between male genetic relatedness and familiarity on female reproduction and survival. Relatedness among males increased female lifespan, reproductive lifespan and overall reproductive success, but only when males were familiar. By showing that both male relatedness and larval familiarity are required to modulate female harm, these findings reconcile previous studies, shedding light on the potential role of indirect fitness effects on sexual conflict and the mechanisms underpinning kin recognition in fly populations.

Genetics and developmental biology of cooperation

Despite essential progress towards understanding the evolution of cooperative behaviour, we still lack detailed knowledge about its underlying molecular mechanisms, genetic basis, evolutionary dynamics and ontogeny. An international workshop "Genetics and Development of Cooperation," organized by the University of Bern (Switzerland), aimed at discussing the current progress in this research field and suggesting avenues for future research. This review uses the major themes of the meeting as a springboard to synthesize the concepts of genetic and nongenetic inheritance of cooperation, and to review a quantitative genetic framework that allows for the inclusion of indirect genetic effects. Furthermore, we argue that including nongenetic inheritance, such as transgenerational epigenetic effects, parental effects, ecological and cultural inheritance, provides a more nuanced view of the evolution of cooperation. We summarize those genes and molecular pathways in a range of species that seem promising candidates for mechanisms underlying cooperative behaviours. Concerning the neurobiological substrate of cooperation, we suggest three cognitive skills necessary for the ability to cooperate: (i) event memory, (ii) synchrony with others and (iii) responsiveness to others. Taking a closer look at the developmental trajectories that lead to the expression of cooperative behaviours, we discuss the dichotomy between early morphological specialization in social insects and more flexible behavioural specialization in cooperatively breeding vertebrates. Finally, we provide recommendations for which biological systems and species may be particularly suitable, which specific traits and parameters should be measured, what type of approaches should be followed, and which methods should be employed in studies of cooperation to better understand how cooperation evolves and manifests in nature.

The importance of the altricial - precocial spectrum for social complexity in mammals and birds - a review

Various types of long-term stable relationships that individuals uphold, including cooperation and competition between group members, define social complexity in vertebrates. Numerous life history, physiological and cognitive traits have been shown to affect, or to be affected by, such social relationships. As such, differences in developmental modes, i.e. the ‘altricial-precocial’ spectrum, may play an important role in understanding the interspecific variation in occurrence of social interactions, but to what extent this is the case is unclear because the role of the developmental mode has not been studied directly in across-species studies of sociality. In other words, although there are studies on the effects of developmental mode on brain size, on the effects of brain size on cognition, and on the effects of cognition on social complexity, there are no studies directly investigating the link between developmental mode and social complexity. This is surprising because developmental differences play a significant role in the evolution of, for example, brain size, which is in turn considered an essential building block with respect to social complexity. Here, we compiled an overview of studies on various aspects of the complexity of social systems in altricial and precocial mammals and birds. Although systematic studies are scarce and do not allow for a quantitative comparison, we show that several forms of social relationships and cognitive abilities occur in species along the entire developmental spectrum. Based on the existing evidence it seems that differences in developmental modes play a minor role in whether or not individuals or species are able to meet the cognitive capabilities and requirements for maintaining complex social relationships. Given the scarcity of comparative studies and potential subtle differences, however, we suggest that future studies should consider developmental differences to determine whether our finding is general or whether some of the vast variation in social complexity across species can be explained by developmental mode. This would allow a more detailed assessment of the relative importance of developmental mode in the evolution of vertebrate social systems.

Was monogamy a key step on the hominin road? Reevaluating the monogamy hypothesis in the evolution of cooperative breeding

Because human mothers routinely rely on others to help raise their young, humans have been characterized as cooperative breeders.(1-9) Several large-scale phylogenetic analyses have presented compelling evidence that monogamy preceded the evolution of cooperative breeding in a wide variety of nonhuman animals.(10-14) These studies have suggested that monogamy provides a general rule (the monogamy hypothesis) for explaining evolutionary transitions to cooperative breeding.(15) Given the prevalence of cooperative breeding in contemporary human societies, we evaluate whether this suggests a monogamous hominin past.

