The Not So Golden Rule: Cooperation Within and Between Levels of a Multilevel Society in Palmchats (Dulus dominicus)

Abstract

Multilevel societies, hierarchical social structures in which smaller subunits cluster to form larger groups, allow organisms to flexibly balance the costs and benefits associated with group living. Previously thought to require complex cognitive abilities unique to a few species, multilevel societies are well-studied in primates and other mammals, but their functions in other taxa are less understood. Palmchats (Dulus dominicus), communal-nesting passerines endemic to Hispaniola, construct multi-chambered compound nests that are clustered among neighboring palms and separated into distinct nest ‘neighborhoods.’ This forms the physical structure of a multilevel society with hierarchically nested tiers expanding from chambers to nests to neighborhoods to populations. This study sought to investigate whether palmchats tailor their investment in the cooperative behavior of mobbing based on their level of social association within this multilevel structure. Playback experiments were conducted at active nest sites and behavioral response variables were recorded for focal individuals and combined into an additive composite score measuring response strength. Distress calls from group mates elicited the strongest responses (mean additive composite score: 11.57 ± 2.76), those from unfamiliar individuals elicited intermediate responses (9.82 ± 3.71), and those from neighbors elicited the weakest responses (8.86 ± 4.26). Although limited sample sizes prevented most results from obtaining significance, the multivariate effect of social category approached significance (Pillai’s trace = 0.668, p = 0.060), primarily driven by differences in time to first flight after playback began. Future experiments are planned to increase the sample size. If this pattern holds, it would show that the ordering of response strength departs from a simple gradient that declines with social association and instead suggest that cooperative investment is shaped by the distinct dynamics at each social level: elevated for group mates due to higher relatedness, cooperative breeding, and shared investment, suppressed for neighbors due to territoriality and public information redundancy, and baseline for unfamiliar individuals. These findings provide preliminary evidence that vocal individual recognition mediates cooperation across the tiers of this avian multilevel society and contribute to the growing recognition that multilevel societies, and complex functioning within them, are not restricted to large-brained mammalian taxa.