Behavioural processes affecting development: Tinbergen's fourth question comes of age

Interest in relationships between behaviour and development has been spurred by research on related topics, including phenotypic plasticity, parental effects, extragenetic inheritance, individual differences and trait syndromes. Here, I consider several emerging areas of research in the interface between behaviour and development, with a focus on behavioural processes that are likely to affect the development and maintenance of interindividual variation in a wide array of morphological, physiological and behavioural traits. Using a norm of reaction approach, I introduce and illustrate the complexities of phenotypic development. Next, I consider the implications of environmental selection and niche construction for phenotypic development, and consider why these behavioural processes are likely to encourage the development and maintenance of repeatable, stable individual differences and trait syndromes. Parental effects involving behaviour also affect the development of a wide array of phenotypic traits; differential allocation is a currently underappreciated type of parental effect, by which males can affect the development of their offspring via nongenetic means, even if those males have no contact with their young. Behavioural parental effects also contribute to extragenetic inheritance, and recent studies suggest that this phenomenon may be more widespread than previously suspected. The effects of behavioural processes on phenotypic development have interesting implications for problems in related disciplines (e.g. ecology, evolution and conservation biology), providing additional impetus for future research on the effects of behavioural mechanisms on the development of behavioural and other traits. (C) 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.