Frogs have generations too? Not every frog gets to be your son; some are your grandpa frog

Summer has arrived, and the sound of frogs is rising. I wonder if you've been flooded these past couple of days by the World Environment Day"Travel Frog Charity Journey"? The hit Travel Frog from the start of the year traveled to China and met all kinds of endangered animals native to China --

for example, the Chinese horseshoe crab (Tachypleus tridentatus), which Species Calendar once introduced:

……

The frog "son" -- is there some misunderstanding here?

The introductions to the native endangered animals in the game all come from Species Calendar, so I won't go into them here. Let's take an extra look at the frog "son," originally considered unremarkable, that has once again drawn everyone's attention.

Wait a minute,who are you calling "son"? Who are you calling unremarkable?

As the largest group among amphibians, frogs number nearly five thousand species worldwide, and they display their adaptation to different habitats in all kinds of amazing ways. Some species have even lived long enough to become...your grandpa frog, for example the poison frogs.

An example of adaptive evolution: Wallace's flying frog (Rhacophorus nigropalmatus) in the rainforests of Southeast Asia can glide between branches using the webbing on its feet when it encounters danger or forages for food. It is named after the great naturalist Alfred Wallace. Image: endangeredliving.com

When we talk about poison frogs, the first thing that comes to mind is certainly the roughly 170 species of the family Dendrobatidae.Poison dart frogsdo not synthesize toxins themselves, but insteadobtain toxins from the arthropod prey they capture and accumulate them, and they develop bright body colors as warning coloration to tell predators that you're always going to be you and they can't eat them.

The most toxic species, the golden poison frog (Phyllobates terribilis), has alkaloid toxins on its skin surface with a subcutaneous lethal dose for mice of only about 0.1 microgram. Image: Brian Gratwicke / Wikimedia Commons

This inevitably brings to mind the "no longer than two cun, whole body crimson as blood" creature from Demi-Gods and Semi-Devils --the Manggu Zhuha, the so-called "king of ten thousand poisons" -- Zhong Ling's pet marten took one taste and died on the spot.

But there is a problem here: aside from being "unfriendly to foodies," the Manggu Zhuha can also "spray poisonous mist" to actively attack enemies, something no poison dart frog, however toxic, can do.

In biology, the phenomenon of being full of toxins oneself, like poison dart frogs, is called"poisonous"(poisonous), while being able to inject secreted toxins into other organisms by some means and actively cause harm is what makes one able to"deliver poison"(venomous). A typical example is various venomous snakes. The difference between the two is clear at a glance in the figure below:

That is, the difference between "you bite me and you die" and "I bite you and you still die." Image: snakebuddies.net

Being "poisonous" is nothing rare -- you've surely heard the story of indigenous peoples using the venom of poison dart frogs to coat their arrowheads, and it is precisely for this that the family Dendrobatidae is famous. Butgrandpa frogs that can "deliver poison"are rarely heard of.

Are there really frogs that can deliver poison?

So far, only two species among the Anura (that is, "frogs") have been confirmed as "venomous": Greening's frog (Corythomantis greeningi) and Bruno's casque-headed frog (Aparasphenodon brunoi).

A is Bruno's casque-headed frog, B is Greening's frog. Image: Jared et al. / Current Biology (2015)

These two frogs come from the eccentric South American continent,and like the Japanese tree frog, the protagonist of Travel Frog, they belong to the tree frog family (Hylidae) within the Anura。Bruno's casque-headed froglives in the tropical / subtropical moist broadleaf forest (Restinga) along the Atlantic coast within Brazil, and like other species of the same genus it dwells in the water pools at the center of tank bromeliads; whileGreening's froghas adapted to the semi-arid savanna region (Caatinga) of northeastern Brazil, relying on seasonal ponds during the rainy season to reproduce.

Bruno's casque-headed frog perched in a tank bromeliad. Image: Instituto últimos Refúgios / iNaturalist.org

Both of these frogs are, in the truest sense,strangely built in the bones-- compared with the round-headed "croaking babies" we usually picture, these two have flat heads, upper jaws protruding forward, and skin that has ossified and fused with the skull. The genus name of Bruno's casque-headed frog contains the Greek root spheno, meaning "wedge," perhaps because of the shape of its head; while the genus name of Greening's frog begins with corytho, meaning "helmet." In fact, the heads of both frogs are "protruding in front and raised behind," forming a disc-like structure.

