By Paul Donovan
You could be forgiven for thinking that a country whose land mass is comprised of 84% of the Kalahari Desert, and is arid as a piece of sandpaper, you’d be hard pressed to find anything in the way of a reasonable amphibian population. And that’s what I thought, until I embarked on an 1100km (684 mile) bike ride from the arid South to the wet North to see just what I could find.
Adapted To a Dry Existence
A lot of frogs and toads that live in the south, where it is dry and sandy, have adapted to living in such a dry environment by burrowing down into the sand to the cooler underlying layers. Emerging at night as the temperature drops, they begin to forage for nocturnal insects.
Interestingly, unlike the majority of frogs and toads, these burrowing species are not reliant upon a water source to survive. They have adapted themselves to live in a more hostile environment, by undergoing several important biological changes. By far the most important of these is the ability to retain water.
An amphibian living in the desert, is a pretty precarious creature as they will dehydrate faster than an ice cream melting on a hot summer’s day. When you unearth some of these frogs and toads, they resemble bloated bags of water. If you pick them up (which you should not do), they then release jets of water from the anus, and begin to shrink like a deflating balloon. This ability is important for the toad or frog, as it prevents them from dehydration, and allows them to remain deep in the ground for days, if not weeks on end while the surface is baking hot. They slow their metabolism right down.
A further important area where biological adaptation has occurred, is with reproduction. In many expanses of their range, water may be a very scarce commodity, and only when the rains come, do many of Botswana’s amphibians have a short window of opportunity to breed. Come the rains, males and females will congregate in temporary pools of water and begin to spawn. The eggs of some species may hatch in as little as 24 hours, and metamorphosis (transformation from an immature form to an adult form) can occur in a matter of a few days.
My First Find
The first frog I found was a species that I knew well, as I find plenty of them around my home, the Grey Tree Frog, Chiromantis xerampelina. This is quite an interesting frog, because it changes color. When found in a dark environment they take on, as the common name implies, a grey color. However, move them into the light, and within a few minutes they have turned snow-white. Put them back in the dark, and they revert to their grey color. I know of no other amphibian in Botswana with the ability to change color like that. The behavior is likely used for defence.
Another species known to me is the Red Toad, Schismaderma carens. I covered this species in depth in an article in the April 2021 issue of HM. Occurring in grasslands and savannah areas, it is known to forage far from water. During the day it holds up in burrows, or in the hollows of trees where the temperature is significantly lower, and humidity higher, thus preventing dehydration. It has a range extending from South Africa, through Botswana into Zimbabwe. It is quite a large toad reaching a length of seven to nine centimeters.
Females can be identified by their reddish brown coloration and two prominent black spots on the lower region of the back. Males, in contrast, are a greenish grey with a dark line running down the flanks, the throat is wrinkled and the back is granular. The toes of both sexes are webbed.
During the evening when males sing to attract a mate, they have a deep croaking call. The call is particularly noticeable following a bout of rain (during the day or evening) when the males sing as a chorus.
Mating typically takes place in deep water during the rainy season. The tadpoles are quite large, reaching three centimeters in length, and congregate just below the water’s surface in football-sized clumps.
One area where this toad is frequently encountered, is in the urban garden. It is attracted there by poolside lights or lit windows where insects congregate. Although it cannot climb like Chiromantis xerampelina, it takes insects which fall to the floor such as beetles. It is considered the gardener’s friend for it preys on a great many garden pests. Wake up in the morning and it is not uncommon to find several toads taking a swim in the swimming pool.
A Natural Pregnancy Test
The African Clawed Frog, Xenopus laevis, is a rather obscure looking, somewhat flattened representative of the family Pipidae, and is a commonly kept frog in the hobby – in its albino form. It reaches a length of eight to thirteen centimeters, and extends from South Africa into Central and Eastern Africa where it occurs in lakes, swamps, ponds and slow-flowing rivers.
It has a rather broad, flattened body with powerful hind limbs and strongly webbed rear feet. The head is small and flattened with a wide mouth. This profile allows them to secrete themselves into the tightest of gaps.
Simply referred to as Xenopus, this frog was once used as a test to determine whether a woman was pregnant or not. The patient’s urine was injected into the frog, for which the frog would react to the hormones in it. Within a few hours of being injected, if the woman was pregnant the frog would begin to spawn. If she was not, no spawning would take place.
As with all Pipidae, Xenopus is a tongueless frog. What this means, is they do not have an extendible tongue with which to capture their prey as other frogs and toads do. Xenopus is a totally aquatic frog that is common to bodies of water with large amounts of vegetation, and muddy bottoms. During the rainy season, they may move from one water source to another by ‘swimming’ across open ground. They are supremely adapted to an aquatic lifestyle. Their natural greyish or brownish color with dark web-like markings on the back, help blend them in on the muddy floor. And, with their bulging eyes set high on the head, they are able to detect predators approaching from above while remaining concealed. Furthermore, running along either side of the flanks is a line of white ‘stitch-like’ markings. These ‘stitches’ house sensory organs that pick up the vibrations of approaching predators, or prey.
