DOI: 10.5061/dryad.69p8cz997
title: Baseline glucocorticoids alone do not predict reproductive success across years but in interaction with enzymatic antioxidants

description: Glucocorticoids are known to adjust organismal functions, such as
 metabolism, in response to environmental conditions. Therefore, these
 hormones are thought to play a key role in regulating the metabolically
 demanding aspects of reproduction, especially in variable environments.
 However, support for the hypothesis that variation in glucocorticoid
 concentrations predicts reproductive success is decidedly mixed. Two
 explanations may account for this discrepancy: a) glucocorticoids might
 not act independently but could interact with other physiological traits,
 jointly influencing reproduction, and b) such an association could become
 apparent primarily in challenging environments when glucocorticoid
 concentrations increase. To address these two possibilities, we determined
 natural variation in circulating baseline glucocorticoid concentrations in
 parental great tits (Parus major) alongside two physiological systems
 known to be related to an individual's metabolism: oxidative status
 parameters (i.e., concentrations of pro-oxidants, dietary, and enzymatic
 antioxidants) and body condition. These systems interact with
 glucocorticoids and can also influence reproductive success. We measured
 these variables in two breeding seasons that differed in environmental
 conditions. When accounting for the interaction of baseline
 glucocorticoids with other physiological traits, we found a positive
 relationship between baseline glucocorticoids and the number of fledglings
 in adult great tits. The strength of this relationship was more pronounced
 for those individuals who also had high concentrations of the enzymatic
 antioxidant glutathione peroxidase. When studied independently,
 glucocorticoids were not related to fitness proxies, even in the year with
 more challenging environmental conditions. Together, our study lends to
 support the hypothesis that glucocorticoids do not influence fitness
 alone, but in association with other physiological systems
description: # Baseline glucocorticoids alone do not predict reproductive success
 across years but in interaction with enzymatic antioxidants
 [https://doi.org/10.5061/dryad.69p8cz997](https://doi.org/10.5061/dryad.69p8cz997) In this dataset, you can find three files: the R code ('Mentesana_Adults_data') for the statistical analysis, as well as two Excel files ('Mentesana_Adults_db' and 'Mentesana_Chicks_db') containing information on 'adult' and 'chick' great tit (*Parus major*) samples, respectively, during the breeding seasons of 2015-2016, from a free-living population located in southern Germany. The column names in both Excel files stand for: * ID (both files): unique identification for the cell. * Date (both files): the date when we captured the birds. * Year (both files): Year of sampling. * Nest (both files): nest where the bird bred. * Life_history (only in the database for adults): breeding season, when chicks were 9-10 days old after hatching. * Sex (only in the database for adults): 1 male, 0 female. * Time of capture (only in the database for adults): time when the bird was captured. * Band_number (both files): unique ID used in the field. * Time_c0 (only in the database for adults): the time it took us to take a blood sample to measure baseline glucocorticoid concentrations. * Time_end_sampling (only in the database for adults): the time when we finished processing the bird (i.e., once it was released back into the nest box). * Mass (both files): in grams. * Tarsus (both files): cm. * Wing (both files): cm. * Clutch_number (both files): 1st or 2nd clutch. For this study, we only worked with 1st clutches. * Clutch_size (both files): number of eggs laid in that nest. * Hatch_number (only in the database for adults): number of chicks that hatched. * Fledge_number (only in the database for adults): number of chicks that left the nest after ~20 days since hatching. * Mean_temperature_nestling_phase (°C; only in the database for adults): average temperature experienced by the chicks during the time they stayed in the nest. Temperature data was recorded every hour using i-buttons. * Mean_rain_nestling_phase (mm; only in the database for adults): average rain experienced by the chicks during the time they stayed in the nest. Rainfall data was recorded every 5 minutes by a meteorological station located less than 5 km from our field site. * c0 (only in the database for adults): baseline glucocorticoid concentrations (ng/ml). * OXY (only in the database for adults): non-enzymatic antioxidants (expressed as mM HOCl neutralized) * GPX (only in the database for adults): enzymatic antioxidant glutathione peroxidase (expressed as U/ml) * ROMs (only in the database for adults): reactive oxygen species (expressed as mM H202 equivalents)