Videos show that tapping frogs’ toes can be a strategy to attract prey.

It is well known that some species of frogs tap their hind toes when searching for prey; this can be seen in many videos posted online. However, only a few studies to date have looked at the tapping habit itself, and not much is known about the reason behind it.

Now, two researchers from the University of Illinois at Urbana-Champaign have analytically studied the behavior of poison dart frogs (Dendrobates tinctorius). Their findings are published on the bioRxiv preprint server.

D. tinctorius is a species of tiny poison dart frogs (less than 5 cm long and weighing about 3 g) typically found in rainforest areas of northern South America. These frogs protect themselves from predator threats by secreting a paralyzing and potentially fatal poison from glands in their skin. In the wild, they eat insects and non-insect arthropods like spiders.

Due to the speed and small size of these frogs’ prey, it is essential that they be able to detect their movements for successful capture.

Previous studies have reported on the highly developed vibrational sensitivity of frogs and toads, but mainly by discussing their ability to escape their own predators. The researchers in this new work note that cannibal toads “vibrate their toes at the frequency most effective in attracting conspecific prey,” according to the results of a 2008 study. Additionally, it is also known that frogs’ arthropod prey are very sensitive to and react to vibrations.

This led researchers to hypothesize that the vibrations of D. tinctorius’ toes would stimulate the movement of potential prey, making it easier for the frogs to find; and that frogs generally modulate typing speed as needed, depending on the situation.

The team investigated whether frogs typed more slowly in situations that did not allow them to make vibrational contact with their prey; whether they tapped more quickly on surfaces that facilitate vibration; and whether listening helped them better capture their prey.

Tapping test under various conditions

The researchers conducted their tests with individuals of D. tinctorius raised in breeding pairs in terrariums at their university research center. Three times a week, the frogs fed on fruit flies (Drosophila hydei or Drosophila melanogaster) sprinkled with vitamins. With these frogs, the researchers conducted three tests.

First, they took high-speed video of individual frogs during a feeding period in which they dropped half a teaspoon of fruit flies into the terrarium, then took video at high speed of the same individuals during a period without feeding, noting the proximity of the partner. during each recording.

In the second trial, they assessed whether the frogs adjusted their tapping behavior when their prey moved across a separate, inaccessible surface. To do this, the researchers placed the fruit flies in transparent Petri dishes in the frogs’ terrariums, recorded high-speed video as the frogs attempted to strike the prey, and repeated the test the next day with the same frogs and free prey. .

Finally, the team tested the frogs’ strike rates on four different surfaces varying in their nature and flexibility:

• Leaf litter (natural, soft)
• Soil (natural, not very flexible)
• Gel (non-natural, soft (1% agar))
• Glass (unnatural, not very bendable).

They placed the frogs individually in test terrariums to acclimate them to each surface, then fed them and collected data during the feeding and non-feeding periods as in the first trial, this time also noting the number of moves made by frogs and success rates. .

Diet, prey accessibility and surface area play a role in exploitation rates

Diet made a difference, the first trial confirmed. The team found that the frogs increased their tapping when their prey but no partner was present (average 389 taps/minute), and even more when they were feeding while a partner frog was present (average 684 taps/minute). Without a frog partner nearby and without prey, tapping decreased significantly (on average 50 taps/minute), and even more so with a partner nearby but in the absence of prey (on average 43 taps/minute).

The researchers observe: “We emphasize that, with a maximum of almost 500 taps per toe per minute, this behavior is incredibly fast for any vertebrate muscle movement. In short, these results demonstrate an association between tapping and eating and provide interesting avenues for further studies. study.”

The second trial showed that prey accessibility was important. When the frogs could see but could not reach the flies in the petri dishes, they tapped an average of 50 strokes/minute, while continuing to attempt to grab the flies. When the flies moved freely, the frogs tapped significantly faster (on average 166 taps/minute).

Based on these results, the team notes: “When the frogs could see but not capture the flies, they swatted much less but still hunted. We suggest that this change in strike rate may be related to changes in vibrational stimuli and/or prey capture feedback. success. This observation suggests that frogs might modify their tapping behavior based on the responses of their prey.

The final test found that strike rates varied by surface type, with higher rates on soft surfaces:

• Leaf litter (natural, soft): on average. 255 taps/minute
• Soil (natural, not very flexible): on average. 98 taps/minute
• Gel (non-natural, soft (1% agar)): avg. 118 taps/minute
• Glass (non-natural, non-bendable): on average. 64 taps/minute.

Interestingly, the frogs primarily tapped at lower average rates in the test terrariums than in their home terrariums. However, the researchers noted that the different surface types in the final trial did not affect the frogs’ total number of prey attacks or success rates. As the frogs tapped faster, they increased their striking rate, but their overall prey capture success could not be associated with their striking rate.

Commenting on the results of these third trials, the researchers write: “…we found that frogs tapped less on the ground than on leaves, although both surfaces were familiar to the frogs. Taken together, these observations demonstrate that frogs modulate their tapping behavior depending on substrate type and independent of prey capture attempts.

Overall, the team suggests that further research into the biomechanics of tapping and the sensitivity of frogs and their prey to vibrations, as well as measuring surface vibrations resulting from tapping, will provide a better understanding of this behavior in frogs.

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