Behavioural causes and consequences of sexual size dimorphism in an apex predator species.

Abstract

Individual differences within populations in a range of phenotypic traits are hypothesised to have important ecological and evolutionary implications. Variation in individual growth rates that result in corresponding variations in body sizes, including size dimorphism, is a particularly widespread feature of many animal populations. The increasing characterisation of consistent individual behavioural variations, unrelated to age or sex, is equally considered to have important fitness consequences. Our understanding of behavioural causes of size dimorphism remains weak, and few studies have investigated the relationship between individual behavioural consistency and growth variations in size dimorphic populations. The overall aim of this thesis is to identify the behavioural drivers that underpin observed growth variations and result in size dimorphism by using pike (Esox lucius) as a model species. The results show that early life growth is an important driver of sexual size dimorphism in this species. A subsequent focus on the juvenile life stages revealed that individual differences in movement and dispersal tendencies were related to growth and body size in wild pike. The findings indicate that intraspecific interactions such as size-dependent interference competition during the first year of life plays a key role in maintaining intraspecific size variation and size dimorphism in the wild population. Experimental work revealed the occurrence of a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. This suggests that individual competitive ability is underpinned by a variation in boldness to forage under risk. The importance of a heterogeneous environment and presence of intraspecific competition pressure for driving habitat and resource segregation, and subsequently sexual size dimorphism, is discussed.