Ruff Persistence : the Role of Pre-sexual Selection and Male-male Partner Choice in Maintaining Intraspecific Diversity

Abstract

The Ruff, Calidris pugnax, a Eurasian wader, is an ideal study system for researching intraspecific diversity. Ruffs have drastically different sex roles during reproduction. Most males spend the breeding season intensively competing over matings with females, providing no parental care for chicks. Whilst most females choose mates from display grounds, incubate nests, and rear chicks. Ruffs also have extreme size and appearance variation across demographic classes. Males can be up to 70% heavier than females and most males develop elaborate nuptial plumage in comparison to females. Further, males have three phenotypically distinct mating morphs: first, Independents, aggressive males that jealously guard display courts on leks. Second, Satellites, non-aggressive males that cooperatively display with Independents to gain fertilisations. Third, Faeders, males that sneak copulations on leks through female mimicry. Three supergene haplotype combinations underlie the different morphs. Independents carry two copies of the ancestral haplotype whereas Satellites and Faeders carry one ancestral haplotype and their own morph-specific inversion haplotypes. A break point within the gene CENPN on both inversion haplotypes leads to homozygote lethality. Adult Satellites and Faeders are also rare in the Ruff population, possibly because of this genetic handicap. However, the Satellite and Faeder haplotypes are thousands to millions of years old indicating that the inversion morphs must have a way of balancing their fitness with Independents. I examined two processes that may help the maintenance of Ruff morph diversity. First, I examined the hypothesis that sex- and morph-specific constraints on development during ontogeny of chicks, leads to sex- and morph-specific differences in pre-sexual selection, which in turn may favour females and the Faeder male. Second, I examined the hypothesis that Satellite males are able to use co-display partner choice to optimize their mating success. I split my thesis into two parts, one for each of my two hypotheses. Part I, including Chapter 1, covers the pre-sexual selection in the Ruff sexes and morphs. Specifically, Chapter 1 examines how delays in Independent male development and fledging ages leads to demographic changes in Ruff cohorts from hatching to post-fledging. My co-authors and I used multiple simulations of a single cohort matrix model informed with wild, aviary, and literature data to project both changes in the sex ratio, and pre-sexual selection. Further, we used two forms of sensitivity analysis to examine the impacts of demographic complexity in our model and the demographic vital rate contributions to the post-fledge sex and morph ratios. Our model predicted a 20% decline in male frequency from hatching to fledging which is consistent with independent empirical observations of southward migrating juveniles. The sensitivity analyses of demographic complexity revealed that sex differences in vital rates explained more variation in the sex and morph ratios than vital rate differences across morphs and within the sexes. The most important demographic contributions to the sex and morph ratios were sex-specific intrinsic mortality and fledging ages between Independent males and females. The increase in Satellite male and stability in Faeder male frequencies indicates that variation in viability selection before maturation may help to maintain morph diversity in the Ruff. Part II, of my thesis, includes Chapters 2 and 3, which are dedicated to male-male co-display between territorial Independents, aka Residents, and Satellites. Chapter 2 utilizes a community game model informed with literature data to predict Resident and Satellite partner choice for co-display. Because it is unclear how co-display alters female choice, we included four scenarios for this parameter in our model of Resident/Satellite co-display. We present the ‘Skew’ scenario, in the main text, because under that scenario Satellites obtained the percent of lek copulations that were closest to empirical estimates from the literature and could do so on all modelled lek sizes. Here, Residents, in general, would co-display with Satellites instead of opting for single-display. Further, Satellite males should choose Resident co-display partners of mid- to low-rank on medium lek sizes in order to maximize their mating success and achieve copulation rates equal to those in natural populations. In Chapter 3, I used data from a captive Ruff population to examine the formation probability, duration of co-displays, and variation in Satellite co-display rewards. Further, I tested the key prediction, from Chapter 2, that Satellite copulation success increases with descending Resident mating rank, and a key assumption, of the game model, that a Satellite’s ability to monopolize copulations within a co-display increases with descending mating rank of their Resident partner. Co-display formation probability and duration, and Satellite rewards increased with descending mating rank. These results did not match the predictions under the ‘Skew’ scenario in Chapter 2, but did match the prediction under another scenario called the ‘Uniform Proportion’ scenario. Further one of the assumptions of the model in Chapter 2, i.e. the max proportion of copulations a Satellite could obtain within a co-display did not match the results from Chapter 3. This suggests that further experiments and observations of natural Ruff populations are needed to fully understand Resident/Satellite co-display dynamics.