Abstract

Understanding how reproduction is partitioned between group members is essential in explaining the apparent reproductive altruism of cooperatively breeding systems. Here, we use genetic data from a population of cooperatively breeding pied babblers (Turdoides bicolor) to show that reproduction is highly skewed toward behaviorally dominant birds. Dominant birds monopolized reproduction, accounting for 95.2% of all chicks. Inbreeding avoidance appears to constrain subordinate reproduction because the rare incidences of subordinate reproduction occurred only with unrelated members of their groups. However, even when unrelated potential breeding partners were present in the group, subordinates rarely bred. Although half of chicks hatched into groups where subordinates could potentially breed, only 9.6% of these chicks had a subordinate parent, indicating that additional factors limit subordinate reproduction, such as reproductive conflict with dominants. Groups were highly kin structured and most subordinates were closely related to one another such that help was almost invariably directed toward close relatives. Consequently, helping in this species confers indirect fitness benefits on subordinates, which are likely to play an important role in the evolution and maintenance of cooperative helping behavior.

INTRODUCTION

Cooperative breeding occurs when more than 2 individuals help to raise young in a single brood, and some of the helping (often subordinate) individuals do not breed (Cockburn 2004). Many cooperatively breeding systems are characterized by high reproductive skew in one or both sexes, such that a small proportion of individuals monopolize reproduction (Keller and Reeve 1994; Cockburn 2004; Raihani and Clutton-Brock 2010). A first question to investigate is why subordinates refrain from breeding. Although subordinates may be sexually mature, they may not be in sufficiently good physical condition to breed (Clutton-Brock et al. 2001) or may be tolerated in the group only as helpers (Kokko et al. 2002). Alternatively, they may be constrained from breeding in the natal group by the absence of unrelated breeding partners (Koenig and Pitelka 1979) and the attendant risk of inbreeding depression should they breed with relatives (Pusey 1987; Pusey and Wolf 1996). Parentage must therefore be verified to be sure that subordinates are not in fact breeding and that they do not do so with kin. A second question to investigate is why subordinates help. Subordinates may help nondescendant kin to facilitate the success of shared genes and gain indirect fitness benefits, the primary tenet of kin selection theory (Hamilton 1964; Maynard-Smith 1964, reviewed by Griffin and West 2003). Alternatively, helpers may acquire direct fitness benefits, either in addition to or in the absence of kin-selected indirect benefits. Direct benefits may be gained when help increases group size (“group augmentation”—Brown 1987; Kokko et al. 2001), leading to increased chances of survival due to better vigilance (Pulliam 1973; Ridley et al. 2008) and larger coalition sizes at dispersal (reviewed by Heinsohn and Legge 1999; Russell 2004). In interpreting the inclusive fitness consequences of helping, it is again important to verify parentage since subordinates may help because they are the parents of some of the offspring (Richardson et al. 2001, 2002).

To answer these questions and better understand cooperatively breeding systems, molecular techniques must be used to provide reliable information about parentage within the group. Such techniques have revealed that extrapair reproduction in cooperative breeders is common (Mulder et al. 1994; Li and Brown 2000; Richardson et al. 2001). Mating systems among avian cooperative breeders include polygyny (where several group females breed in a single nest), polygynandry (where several group members, both male and female, breed in a single nest), hidden leks (where a very high rate of extra-group paternity is the norm), and finally, true monogamy, which has been estimated to occur in only a few cooperatively breeding birds (reviewed by Cockburn 2004). Molecular techniques have also been used to investigate the extent of inbreeding in cooperatively breeding species (reviewed by Cockburn 1998; Griffith et al. 2002). Thus, genetic information is now essential to understand 1) the direct fitness benefits of obtaining dominance, 2) the direct fitness benefits obtained through reproduction as a subordinate, 3) the potential indirect fitness benefits of helping as a subordinate group member, 4) the extent of inbreeding and whether inbreeding avoidance restricts subordinate reproduction, and 5) the extent and outcome of reproductive conflict within groups where subordinates have the opportunity to breed.

The pied babbler (Turdoides bicolor) is a cooperatively breeding passerine (70–95 g) that lives in family groups comprising a dominant pair and a variable number of adult helpers of both sexes. Based on behavioral observations, it has been speculated, but not empirically demonstrated, that dominant individuals monopolize breeding opportunities and that inbreeding is rare (Raihani 2008). We used microsatellite DNA genotyping and field observations of courtship, copulation, and group composition to answer 4 questions: 1) which group members are breeding, 2) how related are group members to one another, 3) does inbreeding occur, and 4) what reproductive opportunities are available to subordinates?

MATERIALS AND METHODS

Study site and population

Comprehensive observations of pied babbler group life histories and breeding attempts were collected from July 2003 to May 2008 at the Kuruman River Reserve, South Africa (lat 26°58′S, long 21°49′E) (for information on climate and vegetation, see Raihani and Ridley 2007). Twenty-three wild pied babbler groups were habituated to the close presence of a human observer at a distance of 2–3 m, allowing observational data to be collected without disturbing natural behavior (for habituation techniques, see Ridley and Raihani 2007a). Each individual in the population was ringed with a unique combination of colored leg rings.

