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Body size is associated with yearling breeding and extra-pair mating in the Island Scrub-Jay


Desrosiers, Michelle A. et al. (2021), Body size is associated with yearling breeding and extra-pair mating in the Island Scrub-Jay, Dryad, Dataset,


Large body size is an important determinant of individual fitness in many animal species, especially in island systems where habitat saturation may result in strong intraspecific competition for mates and breeding territories. Here we show that large body size is associated with benefits to yearling breeding and extra-pair mating in the Island Scrub-Jay (Aphelocoma insularis), endemic to Santa Cruz Island, California. This species is approximately 20% larger than its mainland congener, consistent with the island syndrome, indicating that body size may be a trait under selection. From 2009 to 2013, we quantified the reproductive success of a marked population of Island Scrub-Jays, tracked which yearlings acquired a breeding territory and bred, and measured the occurrence of extra-pair paternity. Two potential contributors to fitness were positively related to body size. Larger yearling males were more likely to breed, possibly due to greater behavioral dominance during aggressive encounters. Larger males were also less likely to lose paternity to extra-pair males and, anecdotally, extra-pair males were larger than the social male cuckolded. This study provides evidence that larger males may have a fitness advantage over smaller males by breeding earlier and avoiding paternity loss, but estimates of lifetime reproductive success are ultimately needed for Island Scrub-Jays and other long-lived species.


Morphologic and genetic data (blood) were collected as island scrub-jays were captured on three study plots on Santa Cruz Island, California. We extracted DNA from blood samples with a QIAGEN DNEasy Blood and Tissue Kit following the manufacturer’s protocol and amplified 9 variable microsatellite loci (AIAAGG13, ApCo2, PJGATA3, PJAAAG9, PJGATA2, CmAAAG30, CmAAAG11, CmAAAG25, CmAAAG6). We amplified microsatellites in 10 µl multiplex reactions using QIAGEN's type-it microsatellite PCR kit and a Vapo-Protect Mastercycler 6321. PCR products were run on an Applied BiosSystems 3730xl DNA analyzer. Florescent labels and known allele size differences were used to automatically score electropherograms using GeneMarker software. All scores were verified manually. We used CERVUS 3.0.7 to determine parentage and the presence of extra-pair young. Paternity assignment analyses were seperated by plot and year. We radio-tracked sexually mature yearling island scrub-jays throughout their first breeding season, and observed their reproductive status. We also noted any additional yearling individuals (known from banding records) that were able to aquire a territory and breed their first breeding season.

Usage Notes

Input files for CERVUS are included and labeled by plot (CPC, FS or PZ), and year (2010, 2011, or 2012). 

Allele frequency analyses were performed with genotype files with the naming convention PLOT_YEAR_genotypes.csv.

Simulations of paternity analysis were performed with the resultant allele frequency file from above. Parameters for these analyses (i.e. number of candidate fathers, proportion of candidate fathers sampled) can be found in included simulation output files with the naming convention PLOT_YEAR_SimulationOut.txt.

Parentage analyses were performed with the same parameters as the simulations. Offspring and candidate mothers can be found in files named PLOT_YEAR_trios.csv, candidate father files are named PLOT_YEAR_males.csv, and genotypes of offspring, candidate mothers, and candidate fathers can be found in files named PLOT_YEAR_genotypes.csv.

Additionally, the excel file "ISSJ Offspring Sample Key" will be useful for interpreting parentage assignments (i.e. whether the assigned male is the social male). 


Nature Conservancy

Smithsonian Institution

Colorado State University

National Science Foundation, Award: GRFP-2006037277

National Science Foundation, Award: DDIG-1210421

National Science Foundation, Award: DEB-1754821