The Keeper or the Kept?
The story behind tarantulas with “pet” frogs
Every now and again a bizarre scientific story captures the attention of the masses. In some cases, these stories seemingly resurface every few years for a bewildered generation. Usually, these waves of intrigue are spurred on by social media trends and the latest story to go viral on TikTok is that of giant tarantulas keeping “pet” frogs. Symbiotic relationships are common in the animal kingdom and therefore, this anthropomorphic tale of two unlikely friends seems reasonably plausible. Is this a far-fetched Pixar-esque an anthropomorphic pipe dream, or is there some science behind the story?
Chubby frog (Kaloula pulchra)
The frog and the spider
The “pet frogs” that are frequently seen in close proximity to tarantulas are generally from a specific group of amphibians called “Microhylidae”. Microhylid frogs are commonly known as “narrow-mouthed frogs.” There are 683 species of these frogs found all over the world. Some are beautifully patterned and grow up to 10cm long while others are barely visible creatures, never exceeding 10mm in length. Some species are frequently kept in captivity such as the very charismatic “chubby” frog (Kaloula pulchra). Most Microhylids are very small, terrestrial frogs that occupy leaf litter in tropical and subtropical forests, although it is almost impossible to summarise the family. Some thrive in arid landscapes, burrowing beneath the soil for large parts of the year whilst others are arboreal with toe pads and breed in tree trunk holes in the forest canopy.
Just as the “pet frogs” are from a very diverse group of animals, so are the tarantulas. Theraphosidae contains around 900 different species of tarantula and they can be found across most warm climates. Although most records of spiders with “pet” frogs focuses on one genus of South American tarantula, the family is extremely broad and convergent evolution is reasonably common, indicating this phenomenon may not be restricted to a particular species or genus.
Symbiosis
The first published observation of a “spider keeping a pet frog” dates back to 1989. Whilst studying entomology in Peru, Reginald Crocraft and Kieth Hambler observed a “close association” between a Microhylid frog (Chiasmocleis ventrimaculata) and a Colombian lesser back spider (Xenesthis immanis). They claimed that the spider seemed to allow the frog to share its burrow, despite being perfectly capable of killing and feeding on the frog. They even observed the spider seemingly moving its legs to allow the tiny amphibian to pass in and out of the burrow. Both species are nocturnal and when the spider would emerge from its hole to hunt, the frog would follow.
Initially, it was thought that the spider was simply ignoring the frog. Since Crocraft and Hambler’s discovery, others have observed young spiders picking up the tiny frogs, examining them with their mouthparts and putting them back down again, suggesting that the frogs may have some form of chemical defence. The frog benefits from the protection of the spider, while the spider must tolerate its new tenant. This form of relationship is called commensalism, where one organism benefits from the other one but neither is harmed. However, further studies suggest that there may be further benefits for the unlikely duo.
It appears that the spiders choose not to eat the frogs as they also offer protection for the spiders’ eggs. The frogs will prey on tiny insects that are too small for the tarantula to fend off. These little bugs (including ants, mites, etc) are attracted by the leftovers of the spiders’ much larger prey but would undoubtedly enter the spiders’ burrow to feed on its egg sac and newly hatched offspring. Because it seems that both species are benefitting from the symbiotic relationship, it is described as mutualism. Across the animal kingdom, inter-species mutualistic relationships are well-documented. In extreme cases, these relationships have prompted some species to become entirely reliant on another species for survival.
In 2002, Jolene Csakany suggested that Crocraft and Hambler’s original description of Xenesthis immanis may be incorrect. The “Colombian” lesser back spider’s range does not stretch as far south as Peru and so Csakany set out to find out which species actually exhibit these strange behaviours. She discovered the black spider referenced in 1989 was possibly an undescribed species of chicken spider (Pamphobeteus spp.). She tested this theory by taking the skin of a humming frog and putting it onto two different bullfrog species (Leptodactylus) (both species that the tarantula would usually prey upon) and discovered that the tarantula would not eat the animal with the humming frog’s skin. This supported the theory that the frog’s chemical compounds within its skin help the tarantula identify friends from food.
Pamphobeteus sp. from Peru
Chiasmocleis ventrimaculata from Peru
All around the world
As the story of symbiotic relationships between Arachnids and amphibians engulfed the curiosity of entomologists and herpetologists internationally, new cases are continuing to be unearthed each year. Perhaps the most startling came in 2008 from Rameshwaran Island, off the southeast coast of India. Researchers reported an “unusual” relationship between another Microhylid, the Sri-Lankan painted frog (Uperodon taprobanicus) formerly, Kaloula taprobanica and a member of the Poecilotheria genus of tarantulas. Any invertebrate keeper will know just how popular Poecilotheria are in captivity. They include the Gooty-sapphire ornamental tarantula (P. metallica), ornate tiger tarantula (P. ornata) and the Indian ornamental tarantula (P. regalis). Poecilotheria is known for being extremely aggressive, arboreal spiders that pack reasonably potent venom. Yet, despite their unwelcoming characteristics, they have been recorded sharing a hole with the rather unassuming bullfrog.
In 2009 Poecilotheria ornata and Poecilotheria subfusca were also reported to seemingly have close relationships with the Microhylid frog Uperodon nagaoi. The frogs and the tarantulas were recorded sharing tree holes, many of which contained eggs and/or juveniles of the spider, the frog or both “partner” species. The researchers found that the Poecilotheria tarantulas would actively protect the frogs’ eggs from predatory geckos (Hemidactylus depressus). As the eggs (and frogs) are ultimately defenceless without the spiders and the spiders’ eggs are defenceless against small insects, there is reason to believe the two species are reliant on one another.
Herpetologists and Entomologists are also startled by the fact that both cases of mutualism are enacted by species from different lineages on two entirely different continents. Other scientists have also recorded some form of association between the Tungara frog Engystomops pustulosus (formerly Physalaemus pustulosus) and tarantulas of the genus Aphonopelma in Mexico. Whilst research into the topic is still developing, every observation indicates that this relationship has evolved multiple times across the world.
Poecilotheria subfusca
