How do tree frogs stick to things


Sticky Business: Tree Frogs Hang Tight--But How?

Like wall-hugging geckos, tree frogs are capable of gravity-defying feats of the feet. But new research shows that the two species cling to surfaces in markedly different ways.

The "dry" grip of geckos relies on molecular bonds—firm but easily broken—between tiny fibers in the animal's toe pads and the surfaces on which they stand. But scientists found that frogs use a different approach to hold on.

Biologist Jon Barnes of the University of Glasgow in Scotland, who led the research, used an atomic force microscope (AFM), which can provide images on the scale of billionths of a meter, to scan the feet of White's tree frogs. To the naked eye, the frogs' toe pads appear patterned with flat-topped, hexagonal cells surrounded by grooves filled with mucus. On closer inspection, however, Barnes discovered that the tops were not flat at all but rather were covered by tightly packed "nanopillars," each with a small dimple in the end, which generate powerful friction against the surfaces they contact.

"The AFM can also be used to measure the stiffness of the outer layer of the foot," says Barnes, who published the findings in The Journal of Experimental Biology. "It turns out to be of the same order as silicone rubber. Soft materials are important, for they allow the pad to achieve close contact, following the contours of the surface to which the frog is adhering."

Although mucus can be a lubricant, for tree frogs the substance—only 1.5 times more viscous (resistant to flow) than plain water—serves as a "wet" adhesive. The reason: the nanopillars and larger structures on the toe pads come in direct contact with surfaces. As a result, the small amount of wet mucus between these protrusions provides adhesive forces.

Tree frogs can climb most surfaces, from sheer leaves to glass, with ease, although they do not fare so well on dry, rough materials—presumably because they cannot produce enough mucus to create a continuous fluid layer beneath their pads on such a surface, Barnes says. "In support of this idea is the fact that adhesion dramatically improves if the rough surface is wet," he notes.

Walter Federle, a zoologist at the University of Cambridge in England who studies adhesion, says the study sheds light on the material properties of frog toes at the microscopic level and clarifies that nanopillars play "an important role in adhesion." But he notes that the exact function of these tiny columns is still unclear.

Research on both geckos and tree frogs has tantalized materials scientists with visions of smart adhesives for human applications. For example, a paper in the March 2008 issue of the Journal of the Royal Society Interface estimated that a car brake equipped with a modest patch of  synthetic gecko-grip could stop a 2,200-pound (1,000-kilogram) vehicle traveling 50 miles (80 kilometers) per hour in about 16 feet (five meters).

Barnes and his colleagues believe understanding the adhesive properties of tree frog feet could lead to better tire design, and perhaps even a nonslip shoe, although they first need to demonstrate that the adhesion—and, equally important, the rapid disengagement from the surface—is maintained on structures much larger than an amphibian's toe. Another possible application of the work, Barnes says, is the creation of a coating to protect nerves during surgery by holding them delicately out of the way of the scalpel.

Friction helps frogs stick to ceilings

By Roxanne Khamsi

Hypsiboas boans in the wild

(Image: Lynsey Allen)

Researchers filmed the frogs hanging upside-down in a transparent container

(Image: Jon Barnes)

Tree frogs rely on the joint efforts of friction and adhesion to stick to a vertical or ceiling-type surface, new videos reveal.

While the creatures rely on their sticky toe pads to hold them up, new footage of the frogs walking upside-down reveal that they increase their grip through friction, by adopting a spread-out stance similar to that of the famous comic-book crime-fighter, Spider-Man.

The method that frogs employ to stick to a ceiling or vertical tree trunk is different to the Velcro-like foot bristles geckos use, the researchers discovered.

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Tree frogs use wet adhesion to cling to surfaces – the animals secrete a thin layer of sticky mucus to coat their fleshy toe-pads. According to Jon Barnes at the University of Glasgow in the UK, and colleagues, many believe that this layer of mucus prevents friction from forming between the frogs’ feet and a vertical surface – such as a tree trunk – or anything steeper.

Frog flipping

To address the puzzle, the researchers placed tree frogs such as Litoria caerulea in a revolving, transparent container. The aim was to measure the amount of friction between the toes and the glass plate as the angle of the surface steepened, eventually turning upside-down.

The animals adopted a spread-out stance, keeping their legs parallel to a surface, as the floor of the container on which they stood rotated to become the ceiling. Watch a video of a large Hypsiboas boans frog, weighing in at about 50 grams, as it tries to cling to the tilting surface. The researchers are announcing the angle of tilt throughout (4MB, wmv format).