With the aid of the small forelimbs which are furnished with three elongated clawed toes, any food straying close enough is quickly shoveled into the mouth. These frogs have ravenous appetites and will take worms, aquatic invertebrates, tadpoles and small fish.
Female Xenopus can be identified by their visible cloacae lips that form a tube-like organ. As with other frogs, the male develops a nuptial pad on toes one to three during the breeding season. When a pair locate one another, the male simply ‘bear-hugs’ the female in an embrace called amplexus. During this embrace the pair will swim around performing underwater acrobatics while spawning. Spawning usually coincides with the rainy season.
As the eggs are released, they slowly sink to the bottom of the river bed where they embed themselves in the muddy bottom. Within two or three days the eggs will hatch into fish-like tadpoles bearing long threadlike gills. Metamorphosis takes five to seven weeks, with females maturing slightly quicker than males. Sexual maturity can be attained in as little as twelve months.
Poisonous Skin
My favorite frog that I come across every so often, was the Banded Rubber Frog, Phrynomantis bifasciatus. It is a beautifully marked frog of contrasting orange and black and reaches a length of 5.5cm (males), 6.5cm (females). By nature it is a nocturnal frog that can be found some considerable distance from a source of water. During the day it lies up in a shallow scrape beneath a rock, log or amongst crevices where conditions are moist. At night it emerges and begins to forage for small nocturnal insects.
I have read a few accounts of this frog being adept at climbing. I’m not sure how common an occurrence this is, as all the individuals I have ever found have been on the ground. I know quite a few of these accounts were based on captive observations and this may not necessarily translate itself to wild individuals. Captive animals often display behaviors that are not indicative of their wild counterparts and often manifest due to being kept in the confinement of a small space.
Known locally as the Fire Frog, due in part to its red coloration, there are several ways in which this frog differs from other frogs. Firstly, it walks rather than hops. But possibly the most intriguing feature is that it can project its tongue to the side of its head in order to capture food; something other frogs cannot do.
The bold markings act as a warning to would-be predators to ‘leave well alone’. When threatened, this frog inflates its body and raises itself up off the ground with its head pointing downwards. The skin bears quite a potent poison. On unbroken skin it causes no harm, but get it in an open wound and it can cause swelling at the wound area, an increase in heart rate, headache and nausea. As far as I am aware it is not fatal, and the symptoms usually subside in a few hours. However, I suspect that it may be a different story if you were to put your fingers in your mouth after handling an individual, or an animal were to eat a frog.
During the dry season, Rubber Frogs take refuge in burrows, or termite mounds. I have even found them in soft sandy soil, meaning they have the ability to ‘dig’ down into the surface, even though the hind feet lack the ‘digging’ structures indicative of other burrowing species such as Spadefoot Toads (Scaphiopodidae)
Breeding coincides with the onset of the rainy season. Males and females will gather around temporary pools of water and begin mating. Up to 1000 eggs may be laid that hatch in around four days, with tadpoles metamorphosing in around 30 days. Should the rains not come, breeding may be offset until the following year.
A Giant Amphibian
Although I didn’t find any adults, I did stumble across ponds full with young African bullfrogs, Pyxicephalus adspersus. This bullfrog is the largest amphibian found in Southern Africa at 20cm. With its stumpy legs, small head and colossal mouth, this bullfrog is essentially little more than a walking stomach. Well, they don’t do much walking, as they spend much of their time shuffled in a depression in the ground where they lethargically wait to ambush anything they consider remotely edible, including; insects, earthworms, rodents, and even small snakes. Young and adult alike, practice cannibalism. I found numerous small individuals with their mouths bursting at the seams while trying to swallow a sibling as big as themselves.
Singers of the Delta
In the Okavango Delta I encountered numerous tiny multicolored reed frogs, or whistling frogs as they are locally known. You become aware of these little frogs as the sun begins to set. They have a repertoire of delightful whistling songs.
As I lay beneath my tarp, I drifted off into dreamland being serenaded by the chirping of crickets, and these wonderful little frogs whistling. The still night air was awash with an astonishing array of different frogs calling. Each species has its own slightly different song, and once you get to know them, you can identify the species. This, to me, summed up what Africa is all about.
I was surprised with the number of frogs and toads that I actually found; unfortunately many of which I have yet to identify. But it just goes to show you never know what’s around until you begin searching for it. And Botswana threw up a few pleasant surprises when it came to amphibians.
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