Pigeons try to overthrow a stable social order's hierarchy just adding weights to the revolutionaries' bodies; mass loading causes the low-ranked male birds to immediately become more aggressive and rise-up the dominance hierarchy

Artificial mass loading disrupts stable social order in pigeon dominance hierarchies. Steven J. Portugal, James R. Usherwood, Craig R. White, Daniel W. E. Sankey and Alan M. Wilson. Biology Letters, August 5 2020. https://doi.org/10.1098/rsbl.2020.0468

Abstract: Dominance hierarchies confer benefits to group members by decreasing the incidences of physical conflict, but may result in certain lower ranked individuals consistently missing out on access to resources. Here, we report a linear dominance hierarchy remaining stable over time in a closed population of birds. We show that this stability can be disrupted, however, by the artificial mass loading of birds that typically comprise the bottom 50% of the hierarchy. Mass loading causes these low-ranked birds to immediately become more aggressive and rise-up the dominance hierarchy; however, this effect was only evident in males and was absent in females. Removal of the artificial mass causes the hierarchy to return to its previous structure. This interruption of a stable hierarchy implies a strong direct link between body mass and social behaviour and suggests that an individual's personality can be altered by the artificial manipulation of body mass.

4. Discussion

Over a 31-month period, the dominance hierarchy of the pigeon group did not significantly change, with individuals retaining their position within the hierarchy throughout the experimental period. Previously, it has been demonstrated that in animal groupings of less than approximately 10 individuals, stable hierarchies are more commonly observed than in larger groups [27]. The linear dominance hierarchy in the pigeons was significantly related to body mass. There is no clear pattern yet determined as to why body mass is such a strong determinant of dominance in some species but not others [11,28]. It is possible that body mass is significantly correlated with dominance in species where secondary-sexual ornamentations are less pronounced, and as a result, signalling is less clear. In such cases, body mass may become more of an important indicator of fitness. The hierarchy returning to its stable structure upon the removal of the additional mass load suggests that no carry-over or ‘memory' effects of mass loading persist and implies an instantaneous neurological feedback mechanism regarding changes in body mass (e.g. [29]).

It is possible that the addition of the extra mass to the backs of the subordinate birds aggravated or stressed the birds, causing them to exhibit higher levels of aggression. During the addition of the artificial mass, the subordinate birds did not show any obvious signs of aggravation at the lead weights attached to them, nor did they try to peck or remove them, either on themselves or on conspecifics (S.J.P. personal observation), suggesting this is an unlikely explanation for their increased aggression. Similarly, the composition of aggressive behaviours did not change between weighted and unweighted sessions, suggesting behaviours were not more focused on the back, where the weights were attached. An alternative explanation, however, is that the addition of artificial mass—although only for a short period—increased the energetic requirements of the weighted birds, thus requiring them to be more aggressive to ensure adequate access to food [30,31]. Such a theory is akin to ‘lead according to need', an idea which has previously linked to motivation and leadership in group behaviour [32].

Only males responded to the artificial mass loading by significantly increasing their aggressive behaviour, while females did not seemingly respond, suggesting that increasing aggression in response to artificial mass loading is sex specific. Previously, it has been demonstrated that injections of testosterone into male pigeons did not make male pigeons more aggressive or dominant, [33], yet a perceived possible increase in physiological condition through the addition of mass in the present study did elicit a response. This sex-specific response may be linked to competition for females, with female pigeons preferentially selecting males for partnering who hold dominant positions within a hierarchy [34]. An avenue worthy of further investigation is the impact that the pairing status of an individual has on their respective rank, as it has been previously demonstrated in birds that being paired increases your rank within a hierarchy [35,36].

The present study demonstrates the plasticity of aggressive traits, and the rapidity with which they can be modified based on physiological condition. Fruitful future investigations would be to ascertain the attributes that lead to greater body masses in wild-type scenarios and in turn greater dominance. The ‘prior attributes' hypothesis [8,27,28], for example, suggests hierarchies are predetermined by personality or physiological differences in dominance ability. This in turn may be linked to leadership during flocking and associated energy expenditure [37–40]. How natural seasonal variations in body mass [41–43] manifest in terms of dominance and general social behaviour would further explore the interactions between individual physiology, energetics and social behaviour. Moreover, experiments that supplementary feed specific individuals over a longer period of time to increase body mass may yield different results with respect to the changes in their respective ranks. Our study focused on only one flock of birds, and to determine the full nature of these instantaneous changes in body mass, further studies are needed with larger sample sizes, both in terms of number of flocks and sampling sessions where mass was added, and ideally additional species.