An ancient Greek Corinthian helmet, very fitting for describing the casque-headed frogs with their "rebellious bone at the back of the head." Note the two protrusions at the rear of the casque-headed frog's head, forming the back half of the "helmet." Left image: Matthias Kabel / Wikimedia Commons; right image: Instituto últimos Refúgios / iNaturalist.org

What evolutionary significance do such skull specialization and shape have? Casque-headed frogs have"burrow-guarding"(phragmosis) behavior: in relatively arid environments, Greening's frog likes to hide in burrows, and at this time,the bony "buzz-cut head" facing the burrow entrance can seal the entrance perfectly. First, it serves as defense against potential predators; second, it also reduces water loss within the burrow, helping to keep it moist.

Greening's frog in a test tube will also use its big head to plug the opening. Image: Jared et al. / J. Zool. (2005)

Frogs are not the only ones with this kind of behavior -- the soldier ants of the genus Cephalotes have heads specialized into lids for tree hollows, arguably the pinnacle of burrow-guarding behavior. Image: Alex Wild

Besides serving as a tool for blocking doorways, the big heads of these two frogs have an even more formidable use. The front ends of the heads of Bruno's casque-headed frog and Greening's frog have sharp bone spines, and when these spines pass through the skin they pick up the venom secreted by thevenom glands(granular glands) in the skin -- so actually they are the true "poison dart frogs." When they are bitten by a predator or grabbed by a hand, they desperately swing their heads, trying to stab the bone spines coated in venom into the other's body. This venom delivery mechanism has brought these two "poisonous" frogs into the "venomous club";they have truly achieved "delivering poison"。

Scanning electron microscope images of the skulls of Bruno's casque-headed frog (left) and Greening's frog (right). Image: Jared et al. / Current Biology (2015)

Above: the skin surface of Greening's frog with protruding bone spines. Bottom left and bottom right are electron microscope images of the bone spines (marked with *) of Bruno's casque-headed frog and Greening's frog respectively; the arrow at bottom left indicates the opening of the venom gland on the skin surface, and the letter g at bottom right indicates the venom gland. Image: Jared et al. / Current Biology (2015)

It is worth mentioning that compared with other frogs, theheads of both venomous frogs are extremely flexible, and they can even bow 90 degrees in "silent mourning." This is very likely also a result of their body structure adapting to their lifestyle; after all, a flexibly turning head is beneficial for both burrow-guarding defense and bone-spine attacks.

Frog venom is not to be underestimated

The power of their venom attacks is not to be underestimated: the venom secreted by the head of Greening's frog is twice as toxic as the venom of the local South American Bothrops pit vipers, whilethe venom toxicity of Bruno's casque-headed frog reaches 25 times that of the pit viper, and the median lethal dose of the head venom by intraperitoneal injection in mice is only 3.12 micrograms.

A glance back breeds a hundred poisons. For amphibians with only one cervical vertebra, species capable of this posture are rare. Image: Instituto últimos Refúgios / iNaturalist.org

If we are to discuss the venom-delivery experts among vertebrates, it must be the various venomous snakes among reptiles, which possess "professional-grade" venom glands transformed from salivary glands and connected venom fangs.In the venom of casque-headed frogs, researchers have also found hyaluronidase, common in snake venom,an enzyme that can dissolve the intercellular matrix at the wound site and help the toxin spread.

Although their equipment is rather primitive, these amphibians that "strike enemies with their heads" ultimately arrive at the same destination by different paths as the big venom bosses, and through the power of evolution have become both poisonous and venomous --the king of poison among frogs。

There is a poem that goes

Ribs bearing blades, the ribbed newt,

Head broken, poison flowing, the casque-headed frog.

If caught along the journey,

Deliver poison to defeat the foe and make it home~

This article is a special contribution to Species Calendar, from Species Calendar author @Lu Ping.

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