Sperm storage: distinguishing selective processes and evaluating criteria

•Sperm storage is ubiquitous in internally fertilizing animals.•To understand the evolution of sperm storage we must study adaptations that result from selection on males separately from those that result from selection on females.•Adaptations for sperm storage in males can facilitate sperm storage inside females.•Careful consideration of various criteria to demonstrate sperm storage will allow more comparative studies.•We suggest a sperm storage index that will inform such evolutionary studies. Sperm storage, the extended maintenance of viable sperm, probably occurs in most internally fertilizing animals. Because it temporally separates mating from conception, sperm storage can be adaptive in ecologically diverse habitats and affect life histories, mating systems, cryptic female choice, sperm competition, and sexual conflict. Sperm storage can result from different selective forces acting on females and/or males, sometimes resulting in coevolution. The various criteria often used to determine the presence of sperm storage in any given taxon can result from the action of any one or all of these selective forces. Here we discuss the criteria used to study sperm storage and how we can use these to better understand the evolution of diversity in sperm-storage adaptations.