Calling Where It Counts: Subordinate Pied Babblers Target the Audience of Their Vocal Advertisements

For territorial group-living species, opportunities to reproduce on the natal territory can be limited by a number of factors including the availability of resources within a territory, access to unrelated individuals, and monopolies on reproduction by dominant group members. Individuals looking to reproduce are therefore faced with the options of either waiting for a breeding opportunity to arise in the natal territory, or searching for reproductive opportunities in non-natal groups. In the cooperatively breeding Southern pied babbler, Turdoides bicolor, most individuals who achieve reproductive success do so through taking up dominant breeding positions within non-natal groups. For subordinate pied babblers therefore, searching for breeding opportunities in non-natal groups is of primary importance as this represents the major route to reproductive success. However, prospecting (where individuals leave the group to search for reproductive opportunities within other groups) is costly and individuals rapidly lose weight when not part of a group. Here we demonstrate that subordinate pied babblers adopt an alternative strategy for mate attraction by vocal advertisement from within their natal territories. We show that subordinates focus their calling efforts on the edges of their territory, and specifically near boundaries with neighbouring groups that have potential breeding partners (unrelated individuals of the opposite sex). In contrast to prospecting, calling individuals showed no body mass loss associated with this behaviour, suggesting that calling from within the group may provide a ‘cheap’ advertisement strategy. Additionally, we show that subordinates use information regarding the composition of neighbouring groups to target the greatest number of potential mating partners.

Helping in cooperatively breeding long-tailed tits: a test of Hamilton's rule

Inclusive fitness theory provides the conceptual framework for our current understanding of social evolution, and empirical studies suggest that kin selection is a critical process in the evolution of animal sociality. A key prediction of inclusive fitness theory is that altruistic behaviour evolves when the costs incurred by an altruist (c) are outweighed by the benefit to the recipient (b), weighted by the relatedness of altruist to recipient (r), i.e. Hamilton's rule rb > c. Despite its central importance in social evolution theory, there have been relatively few empirical tests of Hamilton's rule, and hardly any among cooperatively breeding vertebrates, leading some authors to question its utility. Here, we use data from a long-term study of cooperatively breeding long-tailed tits Aegithalos caudatus to examine whether helping behaviour satisfies Hamilton's condition for the evolution of altruism. We show that helpers are altruistic because they incur survival costs through the provision of alloparental care for offspring. However, they also accrue substantial benefits through increased survival of related breeders and offspring, and despite the low average relatedness of helpers to recipients, these benefits of helping outweigh the costs incurred. We conclude that Hamilton's rule for the evolution of altruistic helping behaviour is satisfied in this species.

An experimental test of the information model for negotiation of biparental care

BACKGROUND

Theoretical modelling of biparental care suggests that it can be a stable strategy if parents partially compensate for changes in behaviour by their partners. In empirical studies, however, parents occasionally match rather than compensate for the actions of their partners. The recently proposed "information model" adds to the earlier theory by factoring in information on brood value and/or need into parental decision-making. This leads to a variety of predicted parental responses following a change in partner work-rate depending on the information available to parents.

METHODOLOGY/PRINCIPAL FINDINGS

We experimentally test predictions of the information model using a population of long-tailed tits. We show that parental information on brood need varies systematically through the nestling period and use this variation to predict parental responses to an experimental increase in partner work-rate via playback of extra chick begging calls. When parental information is relatively high, partial compensation is predicted, whereas when parental information is low, a matching response is predicted.

CONCLUSIONS/SIGNIFICANCE

We find that although some responses are consistent with predictions, parents match a change in their partner's work-rate more often than expected and we discuss possible explanations for our findings.

