Sexual selection in a lekking bird: the relative opportunity for selection by female choice and male competition

Abstract

Leks are classic models for studies of sexual selection due to extreme variance in male reproductive success, but the relative influence of intrasexual competition and female mate choice in creating this skew is debatable. In the lekking lance-tailed manakin (Chiroxiphia lanceolata), these selective episodes are temporally separated into intrasexual competition for alpha status and female mate choice among alpha males that rarely interact. Variance in reproductive success between status classes of adult males (alpha versus non-alpha) can therefore be attributed to male-male competition whereas that within status largely reflects female mate choice. This provides an excellent opportunity for quantifying the relative contribution of each of these mechanisms of sexual selection to the overall opportunity for sexual selection on males (I males ). To calculate variance in actual reproductive success, we assigned genetic paternity to 92.3% of 447 chicks sampled in seven years. Reproduction by non-alphas was rare and apparently reflected status misclassifications or opportunistic copulations en route to attaining alpha status rather than alternative mating strategies. On average 31% (range 7-44%, nZ6 years) of the total I males was due to variance in reproductive success between alphas and non-alphas. Similarly, in a cohort of same-aged males followed for six years, 44 -58% of the total I males was attributed to variance between males of different status. Thus, both intrasexual competition for status and female mate choice among lekking alpha males contribute substantially to the potential for sexual selection in this species.

Variation in annual and lifetime reproductive success of lance-tailed manakins: alpha experience mitigates effects of senescence on siring success

“…(c) Social status Male status was determined independent of copulation success, and was usually established months or years before females arrived at a display area [17]. In short, alpha males were the most consistently present males at each display area, participated in the vast majority of duet songs and performed alpha-specific components of paired courtship behaviours, including the distinctive 'eek' which ends cooperative dancing bouts.…”

“…Lance-tailed manakins (Aves, Pipridae: Chiropxiphia lanceolata) therefore offer an excellent model for investigating the influences of age and experience on annual siring success. Males of this lekking species generally do not reproduce until attaining alpha status [17], and vary considerably in both the age at which they first become alpha and the length of time they maintain alpha status. Age-specific plumage stages in the first three years of life allow precise assessment of male ages [18].…”

The mismeasurement of sexual selection

“…In practice though, both empiricists and theoreticians show a consistent tendency to interpret opportunities as if they reflect actual sexual selection (Shuster & Wade, 2003;Bjork & Pitnick, 2006;Sword & Simpson, 2008;Duval & Kempenaers, 2008;Vanpé et al, 2008;Box 2). Merely acknowledging the limitations of a metric does not justify its continued usage.…”

“…Furthermore, there was no significant relationship between I s or I male and sperm investment across several species. Duval & Kempenaers (2008) claim that ''The potential for evolutionary change can more generally be quantified via the opportunity for selection, I, which is the variance in relative fitness… In polygynous mating systems, the opportunity for sexual selection is usually calculated as I male … This measure quantifies the potential for selection to operate, separate from the evolutionary response to selective pressure and without requiring precise knowledge of the mechanism by which selection occurs…' ' Vanpé et al (2008) state that ''Measuring the opportunity for sexual selection is crucial for addressing many questions in behavioural ecology (such as the evolution of sexual size dimorphism, conspicuous male traits, alternative mating tactics, and sexbiased parental investment, Andersson, 1994) and population dynamics…'' Box 1: The measurement of sexual selection (2004)(2005)(2006)(2007)(2008) Sexual selection has been quantified using numerous measures (Arnold and Wade, 1984a,b;Arnold & Duvall, 1994;Andersson, 1994;Ruzzante et al, 1996;Kokko et al, 1999;Mills et al, 2007;Jones, 2009). We present and define six of the most frequently used measures in Table 1.…”

“…In practice, I s and the actual strength of sexual selection are regularly assumed to be strongly positively correlated and used interchangeably (Shuster & Wade, 2003;Bjork & Pitnick, 2006;Duval & Kempenaers, 2008;Vanpé et al, 2008;Sword & Simpson, 2008;Box 2).…”