Why breeding earlier is always worthwhile

Delayed reproduction is probably never advantageous (increases fitness) in constant conditions, unless trade-offs exist between the timing of reproduction and other fitness components. Fitness is here defined as rate of increase of a dominant allele which makes the animal breed earlier. Delayed reproduction could be advantageous if future offspring enjoyed better conditions than present offspring. In the absence of trade-offs it can be shown mathematically that delayed reproduction is never advantageous if any of the following conditions is true: (1) F > -μ_k, where F is fitness and μ_k is mortality rate at the time of the kth breeding attempt; (2) mortality rate is either constant, increases or declines with age provided two or more eggs/neonates are produced per breeding (one or more in asexual species) in the first two cases, and in the third case that breeding after age t_k is regularly spaced; (3) semelparity applies, with more than two eggs/neonates produced at breeding; (4) a particular, but plausible iteroparous model applies. Some previous treatments of delayed reproduction are discussed.