Microbial communication and virulence: lessons from evolutionary theory

At the heart of tackling the huge challenge posed by infectious micro-organisms is the overwhelming need to understand their nature. A major question is, why do some species of bacteria rapidly kill their host whilst others are relatively benign? For example, Yersinia pestis, the causative organism of plague, is a highly virulent human pathogen whilst the closely related Yersinia pseudotuberculosis causes a much less severe disease. Using molecular techniques such as mutating certain genes, microbiologists have made significant advances over recent decades in elucidating the mechanisms that govern the production of virulence factors involved in causing disease in many bacterial species. There are also evolutionary and ecological factors which will influence virulence. Many of these ideas have arisen through the development of evolutionary theory and yet there is strikingly little empirical evidence testing them. By applying both mechanistic and adaptive approaches to microbial behaviours we can begin to address questions such as, what factors influence cooperation and the evolution of virulence in microbes and can we exploit these factors to develop new antimicrobial strategies?

Kinship affects investment by helpers in a cooperatively breeding bird

Helping behaviour in cooperative breeding systems has been attributed to kin selection, but the relative roles of direct and indirect fitness benefits in the evolution of such systems remain a matter of debate. In theory, helpers could maximize the indirect fitness benefits of cooperation by investing more in broods with whom they are more closely related, but there is little evidence for such fine-scale adjustment in helper effort among cooperative vertebrates. In this study, we used the unusual cooperative breeding system of the long-tailed tit Aegithalos caudatus to test the hypothesis that the provisioning effort of helpers was positively correlated with their kinship to broods. We first use pedigrees and microsatellite genotypes to characterize the relatedness between helpers and breeders from a 14 year field study. We used both pedigree and genetic approaches because long-tailed tits have access to pedigree information acquired through social relationships, but any fitness consequences will be determined by genetic relatedness. We then show using both pedigrees and genetic relatedness estimates that alloparental investment by helpers increases as their relatedness to the recipients of their care increases. We conclude that kin selection has played a critical role in moulding the investment decisions of helpers in this cooperatively breeding species.

Do common eiders nest in kin groups? Microgeographic genetic structure in a philopatric sea duck

We investigated local genetic associations among female Pacific common eiders (Somateria mollissima v-nigrum) nesting in a stochastic Arctic environment within two groups of barrier islands (Simpson Lagoon and Mikkelsen Bay) in the Beaufort Sea, Alaska. Nonrandom genetic associations were observed among nesting females using regional spatial autocorrelation analyses for distance classes up to 1000 m in Simpson Lagoon. Nearest-neighbour analyses identified clusters of genetically related females with positive lr values observed for 0-13% and 0-7% of the comparisons in Simpson Lagoon and Mikkelsen Bay, respectively, across years. These results indicate that a proportion of females are nesting in close proximity to more genetically related individuals, albeit at low frequency. Such kin groupings may form through active association between relatives or through natal philopatry and breeding site fidelity. Eiders nest in close association with driftwood, which is redistributed annually by seasonal storms. Yet, genetic associations were still observed. Microgeographic structure may thus be more attributable to kin association than natal philopatry and site fidelity. However, habitat availability may also influence the level of structure observed. Regional structure was present only within Simpson Lagoon and this island group includes at least three islands with sufficient driftwood for colonies, whereas only one island at Mikkelsen Bay has these features. A long-term demographic study is needed to understand more fully the mechanisms that lead to fine-scale genetic structure observed in common eiders breeding in the Beaufort Sea.

Cooperation between non-kin in animal societies

Explanations of cooperation between non-kin in animal societies often suggest that individuals exchange resources or services and that cooperation is maintained by reciprocity. But do cooperative interactions between unrelated individuals in non-human animals really resemble exchanges or are they a consequence of simpler mechanisms? Firm evidence of reciprocity in animal societies is rare and many examples of cooperation between non-kin probably represent cases of intra-specific mutualism or manipulation.