The film shows that the mucus layer is actually thin enough to still allow some friction to occur, says Barnes, helping the animals to stick to a surface.

Desperate situations

“This study of behaviour in desperate situations also brings out an important role for friction, even at these extreme angles – near 180° or upside-down,” says Barnes. “Normally, one would expect friction to be important on the flat and on slopes approaching 90°, but useless beyond the vertical as the frog would simply fall off the surface. Our experiments now show that – when combined with adhesion – friction plays an important role.”

The new research highlights how the toe-pads of tree frogs function much like sticky-tape, which is easier to peel at a perpendicular angle than by pulling at one end.

Geckos, also famous for their climbing skills, use dry, microscopic foot bristles that act more like Velcro. These tiny foot hairs rely on weak attractive forces between molecules to work.

Barnes presented the research at the annual meeting of the Society for Experimental Biology earlier in April 2007.

Snub-nosed tree frogs protect themselves from predators with auditory illusions • Alexander Markov • Science news on Elementy • Ethology, Zoology

Mating calls, such as male crickets or frogs, often attract predators as well as females. An original solution to this problem was found by males of the snub-nosed tree frog Smilisca sila living in Central America. They synchronize their songs: the first person to sing is joined by others almost instantly. As it turned out, in this way the "singers" protect themselves from enemies - bats and blood-sucking mosquitoes, guided by the sound of mating songs in search of victims. Experiments have shown that mosquitoes and bats are subject to a well-known auditory illusion, which is that when two identical sounds are heard almost simultaneously from different directions, it seems to the listener that there is only one sound source, and it is located where the first sound came from. Females Smilisca sila are not subject to this illusion, unlike predators and females of other frog species, which are not characterized by song synchronization. Thus, sing-along males, by attaching themselves to the song of a congener, not only escape from enemies, but also do not reduce their chances of successful reproduction.

Many animals under the influence of sexual selection have developed various and sometimes very bizarre mating signals that serve to attract sexual partners. However, the evolution of mating signals is directed not only by sexual selection, but also by ordinary natural selection for survival. The fact is that the animal that gives the signal becomes more noticeable not only for relatives of the opposite sex, but also for predators and parasites, many of which, in the course of evolution, have learned to find their victims precisely by mating signals. Under the pressure of such enemies, some animals in the course of evolution can change their mating signals, making them less noticeable (see: A simple song helps birds avoid predators, "Elements", 06/27/2016), or even completely abandon them (see: What is it better to leave more offspring or save your life?, "Elements", 12.12.2012). In this case, of course, the main function of the signal suffers - the attraction of sexual partners. In theory, evolution in this case should strive for compromise solutions, trying to make the signal less visible to predators and parasites, but still attractive to potential partners.

A study by American scientists published in The American Naturalist showed that pug-nosed tree frogs Smilisca sila living in Panama, Costa Rica and Colombia, our original way of solving this difficult dilemma. As it turned out, male S. sila deceive enemies - bats and blood-sucking mosquitoes - with the help of an auditory illusion. The most interesting thing is that females S. sila are not subject to this illusion, and therefore the reproductive success of cunning males, most likely, hardly suffers.

Researchers were interested in the S. sila males' characteristic way of singing in chorus, more precisely, starting the song just a few tens of milliseconds after the first male sings (typical delay time is 79 ms). The “choral singing” of males S. sila consists of short rounds of friendly croaking, separated by long pauses. The impression is as if no one wants to vote first. Everyone is waiting for someone to lose their patience, but as soon as one male breaks down and starts to sing, the rest immediately join in.

Scientists have suggested that such synchronization of mating signals may be beneficial to “singalongs” due to a well-known auditory illusion, which is called the precedence effect (or priority effect, see: H. Wallach et al., 1949. The precedence effect in sound localization). The essence of this effect is that when a person or another animal hears two identical sound signals that sounded almost simultaneously, this is perceived as a single signal that came from where the source of the first (leading) signal was located. It is assumed that the precedence effect may be an adaptation that helps animals to more accurately localize the sources of sounds, without confusing them, for example, with an echo.

