33 Facts About the Strange Crucifix Frog or Holy Cross Toad
An Odd Amphibian From Australia
The crucifix frog or holy cross toad is an Australian amphibian with an unusual appearance and some intriguing behaviour. The scientific name of the animal is Notaden bennettii. It was given its common name due to the appearance of a cross on its back. This cross is most obvious in the animals that have a light background colour, like the one in the video below. The frog spends much of its life underground and produces a sticky secretion that may be medically useful for humans.
The amphibian is also known as the crucifix toad, the holy cross frog, and the Catholic frog. It has both frog and toad characteristics. The scientific publications that I've read refer to the animal as a "frog", so I'll follow their example. Whatever it's called, it's an interesting animal to investigate. In this article, I describe thirty-three facts about the amphibian that you may not know.
The crucifix frog belongs to the class Amphibia, the family Myobatrachidae (Australian ground frogs), and the subfamily Limnodynastidae. Some researchers feel that the Limnodynastidae category should be classified as a family, not a subfamily.
Distribution, Habitat, and Appearance
1. The crucifix frog lives in parts of Queensland and New South Wales that are dry for most of the year.
2. It's found in semi-arid grasslands and black soil plains that are rich in clay. The ground forms cracks as it dries out and softens during the rainy season.
3. The frog has a round body, short legs, and relatively large eyes. The short limbs reduce the surface area for water loss.
4. The body of the adult is between 1.8 and 2.6 inches in length.
5. Many members of the species have a yellow or light green skin covered with black and red bumps that form a cross shape. The upper part of the cross typically has two bars instead of one.
6. In some animals the background colour is olive green or brown, but the red and black cross is still present.
7. The frog is said to exhibit aposematism, which is the use of bright colours to warn predators that they shouldn't attack.
8. In the case of the crucifix frog, it's not clear why the warning is given. The frog produces a sticky secretion. The secretion may block the predator's mouth and breathing passages when it enters them, it may taste bad to predators, or the frog may be poisonous. As described below, the secretion has properties that make the first explanation seem probable. This doesn't eliminate the possibility that one or both of the other explanations may be correct as well.
9. The Australian Museum recommends that anyone who has handled the frog avoids touching their eyes before washing their hands. The museum says that some scientists who haven't done this have reported "painful stinging and headaches".
10. The frog is classified as a fossorial animal (one that spends most of its life underground).
11. The amphibian buries itself at least one metre deep in the ground. According to some reports, it sometimes hides at a depth of three metres.
12. While it's buried, the frog is encased in a waterproof cocoon (except for its nostrils). The cocoon consists of multiple layers of skin. The cocoon and the depth at which frog is buried prevent the animal from drying out.
13. The animal's body almost certainly has physiological adaptations that enable it to survive. In other burrowing frogs that have been studied, the breathing, heart, and metabolic rates of the animals decrease dramatically during estivation. (Estivation or aestivation is dormancy during hot and dry weather.) Chemical changes within the cells of the animal often occur during estivation as well.
14. The protection from dehydration is apparently very successful, since the crucifix frog survives for almost a year until the next rainy reason. It can stay underground for more than a year if the rain doesn't make the ground sufficiently soft during a season.
15. The percentage of frogs that survive the underground period is unknown, but the population of crucifix frogs isn't believed to be in any trouble.
16. The IUCN (International Union for Conservation of Nature) classifies the animal in its "Least Concern" category. The last population assessment was done in 2004, however. The animal's status badly needs to be updated.
Life Above Ground
17. When the ground has absorbed enough water from rain to become soft, the frog sheds and eats its cocoon, emerges from its hiding place, and searches for a temporary pool of water that has formed in a depression.
18. Some insects take advantage of the temporary pools that form and reproduce there. The frog feeds on the insects and larvae in and around the pools, especially ants and termites. It also eats other small invertebrates that it discovers. The first video in the article shows the frog catching insects.
19. "Pedal luring" has been observed in captive animals when they are fed live crickets. The frogs wiggle their toes to attract the insects, as shown in the video below. This behaviour has also been observed in some other species of amphibians.
20. The frog completes its life cycle rapidly before the land dries out again. The amphibian is active for six to eight weeks. It then returns to its dormancy underground.
On their back foot they have an extra, little hard shovel-y bit that they use to dig backwards into the ground once it starts to dry out again.— Jodi Rowley, Australian Geographic (in reference to burrowing frogs)
21. To attract a female, a male floats on the surface of a pool with his body and legs spread out.
22. He then emits a call that is described as a "hooo" sound. It's said to resemble an owl's call.
23. A female is attracted by the male's call and releases her eggs into the pool. The male fertilizes them with his sperm.
24. The tadpoles that develop from the fertilized eggs grow rapidly and then metamorphose into adult frogs.
25. After eating as much food as they can, the new frogs go underground as the environment starts to dry out.
The video below shows a crucifix or holy cross frog at the start and a burrowing frog with the scientific name Cyclorana novaehollandiae afterwards. The latter species is found in Queensland and New South Wales and is shown in the frame below.
Frog Glue: A Potentially Useful Adhesive in Medicine
26. When the crucifix frog is disturbed by humans or predators, it releases a sticky substance known as frog glue from its skin.
27. The glue may have two functions. It traps insects, which serve as food. When the frog sheds its skin (as the amphibian does periodically), it eats the glue and the trapped insects. The secretion may also repel predators.
28. Researchers have found that the secretion quickly forms a "tacky elastic hydrogel" that is rich in protein. The hydrogel is pressure sensitive and is sticky even when wet.
29. The researchers have discovered that the glue repairs meniscus tears in sheep more strongly than current protein-based adhesives used in medicine.
30. They've also found that tendon-attachment tear repairs in animals are approximately doubled in strength when the frog glue is used compared to conventional treatments.
31. Some researchers have found that the secretion contains "possibly toxic" components as well as potentially useful ones. Most sources say that the glue is non-toxic, however. The conflicting claims need to be resolved.
32. The glue may be irritating to mammals even if it's non-toxic. Scientists injected small pellets of the glue under the skin of mice. The pellets were gradually absorbed by the bodies of the mice, apparently harmlessly. The injection sites showed initial skin necrosis (cell death), but the mouse skin quickly repaired itself.
33. It's unlikely that frog farms containing animals producing the skin secretion will be established. Scientists may be able to create a similar and safe medical adhesive for humans by studying the frog product, however.
Burrowing frogs are very under-recognised by the public and they’re also incredibly poorly known by biologists because they’re so hard to study.— Jodi Rowley, Australian Geographic
A Somewhat Mysterious Animal
Not as much is known about the crucifix frog as might be expected, perhaps because it spends so little time above the ground. Most people rarely see the animal, even when the amphibians live near them. They are curious creatures that may have much to teach us.
The role of the sticky skin secretion in the amphibian's life and the composition and properties of the secretion need to be further studied. It would be wonderful if improved medical adhesives could be created based on the study of the frog glue. The amphibian is interesting in its own right and because of its potential value to us.
- Crucifix frog information from the Australian Museum
- Facts about the frog from the Australian Geographic website
- Information about the glue from the frog from ABC Catalyst (An Australian Broadcasting Corporation site)
- An adhesive secreted by Notaden bennettii (Abstract) from Springer Publishing
- Biocompatibility of the adhesive (Abstract) from the Wiley Online Library
- Notaden bennettii entry from the IUCN
- Information about burrowing frogs from Australian Geographic
© 2019 Linda Crampton