Manatees store intestinal gas in a specialized hindgut and use it to control their buoyancy. By retaining or releasing the gas, they rise or sink through the water column like a biological ballast tank. An individual that needs to dive holds the gas in. One that needs to rise releases it. And yes- a constipated individual is one that cannot dive properly.

This buoyancy control system is one adaptation among many that makes the manatee one of the more unusual large mammals in the ocean. They are the only fully aquatic herbivorous mammals, grazing on seagrass, freshwater vegetation, and algae for up to eight hours per day and consuming up to 10% of their body weight in plant material daily. A large individual weighing 600 kilograms eats 60 kilograms of vegetation every day.

The teeth are replaced continuously throughout the manatee’s life in a system called polyphyodonty, with new teeth forming at the back of the jaw and moving forward as front teeth wear down and fall out, in a slow conveyor belt of dental replacement driven by the abrasive diet of aquatic vegetation. It is a system shared with elephants, which are this aquatic mammal’s closest living relatives on land, a fact that surprises most people who have not thought about its evolutionary history.

The Hearing

These animals have no external ear openings. Sound reaches their inner ear through the bones of the jaw, a system of bone conduction that is more sensitive to low-frequency sounds than the air-conduction hearing of most mammals. They communicate through vocalizations in the range of 1 to 18 kilohertz, with mothers and calves maintaining near-constant vocal contact in the first weeks after birth.

manatee

The Recovery and the Setback

The jaw-bone hearing system is sensitive enough that these creatures can detect the low-frequency sounds of boat engines at considerable distances, but not always sensitive enough to allow them to avoid the boats in time. Propeller strikes from motorboats are the leading cause of mortality for this species in Florida, and virtually every adult individual in Florida waters carries propeller scars on its back, used by researchers to identify individuals in long-term population studies.

Florida populations of this species recovered significantly after protections were introduced in the 1970s, growing from a few hundred individuals to over 7,500 by 2017. In 2021, over 1,100 died in a single year, the largest die-off on record, driven primarily by starvation as seagrass beds in the Indian River Lagoon collapsed due to algal blooms caused by nutrient pollution from agricultural and urban runoff.

The seagrass that these animals depend on requires clear, clean water to photosynthesize. Nutrient pollution clouds the water, algae blooms, seagrass dies, and the herbivores that depend on it starve. The connection between a fertilizer application on a farm and an individual dying of starvation in a lagoon 50 kilometers away is real, measurable, and largely invisible to the people making the fertilizer decision.

What Needs to Change

Manatee conservation depends on water quality as much as it depends on boat speed limits. The seagrass beds that these aquatic mammals rely on for all of their nutrition require clear, clean water to survive. Reducing nutrient runoff from farms and urban areas — the primary driver of the algal blooms that killed the seagrass in the Indian River Lagoon — is the only path to a stable population. Emergency lettuce feeding kept animals alive through the 2022 winter. It is not a substitute for clean water.

Emergency feeding programs were established in 2022, with wildlife managers providing lettuce to these animals at warm-water refuges during winter months. It kept individuals alive. It is not a long-term solution. The long-term solution is cleaner water, which requires changes to land use practices that extend far beyond the lagoon itself.