Dispersal of sibling coalitions promotes helping among immigrants in a cooperatively breeding bird

Kin selection is a major force in social evolution, but dispersal is often assumed to reduce its impact by diluting kinship. In most cooperatively breeding vertebrates, in which more than two individuals care for young, juveniles delay dispersal and become helpers in family groups. In long-tailed tits (Aegithalos caudatus), however, offspring disperse to breed and helpers are failed breeders that preferentially aid kin. Helping also occurs among immigrants, but their origins are unknown and cooperation in these cases is poorly understood. Here, we combine long-term demographic and genetic data from our study population to investigate immigration and helping in this species. We first used a novel application of parentage analysis to discriminate between immigrants and unknown philopatric recruits. We then cross-checked sibship reconstruction with pairwise relatedness estimates to show that immigrants disperse in sibling coalitions and helping among them is kin biased. These results indicate that dispersal need not preclude sociality, and dispersal of kin coalitions may help maintain kin-selected cooperation in the absence of delayed dispersal.

GRANDPARENT HELPERS: THE ADAPTIVE SIGNIFICANCE OF OLDER, POSTDOMINANT HELPERS IN THE SEYCHELLES WARBLER

The possibility that older, often nonreproductive, individuals may engage in kin-directed cooperative behavior has been largely overlooked in the study of cooperative breeding. Here, we describe and investigate the adaptive significance of such "grandparent" helpers in the Seychelles warbler, the first bird species in which this phenomenon has been observed. On Cousin Island, over a period of 24 years, a significant proportion (13.7%) of females, but few males (3.0%), was deposed from dominant positions. Deposed females were replaced by related females. However there was no evidence that older, senescent females were stepping aside to gain greater fitness benefits by increasing the reproductive success of their offspring, rather than breeding themselves; deposed females were not postreproductive, nor was being deposed linked to age or reproductive senescence. Of the deposed females, 68% became subordinates and helped to raise group offspring, accounting for ca. 10% of subordinates in any year. Demoted females were related (r= 0.24) to the group offspring and, consequently, could gain indirect benefits through helping. As direct benefits appeared to be limited, we suggest that indirect benefits have driven the evolution of such "grandparent helpers." This study now provides evidence for a new route to cooperative breeding in birds.

Female choice for genetic complementarity in birds: a review

Data from avian species have played a prominent role in developing and testing theories of female mate choice. One of the most prominent models of sexual selection, the "good genes" model, emphasizes the indirect benefits of female preferences for male ornaments as indicators of a potential sire's additive genetic quality. However, there is growing interest in non-additive sources of genetic quality and mate choice models for self-referential disassortative mating based on optimal levels of genetic dissimilarity. We reviewed the empirical evidence for genetic-complementarity-based female mate choice among birds. We found the evidence for such choice is mixed but in general against the genetic complementarity hypothesis. The lack of evidence for genetic complementarity in many birds may be due to an inability to make the fine distinctions among potential mates based on genes, possibly due to the comparative anosmatic nature of avian sensory system. For some species however there is compelling evidence for genetic complementarity as a criterion used in female mate choice. Understanding the ubiquity of female mate choice based on genetic complementarity and the variation in this source of female preference among and within species remains a challenge.

Host-parasite kinship in a female-philopatric bird population: evidence from relatedness trend analysis

Conspecific brood parasitism (CBP), an alternative reproductive tactic where some females lay eggs in the nests of other females of the same species, occurs in many animals with egg care. It is particularly common in waterfowl, for reasons that are debated. Many waterfowl females nest near their birthplace, making it likely that some local females are relatives. We analyse brood parasitism in a Hudson Bay population of common eiders, testing predictions from two alternative hypotheses on the role of relatedness in CBP. Some models predict host-parasite relatedness, others predict that parasites avoid close relatives as hosts. To distinguish between the alternatives, we use a novel approach, where the relatedness of host-parasite pairs is tested against the spatial population trend in pairwise relatedness. We estimate parasitism, nest take-over and relatedness with protein fingerprinting and bandsharing analysis of egg albumen, nondestructively sampled from each new egg in the nest throughout the laying period. The results refute the hypothesis that parasites avoid laying eggs in the nests of related hosts, and corroborate the alternative of host-parasite relatedness. With an estimated r of 0.12-0.14, females laying eggs in the same nest are on average closer kin than nesting neighbour females. Absence of a population trend in female pairwise relatedness vs. distance implies that host-parasite relatedness is not only an effect of strong natal philopatry: some additional form of kin bias is also involved.