Pied babbler groups ranged in size from 2 to 11 adults (individuals more than 12 months old—Ridley and Raihani 2008) with a mean group size of 4.3 ± 0.2 adults. Habituated groups were adjacent to one another and defended year-round territories of 1–3 km2. Groups typically comprised a dominant pair with subordinate helpers, the status of which could be determined based on the observation of dominance assertions (pecks and other attacks), to which subordinates responded with submissive behavior (Raihani 2008). Peaceful interactions between babbler groups were very rare (Ridley AR, unpublished data), making it difficult for adult subordinates to associate with nongroup members while simultaneously remaining members of their own groups. Individuals that attempted to immigrate into non-natal groups were often chased and attacked by members of the receiving group (Raihani et al. 2010). An individual could immigrate either as a dominant or subordinate and immigration was not sex biased (Nelson-Flower 2010; Raihani et al. 2010). Average age at dispersal was 1.5 years; older birds were more likely to immigrate into groups as dominants, whereas younger birds were more likely to immigrate into subordinate positions (Raihani et al. 2010). Immigrant subordinates never inherited dominance in these (non-natal) groups. Copulation was witnessed between members of the same group (N = 53 copulations in 16 groups over 5 years, usually between dominants, but sometimes between dominants and subordinates) but was never observed between members of different groups.

Which group members are breeding?

Parentage analysis allows assessment of both the level of reproductive skew and whether subordinates are breeding with relatives within the group. Samples were collected from 319 individuals in 23 groups over 5 years and were genotyped at 9 polymorphic loci (see Supplementary Material). Parentage was calculated using CERVUS v. 3.0.3 (Kalinowski et al. 2007). Allele frequencies used in parentage analysis were based on the entire population rather than a subset (N = 42) of unrelated individuals (for rationale, see Kalinowski et al. 2007) because the parentage predictions resulting from the allele frequencies estimated by the 2 data sets were similar, and the data set of unrelated individuals was missing some rare alleles. For the parentage analysis, only those nestlings that were observed to have hatched into a fully sampled group with known dominance structure (based on behavioral observations of ringed birds) were included. This reduced the sample size to 159 nestlings from 67 broods produced by 12 groups.

For these 159 offspring, only adults were included as potential parents, but the CERVUS program was given no prior information about the social pairings of these potential parents. Pied babblers are highly territorial (Ridley and Raihani 2007b) and it was assumed that extra-group females would not have opportunities to lay eggs in the group nest: Breeding was asynchronous between groups and extra-group individuals were attacked and chased off whenever they approached a babbler group (Raihani 2008). Only heavy females (mean body mass of 88.4 ± 1.0 g, range 83.1–95.1 g) lay eggs (Raihani 2008); those adult females with a preforage (dawn) body mass of greater than 80 g during the period the eggs were laid (2–3 days prior to the onset of incubation) were included as candidate mothers (for weighing protocols, see Ridley and Raihani 2007b). Because group females could potentially copulate with extra-group males (although this has never been observed), all known adult males in the population at the time of an offspring's hatching were included as candidate fathers. Those adult males that immigrated into the population after the offspring hatched were also listed as potential fathers. All males that had dispersed or disappeared from the study population were also included, unless there was proof of their death.

CERVUS v. 3.0.3 simulation inputs were 100 000 offspring using the analysis option “parent pair (sexes known)”; 2 (range 1–3) candidate mothers per offspring with an estimated 99% of candidate mothers sampled; 70 (range 29–76) candidate fathers per offspring, with an estimated 80% of candidate fathers sampled (most chicks in this analysis were born into groups which were surrounded by other groups with fully sampled adults); 89% of candidate mothers related to the offspring in question (almost all group subordinates were related to at least 1 of the breeders) with a mean estimated relatedness of 0.38 (based on relatedness calculations—Konovalov and Heg 2008; see below); and an estimate of 10% of candidate fathers related to the offspring in question (based on behavioral observations) with a mean estimated relatedness of 0.125. The proportion of loci typed was set at 0.982 (generated by the program), and the genotyping error rate was set at 0.01.

How related are group members to one another?

Relatedness was calculated based on the microsatellite loci using an algorithm that is relatively insensitive to allele frequency bias (Konovalov and Heg 2008) within the program KINGROUP v2_090218 (Konovalov et al. 2004). Relatedness was also calculated using the KINSHIP estimator (Goodnight and Queller 1999). However, the relatedness estimator of Konovalov and Heg (2008) recaptured known relationships more accurately (i.e., mothers and offspring in groups without helpers), and only these results are presented. Ninety-five percent confidence intervals (CIs) around the population mean were calculated using bootstrapping (10 000 replicates) in R version 2.12.1 (www.r-project.org). High levels of relatedness among group subordinates can indicate that a single dominant pair is monopolizing reproduction and that indirect fitness benefits are available to helpful subordinates. Relatedness was calculated for each group-year: In order to avoid pseudoreplication, mean relatedness was found for each group as a whole. Relatedness was also calculated between subordinates and the chicks they helped to raise and between dominants and subordinates. Gene flow was quantified between groups using the F-statistic FST (Wright 1951). FST is the proportion of the total genetic diversity that separates groups and ranges from 0 to 1: If there is no population substructure (i.e., no stable groups), FST will approach 0. FST was calculated using analysis of molecular variation in the program GenAlEx 6.2 (Peakall and Smouse 2006). The program tests each F-statistic for each year for significance by comparing the calculated value against a null distribution of 1000 random permutations of the data set. A significant FST value indicates that there is group-based substructure in the population. Only fully blood-sampled groups were included in this analysis (N = 56 group-years, from 18 groups over 5 years).

Does inbreeding occur?

