Welfare biology research: Sexual competition

27 Feb 2019

Primary goals of welfare biology are to improve our understanding of the quality of life of animals in relation to their environments, and to find the most promising ways of helping them. There’s a wide range of topics that welfare biology can address. In studying the wellbeing of wild animals, research in evolutionary biology in general, and in the life history of animals in particular, may be very useful. This field aims to explain patterns of growth, reproduction, and death in animals, in terms of natural selection and in relation to their environments. Such knowledge can be very useful in assessing what the lives of individuals in different populations might be like.

Below is a project draft about sexual competition that illustrates the kind of life history research that could be developed in welfare biology. This research project idea is designed with a broad scope, which is why lists of possible species and welfare proxies worth considering are included.

We hope to inspire researchers interested in sexual selection and competition and other life-history related topics to design their own research projects adapted to their own circumstances. The results could improve our understanding of wild animal suffering.

Project idea: welfare costs of sexual competition

Sexual selection is considered a form of natural selection that favors adaptations for acquiring mates rather than adaptations for survival.1 Although certain traits can enhance an individual’s fitness in terms of both survival and reproduction (e.g., foraging in efficient ways enhances their chances of survival and provides nutrients for the growth of sexual ornaments such as colored feathers), sexually selected traits can also hinder an individual’s survival.

In many animal species, traits that enable individuals to increase their ability to acquire mates are selected in two general ways: mate competition (intrasexual selection) and mate choice (intersexual selection). The first occurs when individuals engage in combat with rivals of the same sex, and favors characteristics of offense (weapons) such as horns, antlers, claws, and aggressive behavior. The second occurs when individuals of the same sex engage in courtship displays to attract individuals of the opposite sex, and favors the development of sexual ornaments such as colors, odors, elaborate dances, and vocalizations.2 Many of these traits have been associated with lowering the chances for survival of individuals of one sex.3 The large antlers of male moose, for example, are bulky and heavy, reducing their capacity for flight from predators, and antlers can become entangled in low-hanging tree branches and shrubs, increasing risk of death in male individuals. Bright colorations and elaborate vocalizations, such as those seen in many male birds and frogs, attract not only females, but also predators.

This suggests that sexual competition may cause individual suffering, either directly, as injuries incurred from fighting over a mate, or indirectly, as when animals engage in courtship behaviors that make them more likely to be predated. However, no studies have yet determined whether there is such a cost or have attempted to quantify it. Doing so is important, for in species with very strong sexual selection, sexual competition may be one of the main causes of mortality and suffering.

General purpose

Determine the direct and indirect effects of sexual competition on the welfare of individuals belonging to species where sexual selection is a dominant selective force.

Direct and indirect effects may be assessed separately in different research projects.


  • Identify species of mammals and birds where the main selective force is sexual selection (focus on these taxa for ease of identifying welfare costs)
  • Identify the ways mate competition and choice affect the welfare of individuals of both sexes
  • Analyze the welfare effects of sexual competition using quantitative and qualitative methods

Species to consider

For direct effects of sexual competition in the wellbeing of animals

  • Elephant seals (Mirounga angustirostris): high size sexual dimorphism has been associated with high male mortality rates4
  • Fallow deer (Dama dama), lechwe (Kobus leche), Roosevelt elk (Cervus canadensis roosevelti), white-tailed deer (Odoicoleus virginianus): males with large antlers fight violently during the rut5

For indirect effects of sexual competition in the wellbeing of animals

  • Guianan cock-of-the-rock (Rupicola rupicola), greater sage-grouse (Centrocercus urophasianus): predation risk associated with leks6
  • Eurasian siskin (Spinus spinus), and other passerine birds where predation risk is associated with plumage brightness7

Welfare proxies to consider

  • Injuries caused by sexual competition
  • All forms of gender-biased hunger (where one sex has greater nutritional requirements than the other)
  • Predation rate (where one sex is more visible than the other)
  • Lifespan
  • Mortality

Expected impacts

  • Greater understanding of the ways sexual competition causes suffering
  • Greater understanding of the extent of suffering imposed by sexual competition
  • Identification of species where sexual selection poses significant welfare costs
  • Greater understanding of the potential for interventions in nature to reduce suffering caused by sexual selection due to above impacts

Further readings

Bonduriansky, R.; Maklakov, A.; Zajitschek, F. & Brooks, R. (2008) “Sexual selection, sexual conflict and the evolution of ageing and life span”, Functional Ecology, 22, pp. 443-453 [accessed on 27 February 2019].

