Movement Behavior of Manatees and Dugongs: I. Environmental Challenges Drive Diversity in Migratory Patterns and Other Large-Scale Movements

Abstract

The seemingly unhurried nature of manatees and dugongs belies their great capacity for undertaking long-distance journeys, often repeatedly in the form of round-trip seasonal migrations, but sometimes as movements independent of seasonal influence. Unique attributes of sirenian biology that interact with features of their environment to mold patterns of movement and habitat use include herbivory, limited thermoregulatory physiology for coping with cold, and, for manatees, an apparent need to ingest fresh water. Manatees and dugongs are remarkably adaptable in their large-scale movement behavior , as manifested by the considerable variation in the occurrence and extent of migrations across populations within species, and among individuals within populations. Some populations and individuals are relatively sedentary year-round, whereas others migrate hundreds of kilometers between seasonal ranges. The environmental selective pressures that drive seasonal movements vary across species, climates, and ecosystems, but are most commonly generated by predictable fluctuations in water temperature (Florida manatee , some dugong populations), rainfall (coastal populations of Antillean and African manatees), or water level (inland populations of all three manatee species living in flood-pulse river systems) over the annual cycle . In each case, there is a season (winter, dry, or low-water) of heightened environmental stress where the animals’ range is restricted to areas around a key limiting resource (warm water, fresh water, or deep water, respectively) and, therefore, forage is less available or of lower nutritional quality. Because dugongs are strictly marine and do not require fresh water, they experience fewer seasonally imposed constraints and are less likely to migrate than manatees. Consequently, the large-scale movements of dugongs seem more stochastic; assessing the status of forage over a wide area through occasional long-distance exploratory forays may represent a behavioral adaptation to periodic extensive declines in seagrass caused by extreme weather events. The available evidence for manatees indicates strong fidelity to seasonal or year-round ranges across years. A common finding from tracking studies is the existence of considerable variation in large-scale movement behavior among and within individuals, which should confer adaptability to environmental change in the short term and provide the raw material for evolutionary change over the long term.