Are Pinyon Jays (Gymnorhinus cyanocephalus) genetically monogamous?
Barber, M., Benford, R., and Balda, R.P.
Avian Cognition Laboratory · Department of Biological Sciences
Northern Arizona University · Flagstaff, AZ

Pinyon jays are socially monogamous and form lifetime pair bonds.  Both females and males invest heavily in parental care.  The likelihood that social parents are genetic parents is high, but the level of genetic monogamy in this species is unknown.  To determine the rates of monogamy and extra pair paternity, observations were made to identify the social relationships of 22 (11 female and 11 male) captive adult pinyon jays in a single flock.  Birds were then paired with their social partners, with individuals that were not their social partners, and in small groups that contained their social partners and other individuals.  Birds were given the opportunity to breed.  DNA was collected from parental birds (n=22) and all embryos (n=24) produced.  Allele frequencies were compared at 6 VNTR loci.  Exclusion analyses were performed to establish a list of putative parents for each embryo.  Relatedness was calculated for each hypothesized parent-offspring dyad, and a list of most likely parent-offspring triads was derived.  Genetic triads were compared to observational data on adult pair-bonding.  In each case, social triads matched genetic triads.  These data support the hypothesis that pinyon jays are genetically monogamous, and suggest that a high level of parental investment from both sexes is justified.

Introduction

Lack (1967) reported that 93% of passerine species are "normally monogamous."  However, since molecular techniques have allowed researchers to determine true parentage, only ``% of passerine species have been demonstrated to be genetically monogamous.  Genetic monogamy, even in socially monogamous species, seems to be the exception rather than the rule.
Pinyon jays are socially monogamous and form life long pair bonds.  Both male and female pinyon jays invest heavily in their offspring (Marzluff and Balda, 1992).  A heavy male investment would provide a direct payoff in fitness if his paternity were assured.  But if the male's paternity were not assured, he could be the victim of cuckoldry, and the evolutionary return on his investment could be diminished.  This study is an attempt to determine whether or not pinyon jays are genetically monogamous.

Methods

To describe the social relationships within a flock of 22 wild-caught, captive pinyon jays, behavioral observations were performed in an outdoor aviary at the outset of each of the 2003-05 breeding seasons.  Birds were observed in three minute intervals for four hours daily, for a one week period before nest-building began.  Observation schedules were randomized, such that birds were observed in different sequences in the morning, mid-day, and afternoon.  Data on courtship, neutral, and agonistic behaviors, described by Marzluff and Balda 1992, among dyads were collected.  Dyads with the greatest number of courtship and the fewest number of agonistic interactions were categorized as social partners.
For the three breeding seasons in the study, females were placed in experimental conditions where they were given the opportunity to breed with their social partner, with other males in a group containing their social partner, and with males in a group that did not contain their social partner.  Eggs that were produced were collected.  DNA was extracted from each viable embryo.  Genetic information from six VNTR loci was obtained for all embryos and adult birds in the study.  An exclusion analysis was performed to establish a list of putative parents for each embryo.  Relatedness of each parent-offspring dyad was also calculated (Queller and Goodnight 1989).  From the list of putative parents, the adult male and female that had the highest relatedness values were classified as the embryo's genetic parents.  Genetic paternity was compared to observational data on social partnerships.

Results

Of the 24 eggs produced, 15 yielded genetic data.  Parentage could be accurately determined for 9 of these eggs.  In all 9 cases, genetic parentage matched social parentage.  Genetic data from 6 parent-offspring triads were incomplete and yielded inconclusive results.  All reproductively active females (n=6) produced eggs with only one male.  In each case, the female produced eggs with the male that was identified as her social partner.  Table 2 presents parentage data for all viable embryos.

Table 1. Number of eggs produced in each experimental condition. Courtship condition 2 = strong, 1= weak, 0 = none.  Proximity condition 2 = same aviary, 1 = adjacent aviaries, 0 = non-adjacent aviaries.  Shaded areas represent conditions that would imply extra pair paternity.  Non-shaded areas represent conditions that imply monogamy.

Table 2 (Below).  Parentage data derived from genetic analyses.  “Putative Parents” are adults that survive exclusion analysis.  “Relatedness” is genetic similarity of adult to embryo. 1.0 = exactly similar, 0.0 = as similar as one would expect by chance by selecting two individuals randomly from the flock, -1.0 = as dissimilar as possible considering allele frequencies present in population.

Discussion

Results from this experiment suggest that pinyon jays do not perform extra pair copulations.  Data on social parentage were consistent with data on genetic parentage in each of the examples considered. The finding that pinyon jays are genetically monogamous seems to justify the high level of parental investment that both males and females make in their offspring.
While this research provided useful information on the reproductive fidelity of male pinyon jays, the findings must be interpreted cautiously.  Pinyon jays in the study were captive, and their options for choosing both social and reproductive partners were restricted.  In the wild, the opportunity for extra pair copulations may be increased because females have a broader range of possible mates and potential accomplices in cuckoldry.  Furthermore, the sample size in this study is prohibitively small.  More breeding events that yield more complete genetic information should be considered before a strong conclusion can be made.
Future research should include a larger number of adults, breeding permutations, and eggs.  Techniques should be improved to provide more valid genetic data at more loci. Finally, genetic data from wild birds should be used to more accurately characterize the mating dynamics of this species.

References

Lack, D.J.  1967.  The Natural Regulation of Animal Numbers, Oxford:  Clarendon Press.
Marzluff, J., & Balda, R.P. 1992. The Pinyon Jay Behavioral Ecology of a Colonial and Cooperative Corvid. London: T & A D Poyser.
Queller, D.C., & Goodnight, K.F. 1989. Estimating relatedness using genetic markers.  Evolution, 43, 258-275

Acknowledgements

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