The effect of social group size on feather corticosterone in the co-operatively breeding Smooth-billed Ani (Crotophaga ani): An assay validation and analysis of extreme social living

Abstract

Living closely with others can provide a myriad of fitness benefits, from shared territory defense to co-operative resource acquisition. Costs of social aggregation are not absent, however, and likely influence optimal and observed groups’ sizes in a social species. Here, we explored optimal group size in a joint-nesting cuckoo species (the Smooth-billed Ani, Crotophaga ani) using endocrine markers of stress physiology (corticosterone, or CORT). Smooth-billed Anis exhibit intense reproductive competition that is exacerbated in atypically large groups. We therefore hypothesized that intra-group competition (measured by social group size) mediates the desirability and physiological cost of social group membership in this species. To test this hypothesis, we captured 47 adult Smooth-billed Anis (31 males, 16 females) during the breeding seasons of 2012-2014 in south-western Puerto Rico, and documented social group sizes. Tail feathers were sampled and used to quantify CORT (pg/mg) in enzyme-linked immunosorbent assays (ELISAs) (n = 50). Our analyses show significant differences in feather-CORT of adults between categorical group sizes, with individuals from atypically large social groups (≥ x + 1SD) having highest mean concentrations (33.319 pg/mg), and individuals from atypically small social groups (≤ x − 1SD) having lowest mean concentrations (8.969 pg/mg). Whether reproductive competition or effort is responsible for elevated CORT in atypically large social groups, however, remains unclear. Our results suggest that living in atypically large groups is physiologically expensive and may represent an evolutionarily unstable strategy. To our knowledge, this is the first study to explore a correlation between stress physiology and group size in a joint-nesting species.

Corticosterone in feathers of laying hens: an assay validation for evidence-based assessment of animal welfare

“…Additional “unresolved technical issues” ( Romero and Fairhurst, 2016 ), such as influences on the deposition ( Romero and Fairhurst, 2016 ) and the so-called small sample artifact ( Lattin et al., 2011 ; Berk et al., 2016 ), which describes the appearance of higher feather corticosterone concentrations in small sample masses, compared with larger ones, are discussed. Therefore, besides studies on effects of stressors influencing feather corticosterone levels, different validation studies have already been performed in different species ( Lattin et al., 2011 ; Carbajal et al., 2014 ; Berk et al., 2016 ; Harris et al., 2016 ; Robertson et al., 2017 ; Freeman and Newman, 2018 ). Furthermore, an official abbreviation for corticosterone should be established ( Raff, 2016 ), such as ACTH for adrenocorticotropic hormone.…”

“…Unlike Bortolotti et al. (2008) who recommend the unit pg/mm for feather corticosterone concentrations, samples used in the present study were standardized on mass, using pg/mg ( Monclús et al., 2017 ; Robertson et al., 2017 ; Freeman and Newman, 2018 ). This seems to be reasonable for the authors as the aim was to compare the same sample under different treatments and to determine their repeatability, rather than investigate feather corticosterone concentrations within a single feather under consideration of its growth rate.…”

“…Generally, body feathers from the interscapular area ( Carbajal et al., 2014 ; Monclús et al., 2017 ), hereinafter referred to as interscapular feathers ( Monclús et al., 2017 ), and rectrices ( Aharon-Rotman et al., 2017 ; Robertson et al., 2017 ; Freeman and Newman, 2018 ), hereinafter referred to as tail feathers ( Aharon-Rotman et al., 2017 ; Robertson et al., 2017 ), were pulled from 11 adult laying hens. Feathers were collected from commercial Lohmann Brown laying hens (Lohmann Tierzucht GmbH, Cuxhaven, Germany), a commonly used genotype in Germany, as soon as discovering the bird's death.…”

“…(2017) . Every feather was thoroughly cleaned (based on Jenni-Eiermann et al., 2014 ; von Eugen et al., 2019 ) with distilled water and degreased by bathing it in HPLC-grade methanol (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) for 2 to 4 s (based on Robertson et al., 2017 ). Based on the studies by Lattin et al.…”

“…In humans, a half-life of circulating corticosterone of about 1 h is reported ( Doggui, 2012 ); however, no values are found for birds. The quantification of hormone levels influenced by the hypothalamic–pituitary–adrenal axis has been applied over y as an indicator for stress and animal welfare in farm animals ( Beuving and Vonder, 1978 ; Dehnhard et al., 2003 ; Rettenbacher et al., 2004 ; Odihambo Mumma et al., 2006 ; Mormède et al., 2007 ; Palme, 2019 ) and others ( Bortolotti et al., 2008 ; Sheriff et al., 2011 ; Fairhurst et al., 2013 ; Schmaltz et al., 2016 ; Robertson et al., 2017 ; Peric et al., 2018 ; Palme, 2019 ). In addition to blood, several matrices, such as feces ( Rettenbacher et al., 2004 ; Möstl et al., 2005 ; Touma and Palme, 2005 ; Weimer et al., 2018 ; Palme, 2019 ) and eggs ( Rettenbacher et al., 2005 ; Schmaltz et al., 2016 ), are used to detect and quantify corticosterone or its metabolites in birds, whereas in other species, the use of saliva ( Mormède et al., 2007 ), urine ( Hay and Mormède, 1997 ), milk ( Tucker and Schwalm, 1977 ), and hair ( Arnon et al., 2016 ; Peric et al., 2018 ) is reported.…”