Reproductive partitioning and the assumptions of reproductive skew models in the cooperatively breeding American crow

Abstract

Understanding the benefits of cooperative breeding for group members of different social and demographic classes requires knowledge of their reproductive partitioning and genetic relatedness. From 2004-2007, we examined parentage as a function of relatedness and social interactions among members of 21 American crow (Corvus brachyrhynchos) family groups. Paired female breeders monopolized maternity of all offspring in their broods, whereas paired male breeders sired 82.7% of offspring, within-group auxiliary males sired 6.9% of offspring, and extragroup males sired 10.4% of offspring. Although adult females had fewer opportunities for direct reproduction as auxiliaries than males, they appeared to have earlier opportunities for independent breeding. These different opportunities for direct reproduction probably contributed to the male biased adult auxiliary sex ratio. Patterns of reproductive partitioning and conflict among males were most consistent with a synthetic reproductive skew model, in which auxiliaries struggled with breeders for a limited reproductive share, beyond which breeders could evict them. Counter to a frequent assumption of reproductive skew models, female breeders appeared to influence paternity, although their interests might have agreed with the interests of their paired males. Unusual among cooperative breeders, close inbreeding and incest occurred in this population. Incest avoidance between potential breeders did not significantly affect reproductive skew. KeywordsAmerican crow; birds; cooperative breeding; copulations; Corvus brachyrhynchos; inbreeding; incest; mate guarding; mating system; reproductive skew Cooperative breeding in birds occurs when more than two individuals contribute to the care of young in a single brood. Although cooperative systems are highly variable (Brown 1987), cooperative groups in many species are characterized by a single breeding pair, assisted by (presumed) nonbreeding 'auxiliaries,' usually adult or subadult offspring from previous broods. Potential benefits derived by auxiliaries (reviewed in Koenig & Dickinson 2004) include enhanced fitness of nondescendent kin (Hamilton 1964), prospecting for extragroup Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. (Young et al. 2007), and territory inheritance (Wiley & Rabenold 1984) or budding (Woolfenden & Fitzpatrick 1984). In some systems, molecular diagnoses of parentage have revealed that apparently nonbreeding auxiliaries also share in direct parentage (e.g. Rabenold et al. 1990). To understand the benefits of cooperative breeding for auxiliaries of different ...

Condition, innate immunity and disease mortality of inbred crows

“…Inbred crows suffer from severe disease-mediated inbreeding depression: survival probability is lower for relatively inbred birds, and birds that die with signs of infectious disease during post-mortem examination have higher inbreeding indices than birds with other fates (Townsend et al 2009a). Proximately, inbreeding in this population occurs because delayed dispersal and short dispersal distances in both sexes lead to interactions between related adults of the opposite sex (Townsend et al 2009b). Inbreeding occurs through extrapair matings (i.e.…”

“…Crows in this population are socially monogamous, and family groups usually contain auxiliaries of either sex, most of which help to provision the incubating females, nestlings and fledgelings. Criteria and methods for classifying family groups, auxiliaries, male breeders and female breeders are described in Townsend et al (2009b). Hatch date was estimated by the shifting behaviour of female breeders when their eggs begin to hatch, and we refined nestling age estimates at the time of banding.…”

“…We sexed all individuals at diagnostic sex-linked alleles (Fridolfsson & Ellegren 1999). Following Townsend et al (2009b), we genotyped each nestling at 10 microsatellite loci, and calculated two microsatellite-based individual heterozygosity indices that are frequently reported in the literature: (i) multi-locus heterozygosity (MLH), which is the frequency of heterozygous loci for each individual, and (ii) internal relatedness (IR), which accounts for background allele frequencies when estimating parental similarity from an offspring's genotype (Amos et al 2001) using IRMACRON4 (http://www. zoo.cam.ac.uk/zoostaff/amos/#ComputerPrograms).…”

Disease-mediated inbreeding depression in a large, open population of cooperative crows

“…We then explored the relationship between this inbreeding index and two indices of fitness: (i) survival within the duration of the study (2-50 months after banding, depending on the year in which an individual was sampled) and (ii) the probability of dying with disease symptoms. Townsend et al 2009). Offspring were marked individually with colour bands, aluminium bands and patagial tags on day 23-30 after hatching.…”

“…American crows in a wild, cooperatively breeding population in Ithaca, New York, exhibit natal philopatry and limited natal dispersal of both sexes, as well as incest and inbreeding ( Townsend et al 2009). We first assessed the appropriateness of our inbreeding index, estimated from a panel of 10 microsatellite markers, by comparing inbreeding indices with available pedigree information and parental relatedness coefficients, and through HHCs.…”