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Release from ecological constraint erases sex difference in social ornamentation

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Abstract

Sex differences in animal ornamentation are thought largely genetically fixed due to stronger sexual selection on males of species with conventional sex roles. But, other types of sex differences are not genetically fixed. For example, several differences in human social behavior result instead from sociocultural or economic constraints on women. Since gregarious animals use ornamentation for various social functions, perhaps some of their sex differences are, similarly to human behavior, due to social coercion or ecological constraint (their closest equivalents to human social and economic constraints, respectively). We found sex differences in ornamentation that disappear plastically in a social species with conventional sex roles. The red bill of common waxbills (Estrilda astrild) is on average more saturated in males, but in our experiment female bill color correlated with night temperature, an important energetic stressor, suggesting that sexual dichromatism disappears when ecological conditions are favorable to females. Female ornamentation may be more adversely affected by ecology because of their life history that requires balancing investment in ornamentation with maintaining reproductive condition. Manipulation of stress-related physiology (ACTH challenge) suggests that this effect was not mediated by stress mechanisms. Social coercion appears to not explain sexual dichromatism: males were not more aggressive than females, aggressiveness was not related to bill color, and manipulation of reproductive axis’ physiology (GnRH challenge, which in many species mediates aggressiveness) did not increase bill color. Our results show parallels to the plastic sex differences of humans in social animals and suggest that studying their ecological vs. social causes provides a biological backdrop for understanding the human case as well.

Significance statement

Many sex differences in human social behavior result from economic or sociocultural constraints on women, while sex differences in the ornamentation of animals with conventional sex roles are thought largely genetically fixed. We show that a sex difference in ornamentation—the redder bills of male than female common waxbills—disappears plastically in an animal with conventional sex roles due to, in part, changes in female ornamentation. Social coercion did not explain reduced female ornamentation: aggressiveness did not predict bill color, and males were not dominant over females. Instead, female bill color was reduced during colder weather, perhaps because females under energetic stress need to balance investment in social ornamentation vs. maintaining reproductive condition. Similarly to humans, some sex differences of gregarious animals may be due to females requiring appropriate conditions to express their full social potential.

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Acknowledgements

We would like to thank the anonymous reviewers for valuable comments that much improved this article.

Funding

This work was funded by grant PTDC/BIA-EVF/116758/2010, and fellowships SFRH/BPD/46873/2008 and SFRH/BPD/110165/2015 from the Fundação para a Ciência e a Tecnologia.

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Authors and Affiliations

  1. CIBIO—Research Centre in Biodiversity and Genetic Resources, University of Porto, Campus Agrário de Vairão, 4485–661, Vairão, Portugal

    Caterina Funghi, Sandra Trigo, Ana Cristina R. Gomes, Marta C. Soares & Gonçalo C. Cardoso

  2. Department of Biological Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia

    Caterina Funghi

  3. Zoological Institute, Universität Hamburg, 20146, Hamburg, Germany

    Caterina Funghi

  4. Behavioural Ecology Group, Department of Biology, University of Copenhagen, 2100, Copenhagen Ø, Denmark

    Gonçalo C. Cardoso

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  1. Caterina Funghi
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  2. Sandra Trigo
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  3. Ana Cristina R. Gomes
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  4. Marta C. Soares
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  5. Gonçalo C. Cardoso
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Correspondence to Caterina Funghi or Sandra Trigo.

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Communicated by K. McGraw

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Funghi, C., Trigo, S., Gomes, A.C.R. et al. Release from ecological constraint erases sex difference in social ornamentation. Behav Ecol Sociobiol 72, 67 (2018). https://doi.org/10.1007/s00265-018-2486-6

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