Mother guarding: how offspring may influence the extra-pair behaviour of their parents

In this paper we propose a novel form of social control of mate choice. Through mother guarding, offspring can help in protecting the paternity of their father by preventing their mother from engaging in extra-pair matings. We present a model that predicts the circumstances under which mothers should be selected to seek or avoid extra-pair matings, and existing offspring should be selected to prevent or promote such matings. In its simplest form, our model shows that offspring are selected to mother guard as long as the viability of extra-pair young is less than twice that of within-pair young; when the relative viability is greater, offspring are selected to promote extra-pair mating by their mother. If the existing offspring are not necessarily sired by their mother's social mate, then the potential for conflict is further reduced. We also consider whether offspring have an interest in the extra-pair reproduction of their fathers. We show that when the costs of the father's infidelity to the mother's brood are high, existing offspring are selected to prevent extra-pair mating by their father; when such costs are low, offspring are selected to promote extra-pair mating by their father. In principle, our model applies to all species where offspring show delayed dispersal and where breeding pairs raise multiple broods or litters. This situation exists in, but is not limited to, the majority of cooperatively breeding species. The significance of this model with regard to our current understanding of the evolution of extra-pair behaviour in such species is discussed.

Learned kin recognition cues in a social bird

In many cooperatively breeding birds, kin selection has an important role in the evolution and maintenance of social behaviour, and 'helpers' can maximize indirect fitness gains by preferentially allocating care to close relatives. Although there is evidence for kin-biased helping behaviour in several species, the mechanism of kin recognition underlying this behaviour is poorly understood. Vocalizations are the most commonly used cues in avian recognition systems, but the effectiveness of vocal signals as reliable recognition cues must depend on how they are acquired. However, there have been no experimental studies of the development of vocal recognition cues in cooperative birds; indeed, the ontogeny of all bird vocalizations other than song is poorly known in any species. Here, we show that cooperatively breeding long-tailed tits (Aegithalos caudatus) can discriminate between kin and non-kin according to the individual-specific characteristics of contact calls, and show experimentally that individuals learn these calls from provisioning adults during the nestling period. Finally, we show that the pattern of cooperative behaviour in this species is consistent with the use of recognition cues learned through association.

The influence of climate on the timing and rate of spring bird migration

Ecological processes are changing in response to climatic warming. Birds, in particular, have been documented to arrive and breed earlier in spring and this has been attributed to elevated spring temperatures. It is not clear, however, how long-distance migratory birds that overwinter thousands of kilometers to the south in the tropics cue into changes in temperature or plant phenology on northern breeding areas. We explored the relationships between the timing and rate of spring migration of long-distance migratory birds, and variables such as temperature, the North Atlantic Oscillation (NAO) and plant phenology, using mist net capture data from three ringing stations in North America over a 40-year period. Mean April/May temperatures in eastern North America varied over a 5 degrees C range, but with no significant trend during this period. Similarly, we found few significant trends toward earlier median capture dates of birds. Median capture dates were not related to the NAO, but were inversely correlated to spring temperatures for almost all species. For every 1 degrees C increase in spring temperature, median capture dates of migratory birds averaged, across species, one day earlier. Lilac (Syringa vulgaris) budburst, however, averaged 3 days earlier for every 1 degrees C increase in spring temperature, suggesting that the impact of temperature on plant phenology is three times greater than on bird phenology. To address whether migratory birds adjust their rate of northward migration to changes in temperature, we compared median capture dates for 15 species between a ringing station on the Gulf Coast of Louisiana in the southern USA with two stations approximately 2,500 km to the north. The interval between median capture dates in Louisiana and at the other two ringing stations was inversely correlated with temperature, with an average interval of 22 days, that decreased by 0.8 days per 1 degrees C increase in temperature. Our results suggest that, although the onset of migration may be determined endogenously, the timing of migration is flexible and can be adjusted in response to variation in weather and/or phenology along migration routes.

