Trade-offs between storage and survival affect diapause timing in capital breeders

Many organisms spend the unfavourable part of the year, such as the winter season, in diapause or dormancy and reproduce in spring shortly after emergence. Reserves are acquired prior to diapause to cover metabolic costs and in some species also reproduction (capital breeding) directly after diapause. Storage is then a component of future reproduction, and capital breeders consequently pay a pre-breeding cost of reproduction as they risk dying while obtaining and carrying the reserves. How large should the reserves be, and to what extent should optimal storage, and thereby timing of diapause, depend on predation risk and reproductive strategy? We present a general and simplistic life history model of an arthropod (e.g. crustaceans or insects) that is exposed to background mortality risk when it accumulates reserves before diapause. The model optimizes diapause timing and resultant reserves for income, mixed and capital breeders, and predicts how mortality risk affects the degree of capital breeding. For income breeders, timing of diapause is insensitive to the risk while obtaining reserves as they, regardless of risk, acquire the minimum amount needed to survive the winter. For capital breeders, the higher the risk the earlier the diapause and less is consequently stored. Mixed breeders diapause late and store as much as pure capital breeders when exposed to low risk, but behave as income breeders and diapause early when mortality is high. Our model shows that the degree of capital breeding impacts phenology of diapause in a risk-dependent manner. This prediction should impact how diapause timing is thought of across a wide range of taxa, including the much studied marine copepods. Timing of diapause, including triggers and cues, can only be understood when the diversity of reproductive strategies and the adaptive value of storage is taken into account.