Nature Notebook - Dar es Salaam, Tanzania
Vernal pool in May, Dar es Salaam, Tanzania
I have a vernal pool in the middle of my pocket-sized nature reserve. I made it about 20 years ago by digging a hole about two meters deep and three meters wide. It is dry whenever the ground water level sinks below two meters, which is for about eight months of the year — mid-July to mid-March. When it is dry, the depression fills with green Commelina; the tiny blue flowers of the Commelina attract butterflies. But a few weeks after the Masika rains begin, the water table rises, and the depression fills with water and becomes a pool.
Because it is dry most of the year, fish do not live in it. It becomes a perfect receptacle for amphibians’ eggs and larvae (mayai na viluilui wa vyura). Four months is enough time for a generation of many species of frogs to mate and lay eggs, time enough for their tadpoles to mature and emerge as tiny perfect froglets before the pool dries up.
As the water fills the depression, the Commelina that grew there before, rots in the water. This nutritious green slime becomes food for young tadpoles. The grasses on the other hand do not rot and are not eaten by tadpoles.
It has been pouring rain for the last few weeks in the coastal regions, and the vernal pool is full of water. When I go to see what’s happening, I see at least two different types of tadpoles in the pool: a small dark one and a large plump one with a white belly and a tail mottled with white, tan and brown.
My eye is drawn to the larger ones, which are about five centimeters long. They are several weeks out of the egg. Some of them have already grown their back legs. They rarely rest on the surface of the water — that would be too easy for any passing bird. They hover in the water below the surface until they need air. Then they move their tail in a sinous “s” motion up to the surface of the water, lift their mouth and sip in some air. They are slow moving, not expending any extra energy, up and down, up and down again.
Hemismus marmoratus tadpole in an aquarium, reflecting on itself
Last year I took some of them into an aquarium and followed them until they became baby frogs. So I know that these are Hemisus marmoratus or Marbled Snout-burrowers. They are common in grasslands in Equatorial Africa. As adult frogs, they are not particularly beautiful. Their snouts are pointed and hardened for digging. Their bodies are round, and their legs are short. When you see them half covered with wet soil, you can hardly tell the difference.
Hemismus marmoratus, young adult
This year, when the Masika rains were just beginning, I looked for eggs at the beginning of the season around the vernal pool. I couldn’t find them, so I turned to the book “Amphibians of East Africa,” by Alan Channing and Kim Howell (UDSM Faculty of Biology and Marine Sciences). There I learned about the part of their life cycle I have not seen.
They wrote, “Breeding takes place during the short rains. The males clasp the females and then are dragged into a burrow, with the female digging. The females lay the eggs in a burrow or under a log or stone and remain with them. About 150-200 eggs are laid in a compact mass…Many empty capsules protect the top of the clutch. Burrows are usually found in wet soil under shade of vegetation or leaf litter, or in the roots of wild bananas, a little back from the water. Continuing rains cause the water level to rise to the level of the tadpoles, which liberates them. Tadpoles appear to be able to develop either when left in a moist mass or when in water. When the water does not rise high enough to allow tadpoles to swim out into the pond, the female may carry tadpoles to the water on her back, or they may follow her by swimming on the wet mud.”
Isn’t that fantastic? Frogs care for their young!
But what is even more fantastic is how the pieces of the ecosystem — the seasons (Masika), the winds bringing the rain from across the Ocean, the way the ground water flows, the plant (Commelina) — are manifested in the Marbled Snout Burrower.
Zee-ee-eet zeeeeeet. That is what you hear if you sit here at the pool quietly for a few minutes. Unless they are in a mating fervor on a moonlit evening, when they call so loudly and steadily it can give you a headache, Marbled Snout Burrowers are timid. They hide under vegetation. If you stay still, the adults regain their courage and call out again. Zeet zeet. Zeet zeet. That is the tadpoles’ parents on the land side, under the vegetation. Zeeeee eeet.
by Anne H. Outwater PhD, RN,
World Learning Advancing Leaders Fellow
Anne Outwater has lived and traveled widely in Tanzania since 1989. She is head of the Department of Community Health Nursing at Muhimbili University of Health and Allied Sciences. Her garden in Dar es Salaam is a pocket nature reserve certified by the National Wildlife Federation. She has been writing Nature Notes since 1990, and many of them emerge from the nature reserve.