Snub-nosed frogs have two eternal enemies that find males S. sila, based on their mating signals: bats Trachops cirrhosus and mosquitoes of the genus Corethrella that suck blood from frogs (Fig. 1). The hypothesis of the authors was that bats and mosquitoes, due to the precedence effect, should mainly attack those males S. sila that sing first. "Singers", thus, get an advantage.

To test this, two speakers played S. sila mating signals to bats and mosquitoes, one of which started sounding 79 ms earlier than the other. At the same time, the bats were in spacious cages, where speakers were placed at a distance of 1.5 m from each other. It was believed that the mouse "selected" one of the speakers when it flew up to 50 cm to it. Mosquitoes were caught at night in the forest with two traps installed near the speakers, and in the morning, scientists looked at which trap caught more mosquitoes.

Similar experiments were carried out with females S. sila , as well as with females of another frog species, Engystomops pustulosus (Tungar frog), whose males do not synchronize their songs (Fig. 2). In the latter case, mating signals E. pustulosus were used as "bait". The researchers caught mating pairs (see: Amplexus), separated the female from the male and quickly tested her (female frogs in this situation are especially susceptible to male mating signals). After the experience, the female was reunited with her cavalier and both were released into the wild.

Experiments have shown that bats, mosquitoes and females E. pustulosus in most cases choose the speaker from which the “leading” signal is heard. 10 bats were tested, each tested 10 times. The leading signal was chosen in 70 cases out of 100. Traps near the "leading" speaker caught an average of 32 mosquitoes per night, and traps near the "lagging" speakers - 20. The leading signal was also chosen by 36 of 40 tested females E. pustulosus . These results suggest a statistically significant preference for the leading signal in all three cases. Thus, the precedence effect works on all three views as expected.

The authors were able to test only 23 S. sila females, of which 15 preferred the leading signal, while the remaining 8 went to the "lagging" speaker. From a statistical point of view, this does not allow us to reject the null hypothesis of no preference. Moreover, the data obtained are sufficient to state that females E. pustulosus significantly outperform females S. sila in the degree of preference for the leading signal. Previously, similar experiments have already been carried out with several other species of frogs with non-synchronous singing, all of which showed a very pronounced effect of precedence. The authors note that if this effect were to the same extent in snub-nosed frogs as in other species tested (including E. pustulosus ), then not 15, but at least 20 out of 23 females would choose the leading signal.

Thus, the results are consistent with the hypothesis that male snub-nosed frogs try not to start the song first, but to follow others in order to protect themselves from bats and mosquitoes using an auditory illusion - the precedence effect. At the same time, the reproductive success of singing-along males, apparently, does not suffer too much, because the females of this species are either not subject to this illusion at all, or are subject to it to a small extent (compared to predators and other frogs).

It can be assumed that the weakened precedence effect in females S. sila is an adaptation that increases the female's chances of finding a singalong male. Perhaps it is safer for a female to be friends with such a male if all the mosquitoes and bats really hover around the singing males. In addition, skilled singalongs, who rarely or never start singing first, are likely to have a survival advantage. If this trait is hereditary, they may pass it on to their offspring. This is another possible reason why it may be advantageous for females to choose a sing-along. However, this is still only conjecture, to verify which it is necessary to decipher the genetic basis of mating behavior and auditory perception S. sila and find out how the variability in these traits affects the fitness of males and females. So far, it is not even known who becomes the lead singer in the male choir S. sila : always the same males, in whom this is “written in the genes”, or different each time.

It is more certain that the ability of females of S. sila to successfully find male sing-alongs is an important factor that allows males of this species to defend themselves from predators in such an unusual way. It is unprofitable for males of other species to synchronize their songs, because not only predators, but also females will not be able to find the sing-along.

However, S. sila is not the only frog species with synchronized singing. For two other species with the same feature ( Dendropsophus ebraccatus , see: K. D. Wells, J. J. Schwartz, 1984. Vocal communication in a neotropical treefrog, Hyla ebraccata : Advertisement calls, and Kassina fusca , see: T. U. Grafe, 1999. A function of synchronous chorusing and a novel female preference shift in an anuran) has also previously been shown to lack a precedence effect, or even to prefer females to a "lagging" signal in some situations. It is possible that males of these species also use synchronization to protect themselves from predators, but this has not yet been proven. The authors of the work under discussion were the first to show that prey can deceive predators with the help of an auditory illusion, which are able to find them by mating signals.

Source: Henry D. Legett, Claire T. Hemingway, Ximena E. Bernal. Prey Exploits the Audience Illusions of Eavesdropping Predators // The American Naturalist . 2020. V. 195. P. 927–933. DOI: 10.1086/707719.

