Let's talk about sex : sexual selection, sex allocation and parental care in animals

Abstract

For eukaryote species, sex is the path to reproductive success. But sex raises quite a few problems: Who am I going to mate with? How many sons and daughters should we produce? Who's going to look after the babies? Here I present a collection of self-contained studies addressing these and similar questions using a combination of field and laboratory experiments as well as theoretical modelling. Chapter 1 introduces sexual selection, sex allocation and parental care. Chapter 2 investigates temporal shifts in female and male mate preferences in the fiddler crab Uca mjoebergi. Using field mate choice experiments with robotic males, we demonstrated that female preferences for male size shift from large to small across the tidal cycle. This presumably occurs due to temporal constraints on female reproductive timing and incubation temperature. Males also adjusted their courtship levels in concert with this shift in female preferences. This is the first time that temporal shifts in both female and male mate preferences have been demonstrated. Chapter 3 examines female preferences for male signal timing in U. mjoebergi. We found that leading claw waves have an attractiveness advantage. This corroborates previous findings suggesting that wave synchrony occurs as an epiphenomenon of male competition. We also found that males waving in complete alternation with a synchronised group were as attractive as those producing leading waves. We discuss a simple sensory process that could lead to such preferences. Chapter 4 explores the effect of early life nutrition on male adult attractiveness in the mosquitofish Gambusia holbrooki. We found that females preferred to associate with males given a better start to life than males who had suffered a period of early life nutritional restriction, even when they were of similar size. This suggests that females use cues other than final body size to assess the growth history of potential mates. Chapter 5 provides evidence that G. holbrooki populations display an asymmetrical pattern of sex-specific generational overlap. We produced a model of sex allocation for these populations. We found a perfect rank-order correlation between observed sex allocation and theoretical predictions, with stronger biases observed as female generational overlap increased. This is the first evidence that sex allocation theory accounts for cases when mating opportunities vary predictably over time. Chapter 6 considers the effect of sex-specific mortality during the period of parental care on sex allocation via the production of formal models. We confirm that biased production of the sex with higher mortality during care is favoured. Crucially, we show that this is only true when juvenile mortality in the period of parental care frees up resources for their current/future siblings. Chapter 7 presents a model showing that, when males limit cuckoldry by guarding eggs post-mating (thereby improving egg survival), paternity protection can kick-start the evolution of male-only parental care. But only from an initial scenario of no parental care. This fits well with data from fishes, where male-only care is associated with external fertilization, whilst female-only care almost always evolves after internal fertilization arises. Show more