The extent of inbreeding can be determined from genetic patterns within the population using the F-statistics FIS and FIT (Wright 1951), which were calculated as described above for FST. Potential inbreeding within social pairs (the dominant pair in each group) was investigated by measuring their relatedness, calculated as described above (the pedigree is not yet deep enough to use pedigree-calculated values). Although dispersal constraints imposed by social status and age are likely to have considerable importance in structuring dispersal in the population (Raihani et al. 2010), relatedness between potential mates may also play a role. To investigate relatedness and mate choice, the relatedness of social pairs was compared with the relatedness between each mated-pair member and all other opposite-sex adults in the population at the time. If the relatedness of social pairs was significantly less than that of all other possible pairings for these individuals, it indicates avoidance of related individuals as mates. If the relatedness of social pairs was significantly higher than that of all other possible pairings for these individuals, then inbreeding may be occurring regularly. If the mean relatedness between mates did not differ from that of all other possible pairings for these individuals, they are choosing mates that are neither more nor less related to them than is average within the population. Because relatedness estimates are by their nature based on pairs of individuals, analyses “sample” each individual more than once and data are thus nonindependent. This nonindependence can be controlled for by using a randomization/permutation test, which generates a distribution of mean differences between 2 groups of values. This distribution is created by randomly assigning the data values into the groups being compared and calculating the mean difference between them. The permutation is repeated N times (recommended 1000 or more—Jadwiszczak 2009) to create a distribution of simulated mean differences, against which the observed values can be compared and tested statistically. A 2-sample randomization/permutation test within the program RUNDOM PRO 3.14 (Jadwiszczak 2009) was used to compare relatedness of social pairs with the relatedness between each member of each pair and all other opposite-sex adults in the population at the time, with 100 000 permutations (sensu Stiver et al. 2008).

We also investigated courtship behavior, which occurs primarily as a prelude to copulation and involves either males aerially chasing females or mutual presentation of nesting material (Raihani 2008). Courtship behavior between relatives could indicate the occurrence of inbreeding. We compared levels of relatedness between individuals observed in courtship behavior and between those sexually mature birds within the group which undertook no courtship. We used a 2-sample randomization/permutation test in RUNDOM PRO 3.14 (Jadwiszczak 2009) with 100 000 permutations to compare relatedness between pairs of courting birds within groups with relatedness between pairs of noncourting, adult opposite-sex birds in groups.

What reproductive opportunities are available to subordinates?

Because inbreeding and extra-group copulations appear to be avoided (see below), the main route to reproduction for subordinate adults lies in immigration of unrelated potential breeding partners to their groups. Individuals that joined one of the fully sampled study groups during the breeding season and stayed for 2 weeks or longer were considered to be immigrants (Ridley et al. 2008). All individuals that were returning former group members, that were offspring of either of the current dominant pair, or that were never blood-sampled were excluded from the analysis. Data from breeding seasons only (September 1 to May 31) are presented. We measured the proportion of dominants and subordinates each season that were new immigrants in groups, the proportion of subordinates that could potentially breed each year, and the proportion of these potentially breeding subordinates that did successfully reproduce.

RESULTS

Which group members are breeding?

Parentage was determined for 145 offspring from groups with sampled adults (91.2% of 159). Of these, parentage of 106 chicks (66.7% of 159) was determined with 95% confidence and a further 39 chicks (24.5% of 159) with 85% confidence. Of these 145 young, 95.2% were the progeny of the dominant pair. Three subordinate individuals were assigned parentage of a total of only 7 offspring (4.8%); in every case, subordinates reproduced with unrelated dominants. Of the 145 offspring, 72 (49.7%) were hatched into groups where the dominant pair were the only unrelated opposite-sex pair in the group and 73 (50.3%) into groups where there were alternate unrelated breeding partners for one or both dominants. Therefore, in groups where dominants could find alternate partners, 9.6% of chicks (7 of 73) were extrapair. Two subordinate females produced a total of 5 chicks. In the first case, a subordinate female shared reproduction for 1 brood with her mother (the dominant female) and produced 1 chick. In the second case, a subordinate female- shared reproduction with the unrelated dominant female for 2 broods in 2 years with 2 different dominant males and produced 2 chicks each year. Finally, 1 subordinate male cuckolded his father by fathering 1 brood of 2 chicks with a newly immigrated dominant female. In no case did subordinates breed together.

Fourteen chicks (8.8% of 159) could not be assigned parents. An acknowledged difficulty in investigating cooperatively breeding systems is that other close relatives (siblings etc.) are present in the pool of potential parents, thus confounding parentage prediction programs (Cockburn 1998; McRae and Amos 1999). Thirteen of the unassigned chicks had high match scores, but these matches were not significantly better than those of the second-best matches. In each case, the dominant pair was one of the predicted sets of parents. Finally, parentage of 1 chick was assigned to a dominant female with a neighboring dominant male (85% confidence). The neighboring male and the group dominant male were related to one another with R = 0.57 and both matched to the offspring. It is likely that the group dominant male was the genetic father and the program chose the neighboring male because of the high relatedness between the 2 candidates. In support of this, the neighboring male was never observed in a nonconfrontational association with the predicted mother.

How related are group members to one another?

Mean intragroup relatedness was 0.250 ± 0.042 (N = 19 groups). This ranged from –0.231 to 0.419 but was significantly greater than the population mean relatedness of 0.051 (95% CI = 0.048–0.053), indicating that most groups were highly kin structured. FST values were positive and highly significant in each year (Table 1), indicating that groups consisted of kin. Mean relatedness between helping subordinates and the chicks they provisioned was 0.38 ± 0.01 (N = 723 pairs from 15 groups). Mean relatedness between dominants and subordinates was also 0.38 ± 0.01 (N = 596 pairs from 21 groups).

