Tinbergen's four questions: an appreciation and an update.

This year is the 50th anniversary of Tinbergen's (1963) article 'On aims and methods of ethology', where he first outlined the four 'major problems of biology'. The classification of the four problems, or questions, is one of Tinbergen's most enduring legacies, and it remains as valuable today as 50 years ago in highlighting the value of a comprehensive, multifaceted understanding of a characteristic, with answers to each question providing complementary insights. Nonetheless, much has changed in the intervening years, and new data call for a more nuanced application of Tinbergen's framework. The anniversary would seem a suitable opportunity to reflect on the four questions and evaluate the scientific work that they encourage.