Same-sex pair-bonds are equivalent to male–female bonds in a life-long socially monogamous songbird

Abstract

Same-sex sexual behaviors are well documented in both captive and wild animals. In monogamous species, these behaviors are often exclusive, each individual having only one same-sex partner. A bias in sex ratio has been proposed as a social context yielding same-sex pair-bonding, but this hypothesis has rarely been tested. Focusing on a life-long pair-bonding songbird, the zebra finch Taeniopygia guttata, we tested whether same-sex pairing results from a shortage of individuals of the opposite sex. By experimentally skewing the sex ratio towards members of one sex, we observed a greater proportion of same-sex pair-bonds of that sex. Moreover, we assessed whether the quality and stability of social interactions were equivalent in same-sex and male–female pairs. Male–male and female–female same-sex bonds display the same behavioral characteristics as male–female ones: they are intense, highly selective, and stable affinitive relationships involving the same behavioral displays already described in wild birds. Moreover, same-sex male bonds were sufficiently strong not to split up when individuals were given the opportunity to reproduce with females. Because the pair-bond in socially monogamous species represents a partnership that may give advantages for survival (e.g., resources defense, fighting against predators, etc.), we propose that same-sex pairing in the zebra finch may result from the pressure to find a social partner.

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Abbreviations

AP:

Allopreening bouts

CB:

Events of clumping and greeting beak fence

CC:

Courtship and copulation

DS:

Directed songs

I RS :

Index of relationship strength

I RSd :

Daily value of I RS

I RStot :

Value of I RS computed over the whole experiment

NS:

Nest sharing

OV:

Outcome variable

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Acknowledgment

The authors warmly thank Colette Bouchut and Nicolas Boyer for all the technical support in the ENES laboratory. We are also grateful to Hédi A. Soula for help with analysis of the data with R. Finally, thanks to Mylène M Mariette, who kindly accepted to read and correct this manuscript.

This study is funded by the French Agence Nationale de la Recherche (A.N.R., project “BIRDS’VOICE”) and Saint-Etienne Métropole. C.V. is supported by a Young Investigator Sabbatical of the Université de Saint-Etienne. N.M. is supported by the Institut universitaire de France and was funded during part of this study by the Miller Institute for Basic Research in Science, University of California, Berkeley (Visiting Miller Professorship). J.E.E. is supported by the French Ministry of Research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Correspondence to Julie E. Elie.

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Communicated by I. Hartley

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Elie, J.E., Mathevon, N. & Vignal, C. Same-sex pair-bonds are equivalent to male–female bonds in a life-long socially monogamous songbird. Behav Ecol Sociobiol 65, 2197–2208 (2011). https://doi.org/10.1007/s00265-011-1228-9

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Keywords

  • Animal homosexuality
  • Zebra finch
  • Same-sex sexual behavior
  • Sex ratio
  • Partnership
  • Pair-bond
  • Social network