Table 1

F-statistics (FST, FIS, and FIT) calculated for the pied babbler population over 5 years of study

Year N Groups FST P FIS P FIT P 
2003–2004 55 0.159 0.001 −0.251 1.000 −0.053 0.957 
2004–2005 78 10 0.134 0.001 −0.225 1.000 −0.061 0.995 
2005–2006 131 14 0.145 0.001 −0.185 1.000 −0.013 0.779 
2006–2007 121 15 0.152 0.001 −0.189 1.000 −0.008 0.655 
2007–2008 103 0.142 0.001 −0.191 1.000 −0.022 0.857 
Year N Groups FST P FIS P FIT P 
2003–2004 55 0.159 0.001 −0.251 1.000 −0.053 0.957 
2004–2005 78 10 0.134 0.001 −0.225 1.000 −0.061 0.995 
2005–2006 131 14 0.145 0.001 −0.185 1.000 −0.013 0.779 
2006–2007 121 15 0.152 0.001 −0.189 1.000 −0.008 0.655 
2007–2008 103 0.142 0.001 −0.191 1.000 −0.022 0.857 

Values were calculated using analysis of molecular variation with significance tested with 1000 permutations of the data set in GENALEX 6.2 (Peakall and Smouse 2006).

Does inbreeding occur?

In each year, both inbreeding coefficients (FIS and FIT) were negative but nonsignificant (Table 1), indicating that there is no systematic inbreeding within the population. The mean relatedness of social (dominant) pairs (−0.009 ± 0.037; N = 37 pairs) was no different from the mean relatedness of pairs that produced offspring together: this was usually the dominant pair but in 4 cases was a dominant with a subordinate (0.018 ± 0.038; N = 31 pairs; 2 sample randomization/permutation test, P = 0.622). The mean relatedness of social pairs was low and was not significantly different from the mean relatedness of the member of each pair with all other opposite-sex adults in the population in any year (2-sample randomization/ permutation test, Figure 1). Dispersing pied babblers chose mates that were neither more nor less related to them than the population average. Nevertheless, the average relatedness between pairs (0.018) was an order of magnitude lower than the average relatedness within groups (0.250).

Figure 1

Relatedness of mated and unmated pairs of opposite-sex adult pied babblers per year over 5 years of study. Means ± standard error of the mean were generated from relatedness values calculated using the Konovalov and Heg (2008) algorithm within the program KINGROUP v2_090218 (Konovalov et al. 2004); sample sizes are shown. A 2-sample randomization/permutation test with 100 000 permutations within the program RUNDOM PRO 3.14 (Jadwiszczak 2009) was used to calculate significance

Figure 1

Relatedness of mated and unmated pairs of opposite-sex adult pied babblers per year over 5 years of study. Means ± standard error of the mean were generated from relatedness values calculated using the Konovalov and Heg (2008) algorithm within the program KINGROUP v2_090218 (Konovalov et al. 2004); sample sizes are shown. A 2-sample randomization/permutation test with 100 000 permutations within the program RUNDOM PRO 3.14 (Jadwiszczak 2009) was used to calculate significance

Courtship between related adults was very rare. Courtship was most commonly observed between the dominant pair (85.1% of 242 observed courting interactions, N = 19 groups). In addition, however, courtship occasionally occurred between dominants and subordinates (12.8%) and, very rarely, between subordinates (2.1%). Levels of relatedness strongly predicted courtship behavior independently of social status; adult courtship occurred most commonly between unrelated group members (2-sample randomization/permutation test, P < 0.001); mean relatedness was significantly lower between courting (N = 46) than between noncourting (N = 480) adult opposite-sex pairs.

What reproductive opportunities are available to subordinates?

Overall, 40 individuals immigrated into non-natal groups (Table 2). Twenty-seven adults immigrated into non-natal groups and immediately became dominant with the result that 24.2 ± 5.5% of dominants (range 11.9–37.5%, N = 4 years) were new immigrants in their groups each year. Over the same period, 13 individuals (3 subadults and 10 adults) immigrated into non-natal groups and became subordinate: only 4.1 ± 0.7% of subordinates lived in non-natal groups each year (range = 3.0–5.6%, N = 4 years). Overall, adult subordinates made up 41.6 ± 9.5% (range 22.5–67.0%, N = 4 years) of group members. Immigration (usually of new, unrelated dominants) created opportunities for 34.1 ± 1.7% of these adult subordinates to breed with an unrelated adult group member (range 30.3–37.5%, N = 4 years). However, while opportunities for adult subordinates to breed were relatively common, only a small fraction (12.5 ± 7.4%) of these potentially breeding subordinates actually successfully reproduced (range 0–33.3%, N = 4 years).

Table 2

Summary of immigration into non-natal groups and subordinate breeding opportunities created by immigration for 4 breeding seasons in the pied babbler population

Year Immigrant dominants
 
Immigrant subordinates
 
Subordinates that could breed
 
Potential subordinate breeders that did breed
 
2004–2005 28.0% 5.3% 32.0% 12.5% 
2005–2006 19.4% 3.0% 10 30.3% 0% 
2006–2007 11.9% 5.3% 23 36.5% 4.3% 
2007–2008 37.5% 2.8% 37.5% 33.3% 
Mean 24.2 ± 5.5%  4.1 ± 0.7%  34.1 ± 1.7%  12.5 ± 7.4%  
Year Immigrant dominants
 
Immigrant subordinates
 
Subordinates that could breed
 
Potential subordinate breeders that did breed
 
2004–2005 28.0% 5.3% 32.0% 12.5% 
2005–2006 19.4% 3.0% 10 30.3% 0% 
2006–2007 11.9% 5.3% 23 36.5% 4.3% 
2007–2008 37.5% 2.8% 37.5% 33.3% 
Mean 24.2 ± 5.5%  4.1 ± 0.7%  34.1 ± 1.7%  12.5 ± 7.4%  

