The life of fiddler crabs : mate choice, mate attraction and territoriality

Abstract

From violence to romance, fiddler crabs are an ideal study system to explore questions and predictions important to behavioural ecology and sexual selection. Fiddler crabs of the genus Uca are members of the Ocypodid family of brachyuran crabs. These highly social crabs occur in dense mixed sex colonies, and display pronounced sexual dimorphism. Males are endowed with one greatly enlarged claw that is used during courtship, as well as a weapon during aggressive interactions. Females lack this claw and instead have two small feeding claws. Fiddler crabs are highly territorial and display both male contest competition and female mate choice. The following nine chapters of this thesis explore the behavioural ecology of fiddler crabs.

Chapter 1 looks at seasonal shifts in female mating preferences for claw size using custom-built robotic crabs. Here I propose that a change in preference for claw size is due to seasonal difference in the value of larger burrows, which strongly influences burrow temperature and thus larval development rate (Milner et al. 2010a). Chapter 2 explores non-independent mate choice and the role of stimulus enhancement in female mate choice. By conducting both natural mate choice experiments and mate choice experiments using custom built robotic crabs I show that the decision of one female to approach a group of males increases the probability that another female will approach and visit a male from the same group (Milner et al. 2011a). In Chapter 3 I show that male fiddler crabs eavesdrop on the courtship displays of nearby courting males to detect mate-searching females and that males appear to adjust their wave rate according to the information available (Milner et al. 2010b). Chapter 4 tests whether courting males adjust their wave rate, and therefore the cost of courtship, to the current level of competition. Here, I show that display rate increases as competition increases and that when competition is removed, males reduce their display rate by 30 per cent (Milner et al. 2011b). Chapter 5 explores how well-armed males and weaponless female fiddler crabs obtain and defend territories. By conducting replacement experiments and collecting observational data, I show that females acquire burrows by seeking out empty ones, while males acquire burrows by evicting male and female burrow owners (Milner et al. 2010c). In Chapter 6 I show that males defend their female neighbours against male intruders. I also show that females sometimes mate with their immediate neighbour. Therefore males who ensure that they retain a female neighbour could benefit through increased opportunities for future reproductive success (Milner et al. 2010d). Chapters 7 and 8 explore the selective pressures imposed by the formation of territorial coalitions. Within these two chapters I show that territory-seeking males choose whom to fight based partly on the size of the potential opponent's nearest neighbour (Milner et al. 2011c; Milner in press). Finally in Chapter 9 I provide evidence of cannibalism and predation in a fiddler crab as well as the first evidence of fiddler crabs hunting shrimp and insects (Milner et al. 2010e). - provided by Candidate.