Intrasexual competition in females of a socially polygynous lizard, Psammophilus dorsalis

Abstract

Competition between individuals of the same sex, intrasexual competition, has received much attention in males, mainly because such competition, which is primarily for mates (i.e. sexual selection), often selects for exaggerated, conspicuous ornaments and displays. In contrast, social interactions among females, particularly in common mating systems such as socially polygynous systems, have received limited attention. This is because sexual selection theory predicts lower variance in mating success in females than in males, and empirical work shows that females do not commonly display striking ornaments and conspicuous traits. However, recent reviews suggest that intrasexual competition in females can be widespread. Furthermore, while such competition is primarily for mates in males, females may compete not only for mates, but more commonly for resources related to enhancing offspring production and survival, including food, nesting sites, water, safety, etc. Reviews also indicate that intrasexual competition in females can have ecological and evolutionary consequences; and highlight the importance of studying intrasexual competition in females. In my thesis, I have focused on studying social interactions and intrasexual competition in females of a socially polygynous mating system. My study system, the Peninsular Rock Agama (Psammophilus dorsalis), is a sexually dimorphic lizard, in which, like most other polygynous systems, the brightly coloured and bigger males have received much attention, while our knowledge about the relatively duller and smaller females is limited. The nature of inter- and intra-sexual interactions can be broadly predicted by studying the space use of individuals and can be the first step towards understanding social interactions of a system. In my first chapter, I infer the interactions between and within sexes by studying their space use patterns, including variation in home range size, shifts in home ranges and intrasexual and intersexual overlaps in home range. I show that both males and females defend territories. Male territories are small in the non-mating season. However, in the mating season, male 13 territories expand and overlap multiple female and male territories. In contrast, female territories are small, exclude other females, do not change size throughout the year and are overlapped, on an average, by a single male’s territory. This suggests that P. dorsalis is a socially polygynous lizard and substantial intrasexual competition exists between females, which defend exclusive home ranges from other females throughout their lifetimes. Signalling is a key feature of intrasexual competition. Signals used in intersexual interactions are expected to be directed towards mate acquisition while those in intrasexual interactions in solitary territorial species like P. dorsalis are expected to influence mate and/or territory acquisition. Such effects on mate and territory acquisition are further likely to have an effect on the individual’s fitness. The extent to which females use signals and the functions of these signals in intrasexual competition are still not well understood. In my second chapter, I have studied patterns in signalling over the lifetime of females and the adaptiveness of these signals. That is, whether signals function to acquire resources in a sexual or non-sexual context and within the sexual context, how signals function in direct competition versus in indirect competition (i.e. mate attraction). Additionally, I have studied the strength of selection acting on these signals by studying correlation of signal use with proxies of fitness. My study shows that females use a complex signalling repertoire in which a set of well-defined signals are used exclusively in the mating context while others are likely used in competition in both the mating and non-mating contexts. In the mating season, different signals were used in direct competition and mate choice (indirect competition). I thus found evidence for multiple signals to be maintained in a population because distinct signals are directed towards different receivers. In spite of a variety of signals being used in intrasexual competition, I did not find any obvious correlation between signal uses and fitness proxies. However, it is important to note that I studied the relationship between signalling and measures of fitness like clutch size, tenure and perch quality. Maternity analysis using genetic methods might be important to examine the correlation between signalling levels and fitness. 14 Intrasexual interactions in the form of direct competition are widely studied in males because of the obvious physical aggression and conspicuous traits that appear to evolve under such selection, but have received minimal attention in females of a polygynous system. We suggest that though aggressive behaviours in females are not very commonly seen in the wild, competition in females should be strong. However, owing to the biological differences between the two sexes and higher costs of competition in females, the nature of intrasexual competition in females should be different from that in males. In my third chapter, I present hypotheses for strategies in direct intrasexual signalling and aggression in females and for sex differences in these strategies. I then test these hypotheses through observations and experiments. Results indicate that, as predicted, females use less conspicuous signals than males in broadcast signalling; however, they remarkably increase the intensity of signalling as the threat from a competitor increases. Though aggressive competitive events in the wild are generally uncommon, females strikingly increase aggression when the threat from a competitor is high. This suggests that perhaps because competition in females is more costly than in males, unlike males, they use low-key, inconspicuous signals in the normal signalling context; however, they are extremely sensitive to the level of threat, and ramp up their signalling and aggressive response at increased level of competitor threat, even more so than do males. Defending mutually exclusive territories can thus be a strategy used by females to avoid recurrent contests. My thesis highlights that intrasexual competition in females of polygynous systems can be substantially strong, however, cryptic. Females typically signal to compete directly as well as indirectly using well defined displays directed towards same and other sex individuals respectively. However, owing to the high costs of competition, females are extremely sensitive to the level of threat and strikingly increase signalling and aggression when level of threat is high, even more so than males. I suggest that female competition is difficult to detect because of its cryptic nature and experimental simulation of competitor threat might be key to studying intrasexual competition in females of a polygynous mating system