DISCUSSION

Although many cooperatively breeding species have high levels of extrapair paternity and subordinates commonly breed within the group (Cockburn 2004), here we provide evidence that in pied babblers monogamous dominant pairs monopolize reproduction. Dominant pairs within groups were socially monogamous (Raihani 2008) and almost completely sexually monogamous, with only rare instances of successful reproduction by subordinates. Where extrapair reproduction did occur, dominants bred with unrelated group subordinates. However, subordinates generally gained little or no direct reproductive success. This level of monogamy for both males and females is comparable to some other cooperatively breeding birds, including laughing kookaburras (Dacelo novaeguineaeLegge and Cockburn 2000), Florida scrub-jays (Aphelocoma coerulescensQuinn et al. 1999), red-cockaded woodpeckers (Picoides borealis—Haig et al. 1994), and Arabian babblers (Turdoides squamicepsLundy et al. 1998). In these species, as in pied babblers, groups contain subordinate helpers which are not related to the opposite-sex breeder but which very rarely reproduce, either within the group or outside of it (reviewed by Cockburn 2004). Outside the group, breeding is likely limited by intense territoriality that severely restricts opportunities for extra-group copulations (Quinn et al. 1999). Within pied babbler groups, high reproductive skew is likely to be maintained by a combination of the inability of subordinates to breed because of inadequate physical condition, inbreeding avoidance, reproductive suppression of subordinates by dominants, or mate choice by dominant females.

Inbreeding avoidance by subordinate pied babblers plays an important role in sustaining the species' high reproductive skew. However, in many groups, subordinates did have access to unrelated breeding partners but still did not breed, although they did engage in courtship behavior with these potential breeding partners. Reproductive skew in pied babblers can therefore only partly be explained by inbreeding avoidance: other factors such as physiological or hormonal state must also be involved in restricting subordinate reproduction. Inadequate physical condition is one factor that can limit subordinate reproduction in cooperatively breeding groups (Creel et al. 1992; Clutton-Brock et al. 2001). In pied babblers, females with low body mass do not breed (Raihani 2008), although there is no evidence that heavier subordinates more often engage in courtship and other prebreeding behaviors (Nelson-Flower 2010). Mate choice by dominant females is likely to reinforce monogamy among pied babblers because females appear to prefer the largest (thus almost invariably dominant) males (Ridley AR, unpublished data). In general, high skew among females is more common in avian than in mammalian cooperative breeders, perhaps because of the relative ease with which a dominant female bird can prevent a subordinate female from laying in a shared nest site (Raihani and Clutton-Brock 2010). In pied babblers, interactions between competitive females at the nest during the dominant female's fertile period are an important influence on the division of reproduction (Nelson-Flower 2010). Aggressive suppression of subordinate reproduction also works to maintain high skew; aggression regularly occurs between dominants and subordinates in cooperatively breeding species (Taborsky 1985; Curry 1988; Emlen and Wrege 1992; Piper and Slater 1993; Cooney and Bennett 2000; Williams 2004; Ratnieks et al. 2006; Young et al. 2006), including between dominant and subordinate pied babblers of both sexes during the fertile periods of dominant females (Nelson-Flower 2010).

Taken together, this evidence indicates that the primary route to reproduction in pied babblers is through acquisition of dominance. In terms of relative and absolute fitness, such positions are therefore extremely valuable and strong selective pressures are likely to be at work on traits that assist individuals in gaining dominance. For example, phenotypic traits such as plumage coloration can influence which individual becomes dominant (Cockburn et al. 2008), as can physical size (Ciszek 2000; Clutton-Brock et al. 2006; Hodge et al. 2008, but see Bolton et al. 2006; Spong et al. 2008). For pied babblers, large body mass (Ridley et al. 2008) and age (Raihani et al. 2010) have also been suggested to be important in acquiring a breeding position. For female pied babblers, aggression is likely to be an important trait influencing the likelihood of gaining dominance; some females gain dominance through aggressive overthrow of dominant females in non-natal groups (Raihani et al. 2010), and aggressive juvenile females are more likely to attempt dispersal early in life (Raihani et al. 2008) increasing their exposure to unrelated potential mates. Aggression may be less important for males because they gain dominance through founding a new group or filling a reproductive vacancy rather than supplanting an existing dominant (Raihani et al. 2010).

The role of kin selection in the evolution of cooperative breeding remains unresolved (Clutton-Brock 2002), but it has recently been suggested that the rate of female promiscuity (and thus intragroup relatedness levels) has driven the transition between cooperative societies and independent breeding (“the monogamy hypothesis,” Boomsma 2007, 2009). Lifetime genetic monogamy has been suggested to be central to the evolution of helping behavior in eusocial insects (Hughes et al. 2008) and shrimps (Duffy and Macdonald 2010). Hatchwell (2009) and Cornwallis et al. (2010) recently suggested that, in birds, close relatedness and thus kin selection were important in the evolution of cooperative breeding. In pied babblers, mean relatedness values and FST values indicate that groups were highly kin structured. This high relatedness among subordinates reflects both the strongly monogamous breeding system and the low immigration rate by unrelated subordinates. Most subordinates were genetically linked to the current brood through at least one of the dominant pair, with a mean relatedness to the chicks of 0.38. It is therefore likely that kin selection has played a role in the evolution of cooperative breeding in this species, and future work will address the question of the indirect benefits gained by subordinate helpers. Whether such selection plays an important role in the maintenance of cooperative behavior in pied babblers is more difficult to discern, owing to the confounding effects of the direct fitness benefits of helping (e.g., cooperative sentinel calling—Hollén et al. 2008; Bell et al. 2009). The direct fitness benefits of group living (Ridley et al. 2008) could additionally or independently serve to maintain cooperative behavior in pied babblers. In general, it is likely that ecological constraints and genetic relatedness operated together during the development of sociality (Cornwallis et al. 2010; West and Gardner 2010), and the same is likely to be true for the evolution of cooperative breeding in the pied babbler.

