Fitt W. 2020. Florida manatees Trichechus manatus latirostris actively consume the sponge Chondrilla caribensis. PeerJ 8:e8443 https://doi.org/10.7717/peerj.8443
the world’s most soothing swimmer MIGHT NOT BE WHAT IT SEEMS.
Let’s start from the top: what are manatees?
“Manatees are the noblest of all creatures, graceful in form, tremendous in intellect, and without rival in their cuteness” -me
Manatees are big herbivorous mammals that live in warm waters around Florida, the Caribbean, South America, West Africa, and even in the Amazon. Along with dugongs, manatees belong to a relatively small order called Sirenia, which is named such because manatees were apocryphally mistaken by sailors for being mermaids. The sirenians’ closest living relatives are actually elephants!
Manatees are also known as sea cows, a moniker which highlights some common perceptions about them: that they’re herbivores, and that they’re slow, gentle grazers, so calming that they’re even a meme. However, some recent research sheds light on an unexpected manatee behavior – the fact that they sometimes eat other animals!
What animals do manatees eat?
They eat sponges! More specifically, Florida manatees seem to like one particular kind of sponge, called Chondrilla caribensis or C. caribensis for short.
Manatees have actually been known for some time to eat other animals. However, this was thought to be just accidental ingestion of invertebrates and other small creatures who got caught up while the manatees were only trying to eat green, chlorophyll-rich plants like seagrass and algae. (Chlorophyll is the photosynthetic pigment that gives plants their green color, which allows them to make their own food out of sunlight, water, and carbon dioxide (CO2)). However, William Fitt at the University of Georgia and his team of researchers observed that manatees would actively seek out C. caribensis, and so they devised an experiment to confirm it for sure.
Fitt and his team attached several different kinds of sponges to bricks, which they then placed at several locations close to a dock that they knew manatees frequented in Key Largo, FL. They also treated some of the sponges with chlorophyll to see if manatees would prefer the sponges that were most similar to the chlorophyll-rich seagrass they typically eat. The researchers then observed what kind of sponges the manatees ate and in what kinds of conditions.
As a result of this experiment the researchers came to two interesting findings:
- The manatees only ate C. caribensis and left the other sponges alone
- That the manatees ate from all C. caribensis samples regardless of how much chlorophyll they contained.
These results suggest that the manatees’ preference for C. caribensis has nothing to do with its chlorophyll content. The researchers were initially surprised by this, since they had expected manatees to prefer the sponges whose properties were most similar to seagrass.
So why do manatees like these sponges so much? One possibility is that they’ve begun to seek out sponges in response to dwindling food supplies as a result of climate change, but the researchers believe the manatees’ behavior is motivated by something less sinister. They hypothesized that manatees seek out C. caribensis in particular because (1) it is a source of nitrogen and (2) according to previous studies as well as the authors’ own analysis, it may have weaker mechanical and chemical defenses than other kinds of sponges.
This study is exciting just because it shows a new behavior and potentially a new source of food for a charismatic species that only recently escaped the endangered list. But even if you’re not as dazzled by manatees as I am, it’s still interesting that the results of this study are counter to the tropes our culture typically applies to manatees. It raises an interesting question for me, which is to ponder the ways that animal’s reputations and our perceptions color the way we think about policy and science.
Manatees are charismatic animals, which means that like polar bears, penguins, or elephants, they’re an animal that a lot of people might classify as their “favorite animal.” Charismatic animals tend to be the face of conservation efforts, but from the point of view of policy, this raises a host of ethical concerns. Seeing too many photos of charismatic animals might make us forget that they’re endangered, and on the other side of the coin, there may be less funding to protect other species that are just as important, but less charismatic.
From a scientific point of view, I found this study to be eye-opening because until today I would have told you, with confidence based on nothing but cultural stereotypes, that manatees only ate plants. Studies like this demonstrate the importance of having research carried out by scientists who are close enough to their subjects that their scientific questions are motivated by lived experience – in this case, happening to observe manatees eating sponges – rather than by cultural or societal norms that are so ingrained we don’t even remember to question them.
Is there something else you can think of that you believe but have never confirmed due to social or cultural norms? Here’s another one that I learned in the process of writing this post: camels don’t actually store water in their humps.