Long-term evaluation of the association between dominance, bib size, sex and age

The association between dominance and phenotypic traits (‘badges of
status’) has been recently questioned, since it has been found to show
more variation across populations and time than expected. Using an
eleven-year dataset, encompassing more than 1,800 individuals, we studied
the association between the size of a melanin-based plumage trait (a black
patch under the beak or ‘bib’), individual attributes (age and sex), and
dominance. Our study model, the Sociable Weaver (Philetairus socius), is a
small passerine that lives year-round in colonies of varying size. Bib
sizes were obtained from pictures and automatically pre-processed with an
object segmentation deep learning network. We recorded 36,346
dominance-related interactions to estimate dominance hierarchies. We then
investigated the effect of sex and age on bib size, and the overall and
between-year association of bib size and age with dominance for each sex.
We found that bib size increased with age within individuals of both
sexes. Age was a strong and consistent predictor of dominance in males,
but not in females. Nevertheless, dominance was not correlated
with bib size in males and in females. Finally, these relationships did
not strongly vary between years. Our results suggest that males and
females may use different factors to regulate dominance
interactions. Furthermore, our longitudinal dataset allowed us to show
that bib size is not a badge of status in our population, even though its
growth encodes information on age. These results highlight the importance
of considering multiple traits over time when studying the morphological
signalling of dominance hierarchy.