See also:
1) What is better - to leave more offspring or save your life?, "Elements", 12.12.2012.
2) A simple song helps birds avoid predators, "Elements", 06/27/2016.
3) Sexual selection creates illusions, Elements, 01/24/2012.
4) Bowerers deceive their girlfriends with the help of optical illusions, "Elements", 07/10/2011.
5) Natalia Reznik. Frog crazy passion.

Alexander Markov

Frog's paws suggested super-sticky material to scientists

Having created a material in imitation of geckos that easily climb walls and ceilings, scientists turned their attention to tree frogs, no less tenacious scouts. The artificial frog foot micro-channels provide 25 times more grip, and its strength can be easily adjusted.

Research by scientists from the Indian Institute of Technology led by Gatak Animangsu led to the creation of a fundamentally new adhesive material. The idea of ​​development, as is often the case, was taken from nature. Scientists are intrigued by a mechanism that allows animals such as tree frogs, ants and flies to crawl up smooth sheer barriers without falling off.

Today, many adhesive materials use a simple and reliable method of gluing to the surface. A substance that has a high "stickiness", or, scientifically, adhesion, is brought into a viscous form. When a force is applied, this substance fills all the voids between the two materials, and the contact area of ​​the two phases becomes much larger than the visible surface area, which creates the prerequisites for a very strong connection. Such a substance is called glue.

However, this method of gluing has its own very significant disadvantages. Due to the high adhesion of the material, an attempt to peel off a tape or sticker leads to a break in the volume of the adhesive mass or damage to the paper sticker, and not tearing one material from another. In addition, adhesives quickly collect various contaminants on themselves, which makes their reuse even more difficult.

It has long been noted that the paws of the gecko, a small tropical lizard, allow it to easily move along overhanging tree branches without falling off. In this case, the animal does not have to make great efforts in order to tear off the paw and take the next step. In addition, the paws of the reptile always remain clean.

Van der Waals forces

- weak forces of intermolecular interaction. Although the phenomenological van der Waals equation is covered in a school course in thermodynamics, the theory of the corresponding intermolecular forces (van der Waals forces) is very complex and is based on...

It turned out that they are covered with thousands of the finest branching hairs. These hairs fill even the smallest irregularities in the structure of the support on which the limb is located, and adhere very tightly to its surface layer. As a result, the gecko is firmly chained to the branch due to the strong van der Waals interaction (electrical interaction between two close molecules that are not in contact with each other). To tear off the paw, it changes the angle of the hairs, instantly nullifying the grip force.

Having studied this phenomenon, British engineers have learned to imitate the work of a lizard's foot, creating plastic Synthetic gecko . And not so long ago, a group of American scientists were able to combine the advantages of a "synthetic gecko" with the advantages of conventional adhesive materials, creating material haeckel .

However, the gecko's paw is not the only role model.

By studying the structure of the tree frog's foot, scientists from India were able to reproduce and apply structured surfaces as adhesive materials. In their work, scientists took a slightly different path than the creators of Haeckel. Instead of increasing the adhesive properties of the material, the researchers created artificial obstacles to the bond breaking process.

The fact is that the frog's leg, which is smooth at first glance, upon closer examination, turns out to be divided into hexagonal sections, similar to honeycombs. In the microchannels dividing the surface, the frog's body secretes a special secret. Despite this, the mechanism of adhesion between the foot and the surface is more of a microcapillary nature than a viscous or adhesive (intermolecular) one.

Biomimetic

is a young branch of science, the main purpose of which is to draw technical ideas from the world of wildlife. The name comes from the Latin words bios - "life" and mimesis - "imitation". This term, fast...

In the case of ordinary adhesive materials, the separation of surfaces starts from the point of application of maximum force and proceeds like an avalanche. When you try to break the connection of surfaces with a microstructure, the capillary (microchannel) gets in the way of the propagation of the gap: in this case, the separation energy is spent on the deformation of the microcapillaries, thereby requiring the application of additional efforts.

To obtain quantitative data, the scientists used as a comparison a conventional polymer film coated with an adhesive layer. The experiments were carried out on similar materials with a microstructured surface. A key role in increasing the resistance to rupture propagation, according to Indian scientists, is played by the ratio of the thickness of the adhesive layer to the width of the capillaries.


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