Sexual selection is responsible for some of the most extravagant traits found in the animal kingdom, as they confer a fitness advantage in terms of access to mates. Large body size in males of many polygynous species should provide a mating advantage through its association with dominance rank and, ultimately, access to females. Recent evidence from multiple species, however, questions this generalization, suggesting instead a weak relationship between male size and reproductive success. That empirical evidence suggests a need to develop new theory to explain patterns of male reproductive success. This thesis meets this challenge by examining if and how sexual selection acts on a polygynous sexually dimorphic species. I used a combination of behavioral, morphological, spatial, and reproductive data on marked individuals from a long-term longitudinal study of eastern grey kangaroos (Macropus giganteus) to explore which ecological and demographic factors influence the association between body size and dominance, siring chances and reproductive success, and relate the effect of paternal body size to offspring sex and maternal allocation.
First, I needed to understand if body size is positively associated with dominance status, as individual rank is assumed to be a decisive factor granting priority access to receptive females. By analyzing the outcome of about 2,300 male-male agonistic interactions across six years (2010-2011 and 2015-2018), chapter 2 shows that kangaroo males formed yearly linear, steep, and stable dominance hierarchies based on body size. Dominance status was, however, only moderately correlated with yearly reproductive success. This finding strongly suggests that body size is not the only factor influencing reproductive success on this species, possibly indicating weak sexual selection on body size.
To determine what factors other than body size may influence siring success, I examined if males had an upper reproductive threshold set by their mating opportunity. If a male does not encounter a female, he cannot father her offspring. Most studies of sexual selection, however, assume that all males in a population have equal access to all females. Chapter 3 thus uses spatial data collected over 9 years (2010-2018) to show that ecological variables such as mating opportunity, quantified by the spatial overlap between each male-female pair, residency on the breeding site, and an accurate estimation of the competitive environment influence individual siring chances.
Next, I quantified sexual selection on body size, its fluctuation across years and according to mating opportunity and residency, and the strength of reproductive inequality. Chapter 4 shows that sexual selection was overall stabilizing and there was no evidence of temporal fluctuation or fluctuation caused by mating opportunity, and only limited variation caused by differences in residency. Despite weak reproductive inequality, sexual selection acted strongly on body size.
Finally, I redirected my attention to recent findings showing that fathers can influence offspring sex and maternal allocation. Chapter 5 thus examined if paternal body size, which is strongly sexually selected, affected offspring sex ratio or maternal differential allocation. The results indicated that maternal and paternal influences modulated each other, as light mothers conceived sons when the father was heavy, conversely, heavy mothers conceived sons when the father was light. Maternal sex-specific allocation was independent of paternal size.
I found that despite a stable linear social hierarchy, the most dominant males did not monopolize paternities, possibly because they did not have the opportunity to do so. It is important to emphasize that strong sexual selection does not necessarily lead to contemporary high variance in reproduction, as the phenotypes selected will determine the strength of selection. By reporting a rare occurrence of non-linear sexual selection on body size experienced by a sexually dimorphic species, this thesis underlines the necessity of simultaneous study of pre- and post-copulatory sexual selection. Moreover, it provides a solid contribution to our understanding of sexual selection by highlighting the importance of underrated ecological and demographic factors such true mating opportunity, generated by spatial overlap, and effective number of competitors faced by each male.