SUPPLEMENTARY MATERIAL

Supplementary material can be found at http://www.beheco.oxfordjournals.org/.

FUNDING

Department of Science and Technology and National Research Foundation Centre of Excellence at the Percy FitzPatrick Institute for African Ornithology, University of Cape Town (426078PFP1074).

We are grateful to Prof. T. Clutton-Brock for assistance in the initiation and continuation of the Pied Babbler Research Project. The Northern Cape Conservation Authority permitted research on pied babblers and the Kotzes and the de Bruins kindly allowed us access to babbler groups on their land. M. Bell, K. Bradley, K. Golabek, J. Oates, A. Radford, R. Rose, and H. Wade helped with maintaining habituation of babbler groups. Dr D. Dawson at the Sheffield Molecular Genetics Facility (University of Sheffield) very kindly provided test primers. J. Bishop, N. Coutts, and N. Muna helped with technical advice in the laboratory. Useful comments and discussion were provided by T. Flower, M. Bell, N. Jordan, D. Lukas, A. Radford, and S. Sharp. The comments of 2 anonymous reviewers greatly improved the manuscript. Prof. N. Davies at the University of Cambridge hosted M.J.N.-F. as an academic visitor while writing this paper. Fieldwork received ethical clearance from the University of Cape Town's Animal Ethics Committee.

References

Bell
MBV
Radford
AN
Rose
R
Wade
HM
Ridley
AR
The value of constant surveillance in a risky environment
Proc R Soc B Biol Sci
 , 
2009
, vol. 
276
 (pg. 
2997
-
3005
)
Bolton
A
Sumner
S
Shreeves
G
Casiraghi
M
Field
J
Colony genetic structure in a facultatively eusocial hover wasp
Behav Ecol
 , 
2006
, vol. 
17
 (pg. 
873
-
880
)
Boomsma
JJ
Kin selection versus sexual selection: why the ends do not meet
Curr Biol
 , 
2007
, vol. 
17
 (pg. 
R673
-
R683
)
Boomsma
JJ
Lifetime monogamy and the evolution of eusociality
Philos Trans R Soc B Biol Sci
 , 
2009
, vol. 
364
 (pg. 
3191
-
3207
)
Brown
JL
Helping and communal breeding in birds
 , 
1987
Princeton (NJ)
Princeton University Press
Ciszek
D
New colony formation in the “highly inbred” eusocial naked mole-rat: outbreeding is preferred
Behav Ecol
 , 
2000
, vol. 
11
 (pg. 
1
-
6
)
Clutton-Brock
TH
Breeding together: kin selection and mutualism in cooperative vertebrates
Science
 , 
2002
, vol. 
296
 (pg. 
69
-
72
)
Clutton-Brock
TH
Brotherton
PNM
Russell
AF
O'Riain
MJ
Gaynor
D
Kansky
R
Griffin
A
Manser
M
Sharpe
L
McIlrath
GM
, et al.  . 
Cooperation, control and concession in meerkat groups
Science
 , 
2001
, vol. 
291
 (pg. 
478
-
481
)
Clutton-Brock
TH
Hodge
SJ
Spong
G
Russell
AF
Jordan
NR
Bennett
NC
Sharpe
LL
Manser
MB
Intrasexual competition and sexual selection in cooperative mammals
Nature
 , 
2006
, vol. 
444
 (pg. 
1065
-
1068
)
Cockburn
A
Evolution of helping behavior in cooperatively breeding birds
Annu Rev Ecol Syst
 , 
1998
, vol. 
29
 (pg. 
141
-
177
)
Cockburn
A
Koenig
WD
Dickinson
JL
Mating systems and sexual conflict
Ecology and evolution of cooperative breeding in birds
 , 
2004
Cambridge
Cambridge University Press
(pg. 
81
-
101
)
Cockburn
A
Osmond
HL
Mulder
RA
Double
MC
Green
DJ
Demography of male reproductive queues in cooperatively breeding superb fairy-wrens Malurus cyaneus
J Anim Ecol
 , 
2008
, vol. 
77
 (pg. 
297
-
304
)
Cooney
R
Bennett
NC
Inbreeding avoidance and reproductive skew in a cooperative mammal
Proc R Soc B Biol Sci
 , 
2000
, vol. 
267
 (pg. 
801
-
806
)
Cornwallis
CK
West
SA
Davis
KE
Griffin
AS
Promiscuity and the evolutionary transition to complex societies
Nature
 , 
2010
, vol. 
466
 (pg. 
969
-
972
)
Creel
SR
Creel
NM
Wildt
DE
Monfort
SL
Behavioural and endocrine mechanisms of reproductive suppression in Serengeti dwarf mongooses
Anim Behav
 , 
1992
, vol. 
43
 (pg. 
231
-
245
)
Curry
RL
Influence of kinship on helping behavior in Galapagos mockingbirds
Behav Ecol Sociobiol
 , 
1988
, vol. 
22
 (pg. 
141
-
152
)
Duffy
JE
Macdonald
KS
Kin structure, ecology and the evolution of social organization in shrimp: a comparative analysis
Proc R Soc B Biol Sci
 , 
2010
, vol. 
277
 (pg. 
575
-
584
)
Emlen
ST
Wrege
PH
Parent–offspring conflict and the recruitment of helpers among bee-eaters
Nature
 , 
1992
, vol. 
356
 (pg. 
331
-
333
)
Goodnight
KF
Queller
DC
Computer software for performing likelihood tests of pedigree relationships using genetic markers
Mol Ecol
 , 
1999
, vol. 
8
 (pg. 
1231
-
1234
)
Griffin
AS
West
SA
Kin discrimination and the benefit of helping in cooperatively breeding vertebrates
Science
 , 
2003
, vol. 
302
 (pg. 
634
-
636
)
Griffith
SC
Owens
IPF
Thuman
KA
Extra pair paternity in birds: a review of interspecific variation and adaptive function
Mol Ecol
 , 
2002
, vol. 
11
 (pg. 
2195
-
2212
)
Haig
SM
Walters
JR
Plissner
JH
Genetic evidence for monogamy in the cooperatively breeding red-cockaded woodpecker
Behav Ecol Sociobiol
 , 
1994
, vol. 
34
 (pg. 
295
-
303
)
Hamilton
WD
The genetical evolution of social behaviour
J Theor Biol
 , 
1964
, vol. 
7
 (pg. 
1
-
52
)
Hatchwell
BJ
The evolution of cooperative breeding in birds: kinship, dispersal and life history
Philos Trans R Soc B Ser Biol Sci
 , 
2009
, vol. 
364
 (pg. 
3217
-
3227
)
Heinsohn
RG
Legge
S
The cost of helping
Trends Ecol Evol
 , 
1999
, vol. 
14
 (pg. 
53
-
57
)
Hodge
SJ
Manica
A
Flower
TP
Clutton-Brock
TH
Determinants of reproductive success in dominant female meerkats
J Anim Ecol
 , 
2008
, vol. 
77
 (pg. 
92
-
102
)
Hollén
LI
Bell
MBV
Radford
AN
Cooperative sentinel calling? Foragers gain increased biomass intake
Curr Biol
 , 
2008
, vol. 
18
 (pg. 
576
-
579
)
Hughes
WOH
Oldroyd
BP
Beekman
M
Ratnieks
FLW
Ancestral monogamy shows kin selection is key to the evolution of eusociality
Science
 , 
2008
, vol. 
320
 (pg. 
1213
-
1216
)
Jadwiszczak
P
Rundom Pro 3.14. Software for classical and computer-intensive statistics
 , 
2009
 
