Introduction

Why do some reproductively mature animals stay at home, delay breeding, and help to rear the offspring of others? Over the past 25 years, this question has been addressed by authors studying a wide variety of animals, from insects (Alexander, Noonan, & Crespi 1991) to mammals (Brown 1987; Emlen 1991). On one level, the answer is simple: An individual should stay at home and help when its inclusive fitness is higher than it would be if it dispersed and attempted to breed (Hamilton 1964; Brown 1987; note: we will refer to alloparental behavior as “help”). Unfortunately, for several reasons, quantifying the inclusive fitness consequences of these alternatives is not a simple matter. For example, the sequence of behavioral decisions that lead to helping may differ among species, as we discuss later. Also, there are four components of inclusive fitness that need to be measured (Brown 1987): current direct fitness, current indirect fitness, future direct fitness, and future indirect fitness. The calculation of some of these components is quite complicated.

To quantify the fitness payoffs to individuals that make these decisions, we first need to understand clearly what decisions they are making. As Brown (1987) has suggested, cooperative breeding often involves three distinct decisions: to remain in the natal territory (as opposed to dispersing), to delay breeding, and to help. In empirical studies, these decisions are often simplified to a dichotomy: philopatry/help versus disperse/breed.