Christe, P.; Keller, L. & Roulin, A. (2006) “The predation cost of being a male: Implications for sex-specific rates of ageing”, Oikos, 114, pp. 381-384.

Liker, A. & Székely, T. (2005) “Mortality costs of sexual selection and parental care in natural populations of birds”, Evolution, 59, pp. 890-897.

Nunn, C. L.; Lindenfors, P.; Pursall, E. R. & Rolff, J. (2009) “On sexual dimorphism in immune function”, Philosophical Transactions of the Royal Society B: Biological Sciences, 364, pp. 61-69 [accessed on 25 February 2019].

Promislow, D.; Montgomerie, R. & Martin, T. E. (1994) “Sexual selection and survival in North American waterfowl”, Evolution, 48, pp. 2045-2050.

Rankin, D. J. & Kokko, H. (2006) “Sex, death and tragedy”, Trends in Ecology & Evolution, 21, pp. 225-226.

Toïgo, C. & Gaillard, J. M. (2003) “Causes of sex‐biased adult survival in ungulates: Sexual size dimorphism, mating tactic or environment harshness?”, Oikos, 101, pp. 376-384.


1 Rice, S. A. (2007) Encyclopedia of evolution, New York: Facts on File.

2 Darwin, C. (1874 [1871]) The descent of man, and selection in relation to sex, 2nd ed., London: John Murray [accessed on 17 February 2019].

3 Owen-Smith, N. (1993) “Comparative mortality rates of male and female kudus: The costs of sexual size dimorphism”, Journal of Animal Ecology, 62, pp. 428-440.

Promislow, D. E. (1992) “Costs of sexual selection in natural populations of mammals”, Philosophical Transactions of the Royal Society B: Biological Sciences, 247, pp. 203-210 [accessed on 27 February 2019].

Promislow, D. E.; Montgomerie, R. & Martin, T. E. (1992) “Mortality costs of sexual dimorphism in birds”, Philosophical Transactions of the Royal Society B: Biological Sciences, 250, pp. 143-150 [accessed on 28 February 2019].

4 Clinton, W. & Le Boeuf, B. (1993) “Sexual selection’s effects on male life history and the pattern of male mortality”, Ecology, 74, pp. 1884-1892.

5 Clutton-Brock, T. H. & Parker, G. A. (1995) “Sexual coercion in animal societies”, Animal Behaviour, 49, pp. 1345-1365.

Ditchkoff, S. S.; Welch, E. R., Jr.; Lochmiller, R. L.; Masters, R. E. & Starry, W. R. (2001) “Age-specific causes of mortality among male white-tailed deer support mate-competition theory”, The Journal of Wildlife Management, 65, pp. 552-559.

Leslie, D. M., Jr. & Jenkins, K. J. (1985) “Rutting mortality among male roosevelt elk”, Journal of Mammalogy, 66, pp. 163-164.

6 Trail, P. (1987) “Predation and antipredator behavior at Guianan Cock-of-the-Rock leks”, The Auk, 104, pp. 496-507.

Boyko, A. R.; Gibson, R. M.; Lucas, J. R. (2004) “How predation risk affects the temporal dynamics of avian leks: Greater sage grouse versus golden eagles”, The American Naturalist, 163, pp. 154-165.

7 Huhta, E.; Rytkönen, S. & Solonen, T. (2003) “Plumage brightness of prey increases predation risk: An among-species comparison”, Ecology, 84, pp. 1793-1799.

Pascual, J.; Senar, J. C. & Domènech, J. (2014) “Plumage brightness, vigilance, escape potential, and predation risk in male and female Eurasian Siskins (Spinus spinus)”, The Auk, 131, pp. 61-72.