Whether Steller's sea cow had any natural predators is unknown.
It may have been hunted by killer whales and sharks, though its buoyancy may have made it difficult for killer whales to drown, and the rocky kelp forests in which the sea cow lived may have deterred sharks.
According to Steller, the adults guarded the young from predators.

Steller described an ectoparasite on the sea cows that was similar to the whale louse (Cyamus ovalis), but the parasite remains unidentified due to the host's extinction and loss of all original specimens collected by Steller.
It was first formally described as Sirenocyamus rhytinae in 1846 by Johann Friedrich von Brandt.
It was the only species of cyamid amphipod to be reported inhabiting a sirenian.
Steller also identified an endoparasite in the sea cows, which was likely an ascarid nematode.

Like other sirenians, Steller's sea cow was an obligate herbivore and spent most of the day feeding, only lifting its head every 4–5 minutes for breathing.
Kelp was its main food source, making it an algivore.
The sea cow likely fed on several species of kelp, which have been identified as Agarum spp.,
Alaria praelonga, Halosaccion glandiforme, Laminaria saccharina, Nereocyctis luetkeana, and Thalassiophyllum clathrus.
Steller's sea cow only fed directly on the soft parts of the kelp, which caused the tougher stem and holdfast to wash up on the shore in heaps.
The sea cow may have also fed on seagrass, but the plant was not common enough to support a viable population and could not have been the sea cow's primary food source.
Further, the available seagrasses in the sea cow's range (Phyllospadix spp.
and Zostera marina) may have grown too deep underwater or been too tough for the animal to consume.
Since the sea cow floated, it likely fed on canopy kelp, as it is believed to have only had access to food no deeper than 1 m (3.3 ft) below the tide.
Kelp releases a chemical deterrent to protect it from grazing, but canopy kelp releases a lower concentration of the chemical, allowing the sea cow to graze safely.
Steller noted that the sea cow grew thin during the frigid winters, indicating a period of fasting due to low kelp growth.
Fossils of Pleistocene Aleutian Island sea cow populations were larger than those from the Commander Islands, indicating that the growth of Commander Island sea cows may have been stunted due to a less favorable habitat and less food than the warmer Aleutian Islands.

Steller described the sea cow as being highly social (gregarious).
It lived in small family groups and helped injured members, and was also apparently monogamous.
Steller's sea cow may have exhibited parental care, and the young were kept at the front of the herd for protection against predators.
Steller reported that as a female was being captured, a group of other sea cows attacked the hunting boat by ramming and rocking it, and after the hunt, her mate followed the boat to shore, even after the captured animal had died.
Mating season occurred in early spring and gestation took a little over a year, with calves likely delivered in autumn, as Steller observed a greater number of calves in autumn than at any other time of the year.
Since female sea cows had only one set of mammary glands, they likely had one calf at a time.

The sea cow used its fore limbs for swimming, feeding, walking in shallow water, defending itself, and holding on to its partner during copulation.
According to Steller, the fore limbs were also used to anchor the sea cow down to prevent it from being swept away by the strong nearshore waves.
While grazing, the sea cow progressed slowly by moving its tail (fluke) from side to side; more rapid movement was achieved by strong vertical beating of the tail.
They often slept on their backs after feeding.
According to Steller, the sea cow was nearly mute and made only heavy breathing sounds, raspy snorting similar to a horse, and sighs.
Whether Steller's sea cow had any natural predators is unknown. It may have been hunted by killer whales and sharks, though its buoyancy may have made it difficult for killer whales to drown, and the rocky kelp forests in which the sea cow lived may have deterred sharks. According to Steller, the adults guarded the young from predators.
Steller described an ectoparasite on the sea cows that was similar to the whale louse (Cyamus ovalis), but the parasite remains unidentified due to the host's extinction and loss of all original specimens collected by Steller. It was first formally described as Sirenocyamus rhytinae in 1846 by Johann Friedrich von Brandt. It was the only species of cyamid amphipod to be reported inhabiting a sirenian. Steller also identified an endoparasite in the sea cows, which was likely an ascarid nematode.
Like other sirenians, Steller's sea cow was an obligate herbivore and spent most of the day feeding, only lifting its head every 4–5 minutes for breathing. Kelp was its main food source, making it an algivore. The sea cow likely fed on several species of kelp, which have been identified as Agarum spp., Alaria praelonga, Halosaccion glandiforme, Laminaria saccharina, Nereocyctis luetkeana, and Thalassiophyllum clathrus. Steller's sea cow only fed directly on the soft parts of the kelp, which caused the tougher stem and holdfast to wash up on the shore in heaps. The sea cow may have also fed on seagrass, but the plant was not common enough to support a viable population and could not have been the sea cow's primary food source. Further, the available seagrasses in the sea cow's range (Phyllospadix spp. and Zostera marina) may have grown too deep underwater or been too tough for the animal to consume. Since the sea cow floated, it likely fed on canopy kelp, as it is believed to have only had access to food no deeper than 1 m (3.3 ft) below the tide. Kelp releases a chemical deterrent to protect it from grazing, but canopy kelp releases a lower concentration of the chemical, allowing the sea cow to graze safely. Steller noted that the sea cow grew thin during the frigid winters, indicating a period of fasting due to low kelp growth. Fossils of Pleistocene Aleutian Island sea cow populations were larger than those from the Commander Islands, indicating that the growth of Commander Island sea cows may have been stunted due to a less favorable habitat and less food than the warmer Aleutian Islands.
Steller described the sea cow as being highly social (gregarious). It lived in small family groups and helped injured members, and was also apparently monogamous. Steller's sea cow may have exhibited parental care, and the young were kept at the front of the herd for protection against predators. Steller reported that as a female was being captured, a group of other sea cows attacked the hunting boat by ramming and rocking it, and after the hunt, her mate followed the boat to shore, even after the captured animal had died. Mating season occurred in early spring and gestation took a little over a year, with calves likely delivered in autumn, as Steller observed a greater number of calves in autumn than at any other time of the year. Since female sea cows had only one set of mammary glands, they likely had one calf at a time.
The sea cow used its fore limbs for swimming, feeding, walking in shallow water, defending itself, and holding on to its partner during copulation. According to Steller, the fore limbs were also used to anchor the sea cow down to prevent it from being swept away by the strong nearshore waves. While grazing, the sea cow progressed slowly by moving its tail (fluke) from side to side; more rapid movement was achieved by strong vertical beating of the tail. They often slept on their backs after feeding. According to Steller, the sea cow was nearly mute and made only heavy breathing sounds, raspy snorting similar to a horse, and sighs.
Steller's sea cow