Reproductive Structures, Strategies, and Patterns

All groups of marine mammals share several features of reproduction that stem from their placental mammalian heritage. This chapter explores the basic conditions of reproduction, which are reflected in the strategies for care and nourishment of the young and in the resulting life histories of these animals. Some species often remain in limited home ranges for both mating and parturition. Consequently, a discussion of social organization in a broader context is also addressed in this chapter. The timing of ovulation and estrus varies among marine mammals. Phocids and otariids have a postpartum estrus, which establishes an annual cycle with copulation in one breeding season and birth in the next. The mating systems of marine mammals include promiscuity and polygyny. Among pinnipeds, all otariids and many species of phocids are polygynous. Almost all pinniped species that breed on land are extremely polygynous and strongly sexually dimorphic. Because polygynous males must compete for reproductive control of females, competition revolves around either the establishment or defense of breeding territories or the establishment of dominance hierarchies. Mating in cetaceans and sirenians is typically promiscuous, although strategies such as mate guarding do exist. Odontocetes exist in social groups, whereas mysticetes are more solitary. Three maternal strategies characterize marine mammals: fasting, foraging cycle, and aquatic nursing. Evidence of fitness costs associated with different reproductive strategies in female seals is mixed. Similarly, there are mixed results concerning differential investment in male and female offspring.