Unlocking secrets of the West African manatee
Helen: The least well known of the sirenia is the West African Manatee. As the name suggests, they live along the west coast of Africa, they have a huge range from Senegal down to Angola, and they can also move hundreds of mile inland along rivers.
Someone who knows more than anyone else about these mysterious marine mammals is researcher and conservationist, Lucy Keith Diagne. She told me how she was first tempted to visit west Africa in search of manatees.
Lucy: I had actually been working with manatees in Florida and the Caribbean for 6 years when I went to a scientific meeting, a marine mammology conference. And just by pure coincidence I met there a humpback whale researcher who was working in Gabon, West Africa. And he mentioned to me that each day as he traveled to his study site out in the Atlantic Ocean in Gabon he was actually passing a lot of West African manatees. From everything he heard they were quite rare but here he was seeing them daily. He thought that was pretty interesting.
Helen: Lucy ended up spending the next 6 years working with manatees in Gabon, and she began visiting other parts of the manatees' range across West Africa, and as she travelled around, saw more manatees and spoke to people in different countries, it became obvious just how mysterious these animals are.
Lucy: There is so little known about this species. We don't even know how long the species lives, the age of sexual maturity; we don't know anything about mother-calf behavior. For example, in Florida when a mother can't give birth the calf stays with it for two years and in Africa we have no idea if that is the case. That's an important time because the mother actually teaches her calf everything it needs to know to survive: where to go to get food, where to go to get fresh water.
Helen: So, to answer some of the big questions surrounding West African manatees, Lucy is conducting a range of studies, including working out how long they live.
Lucy: Manatees, we age them through their ear bones because they have what isknown as marching molars. Their teeth, in their jaw, are basically on a conveyor belt and they use their teeth obviously to crush plants and as they wear them out they lose their teeth and the back teeth move forward throughout their life and therefore we can't really use those to age them. But their ear bone is very similar to say the otolith of a fish or also the rings of a tree where they can be sliced and we can actually count the rings that determine their age.
Helen: Lucy and her colleague Katie Brill from the Smithsonian Institution in Washington DC, have recently, for the first time, have worked out the age of west African manatees using ear bones collected from dead animals. The oldest they found was 39 years old, which is surprisingly old for a manatee - for other species of sirenia in the wild, anything over 20 is considered pretty old. But they also found some younger manatees too.
Lucy: Particularly in Ghana where they are heavily hunted in Lake Volta, the ages were a lot younger. They were the only manatees that were in the single digits like 3, 4, and 5 years old. Which starts to make you concerned that perhaps they are hunting these animals so young that they might not even be able to reproduce because we don't know the age of reproduction in this species, but in Florida its 5 years old. So if they are not even making it to 5 then they're going to hunt them all out before they can even reproduce.
Helen: Another part of the manatee puzzle that Lucy has set out to solve, is the how their populations of are subdivided across their enormous range. It's known that individua manatees don't tend to swim very far, their home ranges are less than 100 miles. And this raises important questions for conservation.
Lucy: Where are the populations' boundaries? Where are areas where genetically these manatees might be very different from other populations and therefore more important to protect, if you want to protect biodiversity of the species? And add to that, you also now have many manmade dams throughout all major rivers in Africa that are now separating some of these populations permanently. In Mali alone, in the Niger River, you have at least 4 or 5 major dams and I just heard about a new one that's going in.
So, you've actually chopped up the population there into these 4 or 5 different groups that can never mix again and interbreed. Going forward in the future, if we hope to conserve these animals it's really important to know how different they are from each other or how similar.
Helen: To work out where those natural and manmade boundaries lie, a genetic study is underway, using tissues samples from wherever Lucy can get them.
Lucy: It's very difficult to get the samples, the manatees either have to be dead, unfortunately, or else captured alive and sampled. And there really aren't many people able to capture, safely, a live manatee. There are a few cases where the animals are rescued and released. But, unfortunately, most of the samples do come from manatees that are dead, either from natural causes or through hunting.
Helen: Another aspect of manatee ecology that Lucy and her team are investigating is what they eat. In parts of their range where there is seagrass then that does form an important part of their diet, they also eat various other types of aquatic vegetation including floating plants like water lilies or when rivers flood, manatees swim out across what used to be dry land, and graze on grasses and shrubs.
But this can be highly seasonal, and rather than having to cope with cold winters like manatees in Florida, the West African manatees have to deal with long dry seasons when there isn't much around to eat. And that could be why, despite being famous vegetarians, Lucy is finding out this isn't always the case, West African manatees will also eat shellfish.
Lucy: I have seen manatees eating clams in Angola, in the freshwater river system, and some of these clam shells are quite hard to crush so it is amazing that they can do it. I'm told by the local people in Angola that they believe that the manatees are eating only the small ones or the dead clams that pop open and the clam meat would be exposed
Helen: To get more information about manatee diets, Lucy is carrying out a stable isotope analysis - this involves taking samples of body tissues, like hair and bone, and examining the chemical fingerprint that's left behind when manatees eat particular types of food. That way she's building a picture of what they eat across their range. Another thing she's discovering is that in some places, manatees and people are sometimes chasing after the same food.
Lucy: Another thing that we got a lot of reports of, and in one place I've verified it to be true, is that the manatees take fish from nets. And, again I think this partially goes back to this dry season fasting or very little food resources available for them in the dry season. They do take fish out of fishermen's nets , they actually suck the fish meat off the fish and leave the head in the net, which of course doesn't make the fishermen very happy.
But in a way I think it's kind of ingenious; they need food, they need protein, and when they don't have many other choices. For example, up in Senegal, in Eastern Senegal in the river during the dry season they are clearly taking fish from nets; there is no other large animal in the system that would be able to do that.
Helen: An important part of Lucy's work involves collaborating with local people, in the case of the fishermen who are having their catches eaten by manatees helping them to avoid competing for the same food. She is also tapping into local knowledge of these illusive animals, and nurturing a network of researchers to help with her work in the many West African countries that manatees live in.
Lucy: They figure, prominently in the cultures where people have hunted them for generations, they are often tied to legends and myths of a spirit called Mamiwata; but those places are often very remote and there isn't a lot of contact with the outside world. So, they may know they live there but those of us who try to do conservation work you literally have to go to places and ask people, they are just not well known where they exist. And, I think that is a problem for conservation because as I said they are heavily hunted throughout most of their range.
One of the things that I've been trying to do is build a network of African Scientists in their own countries. There is a lot of interest to protect manatees, but they don't have a lot of resources to do that. So, people that I've been working with and training over the last 3 years are usually affiliated with a university or a non-profit organization or government. And they want to start doing manatee work but don't know where to begin.
We bring these people for one to two weeks of training where we tell them about manatees but we also take them in the field and teach them how to use sampling equipment and how to search for manatees and how to conduct a village interview in a way that doesn't give the people leading questions.
So I am trying to build this network, not only to get information about the manatees but to collect the samples so that we can start to analyze things like population size, what they are eating, and what they need to be protected.
Find out more:
In search of Mami Wata, Lucy Keith Diagne's research blog
West African manatee research and conservation, Sea to Shore
Collaborative regional network for West African manatee research, Sea to Shore