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Adaptable Cannibals

August 4, 2010

The first good rain of the rainy season is an exciting time and no one gets more excited than the desert Spadefoot Toads. They have been waiting all year for this night. They emerge from their underground aestivation and begin to gather at puddles, pools and playa lakes. Temporary or ephemeral pools are best because they contain no fish or other aquatic predators. Bleating like tiny sheep, the Spadefoot Toads advertise their presence and their willingness to breed. The first night of the rainy season is an orgy of toad sex with thousands of eggs laid and multiple males clamoring over females to fertilize them. By morning all is quiet and only the egg masses left behind are a clue to the previous night’s activity.

This strategy to avoid predators by choosing temporary water presents its own risk. It becomes a race; a race for the eggs to hatch and the tadpoles to develop lungs before the puddle dries completely. In a large playa lake there is no problem but a shallow mud puddle might go dry if no more rain follows the first storm. Spadefoots have adapted well for this strategy, they can go from egg to air-breathing young toad in as little as twenty days. A bullfrog tadpole, in its permanent watery home, may take more than a year to develop lungs. If the Spadefoot Toad tadpole’s puddle is becoming too small and crowded some individuals undergo a remarkable transformation. The normally herbivorous tadpoles grow powerful jaws and begin to feed on other tadpoles to increase their protein intake and accelerate development. Studies have shown they recognize and avoid eating their siblings (unless they’re really hungry). In a larger pond the tadpoles remain on a vegetarian diet.

I once watched a large playa lake the week after the first storms of the rainy season. The first night the Spadefoots were so loud you could not carry on a conversation. By day two, the dragonflies were there in swarms, coursing over the water and laying their own eggs. Shorebirds and wading birds began to arrive and by the end of the day the lake was a fully functioning aquatic ecosystem where 48 hours before had been only dry cracked mud. Dragonfly larvae are voracious predators and consume many tadpoles in the large playa but hundreds more survive to burrow in the mud and await the singles scene next monsoon season.

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