Those who think the only time frogs and spiders end up together is in a witch’s cauldron might be interested to know that there are species from these animal groups that live side by side in the natural world. They occur frequently together, sometimes even sharing the same tree cavity.
This is curious because these spiders are tarantulas—certainly big and aggressive enough to eat these frogs—yet they leave them alone.
The frogs in these associations are microhylids (also called narrow-mouthed frogs), a family of frogs with nearly 600 species. They are not as well-known as other types of frogs, in part because most of them are only half an inch long, but the group is widespread, occurring in Asia, Africa, South America, North America and Australia.
The best studied of the associations between these frogs and spiders occur in Peru, Sri Lanka and India. In those locations, microhylid frogs and various species of tarantulas have been observed living for extended periods of time in the same tree holes.
These frogs and spiders are in a mutualism, which is the scientific term for a relationship that is beneficial to both species. The frogs eat very small invertebrates that are drawn in to the remains of prey that the spiders leave.
The spiders benefit because these frogs specialize in eating ants, which are one of most dangerous predators of the spiders’ eggs. Beyond food, the frogs receive protection from spiders against possible predators.
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Spiders were observed attacking geckos that tried to eat the eggs of the frog sharing their tree hole. Spiders can also offer protection to the frogs against other common predators such as other arthropods and snakes.
There are often eggs and juveniles of both species present. The two species have been seen right next to each other after emerging from these cavities and the spiders show no interest in the frogs. That’s unusual as most large predatory spiders do prey on small frogs. Preliminary work suggests that the spiders identify these microhylid frogs by chemicals on their skin that are produced in poison glands all over their body.
Those chemicals are unpalatable to the spiders and may be toxic to them if ingested. (Experiments have shown that the skin secretions are toxic enough to kill mice.)
In the original description of one of these frog-spider associations, scientists reported seeing young spiders pick frogs up, examine them with their mouth-parts and then let them go without hurting them. The unpalatability of the frogs to the spiders may explain how such a close association developed in the first place. The unacceptability of frogs as prey for the spiders could have allowed the two animals to be near each other without the frogs becoming dinner.
At Halloween, we often see odd couples among the trick-or-treaters. Perhaps Captain Marvel appears at your door accompanied by a butterfly, or an ear of corn shows up with an astronaut. It’s no stranger to see those weird pairings than to see spiders and frogs be best buddies.