The Dugong: Sea Cow The biology, behaviour, and conservation challenges facing the world's last truly herbivorous marine mammal he dugong (Dugong dugon) is the sole living member of the family Dugongidae and the last surviving representative of a group of large herbivorous marine mammals whose evolutionary lineage extends back more than 50 million years. Its closest living relative on land is the elephant, from which it diverged when a common ancestor returned to the sea. The dugong's elongated, spindle-shaped body, paddle-like forelimbs, and broad fluked tail give it an appearance distinctly different from seals and sea lions, which belong to very different evolutionary lineages. Its common name — sea cow — reflects both its grazing lifestyle and the slow, placid manner of an animal that has relatively few natural predators as an adult. Dugongs are obligate seagrass grazers: their diet consists almost entirely of seagrass, the flowering plants that grow in shallow coastal and estuarine marine environments across the Indo-Pacific from the east coast of Africa to Vanuatu and the Solomon Islands in the Pacific. A large dugong consumes approximately 28 to 40 kilograms of seagrass per day, and large aggregations of feeding dugongs can significantly affect the structure and composition of seagrass meadows through selective grazing and the physical disturbance of sediments. Dugongs typically leave characteristic feeding trails — elongated furrows in the seagrass meadow from which every part of the plant including the roots and rhizomes has been consumed — that allow researchers to estimate population size and feeding activity from aerial surveys. The life history of the dugong combines slow reproduction with exceptional longevity: individuals can live for more than 70 years, reaching sexual maturity at around 10 to 17 years of age, and females produce only a single calf every three to five years following a gestation period of approximately 14 months. This reproductive rate — one of the slowest among marine mammals — means that populations recover extremely slowly from any source of mortality above a very low threshold. Models indicate that an annual adult mortality rate above approximately 5 percent is sufficient to cause population decline regardless of reproductive effort. The dugong's dependence on shallow coastal seagrass habitats places it in direct conflict with the human activities that concentrate along coastlines — shipping, fishing, tourism development, and coastal agriculture and aquaculture that drive turbidity, nutrient loading, and direct physical damage to seagrass meadows. Boat strike, in
which dugongs are injured or killed by the propellers of motorised vessels, is a significant source of mortality in areas of high boat traffic; traditional and subsistence hunting remains important in some parts of the range; and entanglement in gillnets and shark nets set for other purposes kills substantial numbers annually in northern Australia, the Persian Gulf, and parts of South-East Asia. The conservation status of the dugong varies considerably across its range. In Australian waters, where the world's largest dugong population — estimated at over 80,000 animals — is concentrated in Shark Bay and the Torres Strait, relatively intact seagrass habitats and legal protection since 1969 have maintained a substantial population, though periodic cyclones and climate-driven seagrass die-offs cause significant short-term mortality events. Across the rest of the dugong's Indo-Pacific range, populations are smaller, more fragmented, and under greater threat from habitat loss and direct take. The dugong is classified as Vulnerable on the IUCN Red List, though in many parts of its range the actual status of populations is poorly known due to the logistical difficulties of monitoring a large, aquatic animal across remote coastal habitats. The ecological role of dugongs in seagrass ecosystems is substantial and complex. Heavy grazing by large aggregations can reduce seagrass biomass locally but may also stimulate regrowth by increasing the turnover of plant tissue and the recycling of nutrients. Dugongs may influence the species composition of seagrass meadows through selective consumption of certain species. The feeding trails left by dugongs create patches of disturbed sediment that are colonised by species-specific assemblages of microorganisms and invertebrates, contributing to habitat heterogeneity. The loss of dugongs from an ecosystem — as has occurred across much of their former range — would therefore represent not merely the removal of a large, charismatic species but the loss of an ecological process that shapes the structure and biodiversity of seagrass habitats.