[cited 2011 January 1]. Available from: http://pjadw.tripod.com
Kalinowski
ST
Taper
ML
Marshall
TC
Revising how the computer program CERVUS accommodates genotyping error increases success in paternity assignment
Mol Ecol
 , 
2007
, vol. 
16
 (pg. 
1099
-
1106
)
Keller
L
Reeve
HK
Partitioning of reproduction in animal societies
Trends Ecol Evol
 , 
1994
, vol. 
9
 (pg. 
98
-
102
)
Koenig
WD
Pitelka
FA
Relatedness and inbreeding avoidance, counterploys in the communally nesting acorn woodpecker
Science
 , 
1979
, vol. 
206
 (pg. 
1103
-
1105
)
Kokko
H
Johnstone
RA
Clutton-Brock
TH
The evolution of cooperative breeding through group augmentation
Proc R Soc B Biol Sci
 , 
2001
, vol. 
268
 (pg. 
187
-
196
)
Kokko
H
Johnstone
RA
Wright
J
The evolution of parental and alloparental effort in cooperatively breeding groups: when should helpers pay to stay?
Behav Ecol
 , 
2002
, vol. 
13
 (pg. 
291
-
300
)
Konovalov
DA
Heg
D
Estimation of population allele frequencies from small samples containing multiple generations. Series on Advances in Bioinformatics and Computational Biology
Proceedings of the 6th Asia-Pacific Bioinformatics Conference
 , 
2008
Kyoto
Japan. London: Imperial College Press
(pg. 
321
-
331
)
Konovalov
DA
Manning
C
Henshaw
MT
KINGROUP: a program for pedigree relationship reconstruction and kin group assignments using genetic markers
Mol Ecol Notes
 , 
2004
, vol. 
4
 (pg. 
779
-
782
)
Legge
S
Cockburn
A
Social and mating system of cooperatively breeding laughing kookaburras (Dacelo novaeguineae)
Behav Ecol Sociobiol
 , 
2000
, vol. 
47
 (pg. 
220
-
229
)
Li
SH
Brown
JL
High frequency of extrapair fertilization in a plural breeding bird, the Mexican jay, revealed by DNA microsatellites
Anim Behav
 , 
2000
, vol. 
60
 (pg. 
867
-
877
)
Lundy
KJ
Parker
PG
Zahavi
A
Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable
Behav Ecol Sociobiol
 , 
1998
, vol. 
43
 (pg. 
173
-
180
)
Maynard-Smith
J
Group selection and kin selection
Nature
 , 
1964
, vol. 
201
 (pg. 
1145
-
1147
)
McRae
SB
Amos
W
Can incest within cooperatively breeding groups be detected using DNA fingerprinting?
Behav Ecol Sociobiol
 , 
1999
, vol. 
47
 (pg. 
104
-
107
)
Mulder
RA
Dunn
PO
Cockburn
A
Lazenby-Cohen
KA
Howell
MJ
Helpers liberate female fairy-wrens from constraints on extra-pair mate choice
Proc R Soc B Biol Sci
 , 
1994
, vol. 
255
 (pg. 
223
-
229
)
Nelson-Flower
MJ
Kinship and its consequences in the cooperatively breeding Southern pied babbler Turdoides bicolor [PhD thesis]
 , 
2010
Cape Town (South Africa)
University of Cape Town
 