Experimental evidence that kin discrimination in the Seychelles warbler is based on association and not on genetic relatedness

In cooperative breeding systems driven by kin selection, effective kin-recognition cues are important. Recognition could be achieved by the direct assessment of the genetic relatedness of individuals or by learning through association. In the Seychelles warbler, Acrocephalus sechellensis, female subordinates maximize indirect fitness by preferentially helping genetically related nestlings. Help seems to be based on the continued presence of the primary female who previously fed the subordinate in the nest but it has, so far, been impossible to discount the direct assessment of genetic relatedness. We used a cross-fostering experiment to separate the two possible cues. Adult birds did not discriminate between their own and cross-fostered eggs or nestlings. Cross-fostering resulted in nestlings that were unrelated to the primary female that raised them, but control nestlings were closely related to their primary females. The proportions of cross-fostered and control female offspring that stayed and became helpers on their 'natal' territory were similar. However, for both groups the chance of becoming a subordinate helper was associated with the continued presence of the primary female and not with any other factor tested. Our study provides strong evidence that helping decisions are based on associative-learning cues.

'O sibling, where art thou?'--a review of avian sibling recognition with respect to the mammalian literature

Avian literature on sibling recognition is rare compared to that developed by mammalian researchers. We compare avian and mammalian research on sibling recognition to identify why avian work is rare, how approaches differ and what avian and mammalian researchers can learn from each other. Three factors: (1) biological differences between birds and mammals, (2) conceptual biases and (3) practical constraints, appear to influence our current understanding. Avian research focuses on colonial species because sibling recognition is considered adaptive where 'mixing potential' of dependent young is high; research on a wider range of species, breeding systems and ecological conditions is now needed. Studies of acoustic recognition cues dominate avian literature; other types of cues (e.g. visual, olfactory) deserve further attention. The effect of gender on avian sibling recognition has yet to be investigated; mammalian work shows that gender can have important influences. Most importantly, many researchers assume that birds recognise siblings through 'direct familiarisation' (commonly known as associative learning or familiarity); future experiments should also incorporate tests for 'indirect familiarisation' (commonly known as phenotype matching). If direct familiarisation proves crucial, avian research should investigate how periods of separation influence sibling discrimination. Mammalian researchers typically interpret sibling recognition in broad functional terms (nepotism, optimal outbreeding); some avian researchers more successfully identify specific and testable adaptive explanations, with greater relevance to natural contexts. We end by reporting exciting discoveries from recent studies of avian sibling recognition that inspire further interest in this topic.

Sex-specific associative learning cues and inclusive fitness benefits in the Seychelles warbler

In cooperative breeding vertebrates, indirect fitness benefits would be maximized by subordinates that accurately assess their relatedness to group offspring and preferentially help more closely related kin. In the Seychelles warbler (Acrocephalus sechellensis), we found a positive relationship between subordinate-nestling kinship (determined using microsatellite marker genotypes) and provisioning rates, but only for female subordinates. Female subordinates that helped were significantly more related to the nestlings than were nonhelpers, and the decision to help appears to be based on associative learning cues. High levels of female infidelity means that subordinates cannot trust their legitimacy through the male line, consequently they appear to use the continued presence of the primary female, but not the primary male, as a reliable cue to determine when to feed nestlings. By using effective discrimination, female subordinates are able to maximize the indirect benefits gained within a cooperative breeding system otherwise driven primarily by direct breeding benefits.