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Bronberg: African Bullfrog haven

After the heavy hailstorm on old year’s eve, the shallow dam filled up quickly.  The dam had stood dry for many months, but now the deafening chorus of frogs’ mating calls filled the air. Amongst the din, the deeply resonant baritone of the African bullfrog (Pyxicephalus Adspersus) was unmistakable.

Two days later thousands of bullfrog eggs (often from several females) hatched on a shallow rocky plate along the edge. At Bronberg Bewaria, we are lucky to have just the ideal habitat and breeding site for this locally endangered species. Although these giant bullfrogs are extremely adaptable amphibians, in Gauteng they barely survive, while their grassland habitat and especially their wetland breeding sites are destroyed by development. For successful breeding, they are completely dependent on these shallow grassy pans, that fill up with water during the rainy season. Permanent dams with predator fish or even reed-lined water bodies are no good for breeding. The frogs leave their underground estivation chambers (10-30 cm deep) after the ground is soaked by about 50 mm of rain. Within 200 m. from the breeding site, there are several such chambers, where the bullfrogs stay in estivation for periods of more than 8 months at a time.

It is known that they can survive prolonged periods of drought in these shelters, while surface temperatures can be baking hot. In these chambers they shed their outer skin which turns into a tough cocoon in order to preserve body fluids. They also rely on their bladders to re-absorb water.

Their metabolism virtually shuts down, waiting for that rainy trigger.

After emerging from their chambers, the bullfrogs hasten to the breeding site where instinct tells them enough water has accumulated by now. We once, after a prolonged shower, stood guard at the site of a (known) estivation chamber and watched the large male emerge from the ground. We gave him a free ride to the dam in a bucket.  (Some of these males can live for more than 35 years and weigh up to 1.5 kg!)

The males try to establish a territory as quickly as possible. In nature, breeding is a serious matter and there is no time to lose, as the water might dry up too quickly. The water should last for 3 - 4 weeks to give the tadpoles a chance to metamorphose.

In our dam, it’s usually a winner takes all contest. The males fight fiercely for dominance, which sometimes results in serious injury or even death. Since 1976 we only witnessed once that, in a remote corner of the dam, a second male managed to attract a female and raise a small brood. As a rule, the females are mated by a single dominant male.

After mating, the male guards the eggs and then moves around with the tadpoles along the grassy areas of the dam, where the tadpoles voraciously graze on algae. Any terrapin, heron, hammerhead or cormorant that ventures too close (they are attracted by this writhing black mass) gets attacked by the male. We also occasionally saw a few other males hanging around and following the tadpoles from a distance in the hope of snatching a meal. Many visitors, who ventured too close got the fright of their life when the male suddenly hurled himself at them. If he manages to grab you, he is capable of drawing blood with his strong bite (he uses tooth-like projections on his lower jaw). It is quite ironic, that this male, who fiercely defends his brood against any predator, also snacks on his own tadpoles in order to sustain himself during his long vigil. It’s a necessary sacrifice in order to see the bulk of his brood turn into little green striped frogs.

When a pale green stripe becomes visible on the tadpoles’ back, metamorphosis is in full swing. It’s just plain amazing how fast the internal tadpole matter is re-arranged into frog organs and frog features become noticeable. Within a couple more days they are ready to leave the water.

Sometimes, the water in the dam evaporates too quickly which spells certain death for the tadpoles. In such a case, the local benevolent “Rain Gods” help out by feeding additional borehole water in the dam so that metamorphosis can be completed.

When the little frogs start swarming, it’s a long perilous slog uphill for them. We have seen some of the little frogs being cannibalized by the larger ones. Anything that moves and fits into their mouths is fair game. They put on weight as fast as during their tadpole phase, yet very few survive that first year. After a few weeks have passed, we seldom encounter a little frog hopping in the grass. There must be close to 100 % mortality. This is typical for animals that follow a so-called “r-strategy”.  Flood the whole show with thousands of eggs and hope that a few offspring will survive (animals, such as most mammals, whoheavily invest in raising only a couple of offspring follow what biologists call a “K-strategy”).

If there is sufficient water in the dam and it’s still not too late in the season, the male bullfrog often initiates another attempt at breeding. We once witnessed 3 successive breeding rounds in this dam by the same
male. This is surely the maximum that is possible and it is doubtful that the last batch of frogs had sufficient time to eat and fatten up before the onset of winter. We sincerely hope that the conservation of our Bronberg grasslands and breeding site will help these remarkable amphibians, against all odds, survive in the  Gauteng region.

 

Bullfrogs at Bronberg

 

 

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