The world is full of intriguing paradoxes. Does this article still exist when you’re not reading it? If I travelled back in time and killed my grandfather, would I still be writing this now? And how can two socks go into the washing machine, but only one sock comes out? Perhaps the biggest paradox, however, is how can a frog get smaller as it grows up?
Life starts, pretty much as expected for the paradoxical frog (Pseudis paradoxa). The female lays fertilised eggs in the lakes and lagoons of South America and Trinidad. Eggs hatch, tadpoles emerge and then start to eat.
The voracious larvae feed mostly on algae and very soon, start to bulk up. They start small, like other tadpoles, and grow at a similar rate to their non-paradoxical cousins.
If the conditions are right, however, the tadpoles just keep on growing. Bigger than a blueberry. Bigger than a strawberry. Bigger even than a plum. Think ‘satsuma stuffed inside an ankle sock,’ and you’re approaching the right dimensions … and you’ve found your missing sock.
With a chunky, rotund body and a long, muscly tail, paradoxical frog tadpoles can reach lengths of up to 22cm (8.6in). To paraphrase the movie Jaws, we’re going to need a bigger jam jar!
This is around three times bigger than the adult frog the tadpoles will metamorphose into, and most of the development needed to become an adult frog is already complete.
By the time they transition from tadpole to frog, males have well-formed testis and can already produce sperm, while females have the makings of mature eggs.
This is different to the tadpoles of regular frogs, which take time to become sexually mature after metamorphosis, during the juvenile frog stage of the life cycle.
So, how do the giant tadpoles turn into such little frogs? Easy! At least half of the paradoxical frog’s tadpole is tail. Lose the tail and the paradoxical tadpole turns into a normal, if relatively diminutive adult, with a body length of around 7cm (about 2.5in).
This causes the amphibian to ‘shrink’, explaining why it’s also known as the shrinking frog.
Paradox resolved. But just as one conundrum is cracked, another emerges: why does the tadpole put so much energy into becoming so big in the first place?
One possible answer relates to the place and timing of their birth. Paradoxical frogs time mating to coincide with the arrival of the rains.
In Trinidad, this happens around May.
Some eggs are laid in permanent pools, while others are laid in more temporary ponds that will later dry up. Those laid in smaller, transient waters, with less food or more aquatic predators, don’t get to grow so big. But those laid in larger, more permanent ponds, with more food and fewer predators, do.
In this situation, being big may aid survival, as the fish and other animals that predate smaller tadpoles are less likely to take a bite.
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