139 p
Peakall
R
Smouse
PE
GENALEX 6: genetic analysis in Excel. Population genetic software for teaching and research
Mol Ecol Notes
 , 
2006
, vol. 
6
 (pg. 
288
-
295
)
Piper
WH
Slater
G
Polyandry and incest avoidance in the cooperative stripe-backed wren of Venezuela
Behaviour
 , 
1993
, vol. 
124
 (pg. 
227
-
247
)
Pulliam
HR
On the advantages of flocking
J Theor Biol
 , 
1973
, vol. 
38
 (pg. 
419
-
422
)
Pusey
AE
Sex-biased dispersal and inbreeding avoidance in birds and mammals
Trends Ecol Evol
 , 
1987
, vol. 
2
 (pg. 
295
-
299
)
Pusey
AE
Wolf
M
Inbreeding avoidance in animals
Trends Ecol Evol
 , 
1996
, vol. 
11
 (pg. 
201
-
206
)
Quinn
JS
Woolfenden
GE
Fitzpatrick
JW
White
BN
Multi-locus DNA fingerprinting supports genetic monogamy in Florida scrub-jays
Behav Ecol Sociobiol
 , 
1999
, vol. 
45
 (pg. 
1
-
10
)
Raihani
NJ
Cooperation and conflict in pied babblers [PhD thesis]
 , 
2008
Cambridge
University of Cambridge
 
152 p
Raihani
NJ
Clutton-Brock
TH
Higher reproductive skew among birds than mammals in cooperatively breeding species
Biol Lett
 , 
2010
, vol. 
6
 (pg. 
630
-
632
)
Raihani
NJ
Nelson-Flower
MJ
Golabek
K
Ridley
AR
Routes to breeding in cooperatively breeding pied babblers Turdoides bicolor
J Avian Biol
 , 
2010
, vol. 
41
 (pg. 
681
-
686
)
Raihani
NJ
Ridley
AR
Variable fledging age according to group size: trade-offs in a cooperatively breeding bird
Biol Lett
 , 
2007
, vol. 
3
 (pg. 
624
-
627
)
Raihani
NJ
Ridley
AR
Browning
LE
Nelson-Flower
MJ
Knowles
S
Juvenile female aggression in cooperatively breeding pied babblers: causes and contexts
Ethology
 , 
2008
, vol. 
114
 (pg. 
452
-
458
)
Ratnieks
FLW
Foster
KR
Wenseleers
T
Conflict resolution in insect societies
Annu Rev Entomol
 , 
2006
, vol. 
51
 (pg. 
581
-
608
)
Richardson
DS
Burke
T
Komdeur
J
Direct benefits and the evolution of female-biased cooperative breeding in Seychelles warblers
Evolution
 , 
2002
, vol. 
56
 (pg. 
2313
-
2321
)
Richardson
DS
Jury
FL
Blaakmeer
K
Komdeur
J
Burke
T
Parentage assignment and extra-group paternity in a cooperative breeder: the Seychelles warbler (Acrocephalus sechellensis
Mol Ecol
 , 
2001
, vol. 
10
 (pg. 
2263
-
2273
)
Ridley
AR
Raihani
NJ
Facultative response to a kleptoparasite by the cooperatively breeding pied babbler
Behav Ecol
 , 
2007
, vol. 
18
 (pg. 
324
-
330
)
Ridley
AR
Raihani
NJ
Variable post-fledging care in a cooperative bird: causes and consequences
Behav Ecol
 , 
2007
, vol. 
18
 (pg. 
994
-
1000
)
Ridley
AR
Raihani
NJ
Task partitioning increases reproductive output in a cooperative bird
Behav Ecol
 , 
2008
, vol. 
19
 (pg. 
1136
-
1142
)
Ridley
AR
Raihani
NJ
Nelson-Flower
MJ
The cost of being alone: the fate of floaters in a population of cooperatively breeding pied babblers Turdoides bicolor
J Avian Biol
 , 
2008
, vol. 
39
 (pg. 
389
-
392
)
Russell
AF
Koenig
WD
Dickinson
JL
Mammals: comparisons and contrasts
Ecology and evolution of cooperative breeding in birds
 , 
2004
Cambridge
Cambridge University Press
(pg. 
210
-
227
)
Spong
GF
Hodge
SJ
Young
AJ
Clutton-Brock
TH
Factors affecting the reproductive success of dominant male meerkats
Mol Ecol
 , 
2008
, vol. 
17
 (pg. 
2287
-
2299
)
Stiver
KA
Fitzpatrick
JL
Desjardins
JK
Neff
BD
Quinn
JS
Balshine
S
The role of genetic relatedness among social mates in a cooperative breeder
Behav Ecol
 , 
2008
, vol. 
19
 (pg. 
816
-
823
)
Taborsky
M
Breeder-helper conflict in a cichlid fish with broodcare helpers: an experimental analysis
Behaviour
 , 
1985
, vol. 
95
 (pg. 
45
-
75
)
West
SA
Gardner
A
Altruism, spite and greenbeards
Science
 , 
2010
, vol. 
327
 (pg. 
1341
-
1344
)
Williams
DA
Female control of reproductive skew in cooperatively breeding brown jays (Cyanocorax morio)
Behav Ecol Sociobiol
 , 
2004
, vol. 
55
 (pg. 
370
-
380
)
Wright
S
The genetical structure of populations
Ann Eugen
 , 
1951
, vol. 
15
 (pg. 
323
-
354
)
Young
AJ
Carlson
AA
Monfort
SL
Russell
AF
Bennett
NC
Clutton-Brock
TH
Stress and the suppression of subordinate reproduction in cooperatively breeding meerkats
Proc Natl Acad Sci U S A
 , 
2006
, vol. 
103
 (pg. 
12005
-
12010
)

Supplementary data