Behavioural traits are complex, influenced by multiple genes and environmental factors and they can also affect the health and welfare of individuals. Behaviour is equally important for companion animals, cats and dogs. Furthermore, some of their behavioural traits resemble human psychiatric diseases. Despite behaviour’s importance, its biological background is still poorly known in these animals.

This thesis examines the complexity of behaviour by studying the genetic and environmental factors influencing behaviour, as well as behavioural intercorrelations simultaneously in two companion animal species: the dog and the cat. This comparative perspective may reveal insights into the background of behaviour that could also be generalized to human behaviour. Specifically, this thesis aims to study 1) breed differences of dogs and cats and heritability of behaviour in cats, 2) behaviour correlations in both species, and 3) environmental factors influencing aggression in dogs and multiple behavioural traits in cats.

Behavioural and background data was collected from the dog and cat owners through online questionnaires. In both species, the frequency of fear, aggression, and abnormal repetitive behaviour was examined. Sociability and level of activity was also examined in cats and impulsivity/inattention in dogs. Large datasets of 13 715 dogs and 5726 cats were collected and analysed with different methods, including multiple logistic regression for the environmental factors of behaviour and Bayesian multivariate model for heritability analyses.

The results of this thesis show that both dog and cat breeds differ in behaviour, that behaviour is heritable, that many behavioural traits are correlated, and that many environmental factors are associated with behavioural traits. Heritability estimates varied between 0.40 and 0.53 for all behavioural traits and breeds. Especially fear and aggression correlated strongly and these traits were also associated with abnormal repetitive behaviour in both species. Social environment in both early life and at the time of answering was associated with lower incidence of aggression and abnormal repetitive behaviour.