- According to a new study, the Amazonian manatee is a seed disperser; a researcher found germinating grasses in its feces.
- The discovery was made in Lake Amanã, in the Amazon, where 96 samples of manatee feces were collected; surprisingly, they contained 556 intact seeds.
- Manatees eat 40 kilograms (88 pounds) of aquatic plants a day and migrate long distances. This finding shows that the manatee is a crucial vector for dispersing seeds between fertile areas such as floodplains and nutrient-poor areas such as the swamp forests known as igapós; this activity enhances biodiversity in Amazonian environments, especially in times of climate change and environmental degradation.
- Despite its vital ecological function, the manatee is listed as vulnerable to extinction; Illegal hunting, facilitated by periods of extreme drought, is one of the main threats, along with climate change, which affects both their diet and their migratory routes.
MANAUS, Amazonas, Brazil — Located near the confluence of the Solimões and Japurá rivers, Lake Amanã, which means “the path of the rain,” is known as the home of the Amazonian manatee (Trichechus inunguis). During the dry season, when river levels drop, this mammal finds refuge and food there. It was on the beaches of the Amanã that biologist Michelle Guterres made a surprising discovery: manatee feces with strands of germinating grass. This observation was an important step toward confirming a hypothesis that so far had been only speculated about: the manatee is capable of dispersing seeds, thus helping plants migrate between different habitats.
“I remember that, at the beginning of the research, my adviser insisted that the manatee was not a seed disperser,” Guterres says, recalling previous studies that had not found viable seeds in fecal samples. “And I told her: not that we know.”
Guterres collected 96 fecal samples, 19 of which had plants germinating in them. One of the major challenges in the research was finding a significant amount of feces. As soon as a manatee defecates, its feces either dissolve in the water or are eaten by fish. That is why these animals were already known for fertilizing the waters, but their role as seed dispersers was a groundbreaking discovery.
The extreme drought of 2023 made it easier to collect the samples, which were found on the beaches formed by low water levels. In October, Lake Amanã was only 20 centimeters (8 inches) deep.
After that, the samples had to be broken up with tweezers, due to their fibrous texture. In total, 2,033 seeds were found, 556 of which were viable with embryos. “That’s an impressive number,” says Maria Tereza Piedade, a researcher at the National Institute for Amazonian Research. “It is more than enough to confirm that the manatee is a seed disperser.”
How important is this discovery?
In the Amazon, seed dispersal and plant distribution are intrinsically linked to rivers’ flood-and-drought cycles. Piedade explains that, during floods, water is the main disperser in floodplains and igapós (blackwater-flooded forests), especially of aquatic and semiaquatic plants.
“The plants bear fruit, and their seeds fall into the water. Then fish eat them and take them to other habitats located upstream or in nearby areas,” she explains, referring to studies that have found seeds in the digestive tracts of fish.
But knowing that the manatee plays a similar role is significant. Weighing up to 550 kilograms (1,200 pounds), manatees eat up to 40 kg (88 lbs) of aquatic plants per day and have been recorded traveling up to 115 kilometers (71 miles) between lakes Amanã and Mamirauá. With a long digestive tract, seeds can remain in their intestines for up to a week, increasing their potential as seed dispersers. “In addition, they deliver the seeds with natural fertilizer, a real gardening kit,” Piedade comments humorously.
The manatee’s migratory patterns suggest that it plays a vital role in dispersing seeds between fertile habitats such as river floodplains and nutrient-poor areas such as igapós. “During floods, the manatee stays in the floodplains, where food is abundant,” explains Miriam Marmontel, an aquatic mammal expert at the Mamirauá Institute for Sustainable Development. “When river levels drop, they seek refuge in lakes and flooded areas,” she adds.
While river floodplains are productive areas with some 400 species of plants, the swamp forests known as igapós, with more acidic waters and poor soils, are home to only 10% of those. Therefore, it is possible to assume that manatees transport seeds between these areas, increasing the biodiversity of these environments and securing their own food source for the next drought.
A decades-long debate
A native of Porto Alegre, Michelle Guterres has extensive experience with the Amazon and manatees. Her first stay in the Amazon municipality of Tefé lasted 10 years, from 2000-10, when she worked at the Mamirauá Institute.
In 2012, she published her first work on seed ingestion by manatees. At the time, with more than 200 samples collected, all the seeds found were nonviable — they had no embryos — which led to the conclusion that the manatee was not a disperser.
Other researchers have also investigated this possibility. A 2020 study about manatees in the Caribbean found no viable seeds in their feces. Previous research, such as that of biologist Robin C. Best’s study conducted in the 1980s, had speculated about this role but had no proof.
After moving to Florianópolis and having a break from her career to take care of her children, Guterres resumed her research work in 2019 and during the pandemic, and returned to Lake Amanã. “That question never left my mind,” she says. “But this result was totally unexpected and came from field experience. The Amazon allows you to find things that no one would ever imagine.”
Urgent need for conservation
Currently, the manatee is listed as a vulnerable species by IUCN, the International Union for Conservation of Nature. Its greatest risk of extinction comes from illegal hunting.
In the first half of the 20th century, when fishing was legal and booming, around 19,000 manatees were killed for leather, which was used in rubber production. Their meat is also appreciated by riverine communities. Hunting was banned in 1967, but it is still practiced in some areas.
Published in 2019, a study with riverine populations indicates that hunting is still the main cause of death for manatees in the Amazon. That was confirmed by 62% of respondents interviewed in the Anavilhanas area and by 52% in the Tapajós area.
Severe droughts such as those experienced in 2023 and 2024 also facilitate the capture of these animals. In general, the dry season lasts about three months, during which they remain in the lakes, where they are more easily killed. However, prolonged droughts also extend this period of greater vulnerability.
“Droughts worsened by climate change also affect manatees’ diet,” Marmontel points out. “They feed on organic matter at the bottom of the lakes but end up ingesting mud and sand, which may harm their digestive system. A longer dry season increases the chances of this outcome.”
Guterres sees her finding as yet another argument for manatee conservation. “We always emphasize environmental education to raise awareness about the importance of manatees for the Amazon, including the health and well-being of communities that depend on river, floodplain and forest waters.”
Piedade adds: “By fertilizing the waters and transporting seeds, manatees contribute to biodiversity. Protecting this species means protecting the ecosystem and the populations that depend on it.”
Banner image: Amazonian manatee (Trichechus inunguis). Image courtesy of André Dib.
Citations:
Gil Guterres-Pazin, M., Lopes, A., Fernando Volpato Pazin, V., Garcia de Santana, D., Marmontel, M., & Teresa Fernandez Piedade, M. (2024). A giant gardener of the floodplains: The Amazonian manatee. Journal for Nature Conservation, 82, 126708. doi:10.1016/j.jnc.2024.126708
Guterres-Pazin, M. (2012). Short note: Ingestion of invertebrates, seeds, and plastic by the Amazonian manatee (Trichechus inunguis) (Mammalia, sirenia). Aquatic Mammals, 38(3), 322-324. doi:10.1578/am.38.3.2012.322
Best, R. C. (1981). Foods and feeding habits of wild and captive sirenia. Mammal Review, 11(1), 3-29. doi:10.1111/j.1365-2907.1981.tb00243.x
Crema, L. C., Da Silva, V. M., & Piedade, M. T. (2019). Riverine people’s knowledge of the vulnerable Amazonian manatee Trichechus inunguis in contrasting protected areas. Oryx, 54(4), 529-538. doi:10.1017/s0030605318000686
This story was first reported by here in Portuguese on Nov. 21, 2024.