Abstract
Seasonally breeding mammals exhibit behavioural and physiological responses regulated by internal and external cues, mediating physiological trade-offs between survival and reproduction. Steroid hormones, such as androgens and glucocorticoids, play key roles in regulating reproductive behaviours, energy utilization, and responses to environmental constraints. Studying hormonal and behavioural responses to internal and external factors, provides insight into adaptive strategies in ungulates inhabiting highly seasonal, unpredictable ecosystems. Using the guanaco (Lama guanicoe), a territorial ungulate, we (1) analysed faecal cortisol (FCMs) and testosterone (FTMs) metabolites in males of different social units during the reproductive season; and (2) assessed the relationship between these hormones and aggression, often associated with territoriality and competition. During the 2014 and 2016 reproductive seasons, we recorded 227 focal observations of territorial (family group, solitary) and non-territorial (mixed and bachelor groups) males across three periods (group-formation, mating, and post-mating). We paired each observation with a fresh faecal sample for hormone analysis via group-specific enzyme immunoassays. FCMs and FTMs increased during the mating period, with significant inter-annual differences, likely related to variations in forage quality. Hormone metabolite levels were similar between social units. We found no relationship between aggression and FCMs, potentially due to the predominance of low-cost, low-aggression level displays. The relationship between FTMs and aggression supports the predictions of the challenge hypothesis for polygynous species, suggesting low androgen responsiveness to male-male interactions due to already elevated baseline testosterone. Our findings highlight how reproductive demands and environmental variability shape hormone secretion, emphasizing the interplay between social and ecological factors in regulating endocrine responses in wild mammals.
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Data availability
The datasets generated during and/or analysed during the current study are available in the open repository: Mendeley Data (https://data.mendeley.com/datasets/jmjhtgvn8g/2).
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Acknowledgements
We wish to dedicate this paper to the memory of Dr Matías Pandolfi, an outstanding researcher, who made a profound impact on the paths of the students he mentored throughout his career. We thank the park rangers and field assistants for providing logistical support during fieldwork. We deeply appreciate Verónica Arana, Maximiliano Estravis (National Institute of Agricultural Technology), and Edith Kobletz (University of Veterinary Medicine) for their essential technical support and help in the laboratory. The manuscript benefitted substantially from the critical and valuable comments of the anonymous reviewers; we appreciate them for taking the time to revise our paper. This study was part of the first author’s doctoral dissertation conducted at a national, public, and free university. We will continue to advocate for education and science policies that strengthen our community.
Funding
This study was partially funded by IDEA WILD, ANPCyT (PICT-1305/PICT-0304), CONICET (PIP- 11220100100386), and FONDECYT-CONICYT-PROGRAM (No 3140237).
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AP: Conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, writing – original draft, writing – review and editing; RO: funding acquisition, visualization, writing – review and editing; RP: laboratory analysis, writing - review & editing; PG: data curation, methodology, investigation, writing – review and editing; AM: data curation, investigation, writing – review and editing; FPL: visualization, writing - review & editing; VP: visualization, writing - review & editing; PT: conceptualization, funding acquisition, supervision, writing - review & editing; PC: conceptualization, investigation, funding acquisition, supervision, writing - review & editing. All authors read and approved the final manuscript.
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The Directorate for Renewable Natural Resources of Mendoza Province (Resolution n°: 893/2013 and 1231/2016) provided the necessary permission to work in La Payunia Reserve. The Secretary of Territorial Development and Environment of Neuquén Province provided all the necessary permits for the procedures at Los Peucos (Disp. 02/2020). All applicable international, national, and/or institutional guidelines for the use of animals were followed.
Significance statement
Seasonal changes in reproductive effort arise from trade-offs in energy allocation between reproductive physiology, social behaviour, and maintenance. This study shows that hormone metabolite levels in male guanacos — a territorial, polygynous ungulate — vary across key periods of the reproductive season in association with environmental variability and social context. Testosterone and cortisol levels rise with seasonal reproductive activity and energy demands but are not directly tied to aggressive behaviours. These findings deepen our understanding of how internal and external factors jointly shape hormone regulation and support broader predictions about hormonal dynamics in species with polygynous mating systems. Finally, by using non-invasive methods in a natural setting, this work provides valuable insights into the evolution of reproductive strategies in mammals facing seasonal and variable environments.
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Panebianco, A., Ovejero, R., Palme, R. et al. Hormonal and behavioural correlates of reproductive seasonality in a polygynous territorial ungulate. Behav Ecol Sociobiol 79, 102 (2025). https://doi.org/10.1007/s00265-025-03653-w
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DOI: https://doi.org/10.1007/s00265-025-03653-w