• Arrival timing;
  • behavioural ecology;
  • breeding schedule;
  • emergence;
  • migration;
  • protandry;
  • protogyny

Protandry, the earlier arrival of males to breeding areas than females, is a common pattern of sex-biased timing in many animal taxa (e.g. some insects, fish, amphibians, reptiles, birds and mammals). The adaptive significance of protandry is not fully understood and, since the 1970s, at least seven hypotheses for protandry have been proposed. We describe each of these hypotheses and summarize what is known about each. In three of these hypotheses, the relative arrival timing of males and females has no direct fitness consequences for males or females, but selection for different timing in each sex indirectly produces protandry. In the other four hypotheses, the difference between male and female timing has fitness consequences for males or females and selection directly maintains the fitness-maximizing degree of sex-biased timing. The hypotheses are not mutually exclusive, and the degree of multiple mating by males and the occurrence of male territoriality seem to determine the relative importance of each hypothesis. In order to understand the adaptive significance of sex-biased timing, future studies need to consider all the alternatives and to assess the costs and benefits to males of early arrival relative to calendar date, to other males and to females.