1. The Star-Nosed Mole's Star-Shaped Nose Has 22 Tentacles Covered in Tiny Domes
Around the nose of the star-nosed mole, 22 fleshy tentacles (11 pairs) are arranged. The spread-out shape looks like a star, and its diameter is about 1 cm [1].
On the surface of the star, there are about 25,000 small dome-shaped sensory organs called Eimer’s organs in a typical star. The size varies by source, but the diameter is about 0.03 to 0.06 mm, so it is fine to think of it as around 0.05 mm [2]. Eimer’s organs were first described in European moles in 1871 [7].
The nerves heading from here to the brain consist of about 100,000 myelinated fibers. This corresponds to about five times the number of touch nerve fibers in a human hand [2]. When comparing the 1 cm star to a palm, the star is much smaller, yet it has many more pathways for sending information to the brain.
So, how does the mole actually move this star equipped with so many sensory organs?
2. How the Star-Nosed Mole Taps Its Star Against the Ground 10 to 15 Times a Second
When searching for prey, the star-nosed mole presses the star against the ground and lifts it off, touching about 10 to 15 spots per second. Each contact lasts only 20 to 30 milliseconds, which is 0.02 to 0.03 seconds [1]. Because sources vary between "10 to 13 times" and "10 to 15 times," it is safe to consider it as "about 10 times."
If it is 10 times per second, that is one time every 0.1 seconds. Even if you close your eyes and tap a desk lightly, continuing to change locations at that speed is not easy. Since the tentacles of the star have 25,000 sensory organs, fine clues from the ground enter with each tap.
However, not all 22 tentacles perform the same function.
3. The Smallest Pair of Tentacles Works Like the Eye's Fovea to Examine Prey
On the underside of the star, the 11th pair of tentacles (one on each side) is smaller than the others. When other tentacles touch something that looks like prey, the mole quickly moves this 11th pair to that spot and touches it repeatedly to examine it closely [1].
Comparing it to the eye makes it easier to understand. When something moves at the edge of our vision, we turn our face toward it and look closely using the "fovea," the part of the eye for detailed vision. Researchers call this 11th pair of tentacles the "tactile fovea." The roles are divided: other tentacles probe a wide area roughly, while the 11th pair examines found items in detail. However, this is a metaphor used by researchers and does not mean it works exactly like an eye [2].
This division of roles is also seen in the "star map" in the brain. The 11th pair of tentacles uses a very wide area relative to its size, reportedly occupying about 25% of the star map [1]. Note that 25% is the proportion within the star map, not the entire brain.
Find, check, eat. How fast does this process happen?
4. Why the Star-Nosed Mole Can Eat So Fast
The time from the star-nosed mole first touching prey to eating it and resuming the search is reported to be about 0.12 seconds at the shortest and about 0.227 seconds on average. 0.12 seconds is about one-eighth of a second. This is said to be the fastest feeding speed among mammals [1].
Why is it in such a hurry? The star-nosed mole lives in wetlands, and its prey consists of small items like aquatic insect larvae. Prey in wet mud is said to be about 1/20th the size of the prey eaten by other moles living in dry places [2].
If the prey is small, the gain from eating one is also small. Therefore, researchers think that selection for speed worked strongly because the benefit gained increases greatly if small prey can be eaten with almost no time spent [3]. Researchers explain this by linking it to the small insect larvae in wetlands, and the reason for evolution is a hypothesis.
By the way, this wetland is near water. Can prey in water be found just by touching with the star?
5. How the Star-Nosed Mole Smells Underwater by Blowing Bubbles from Its Nose
The star-nosed mole is a semi-aquatic animal living in wetlands from southeastern Canada to the eastern United States, and it is also good at swimming. Its weight is about 70 g [6].
Can prey in water be found relying only on smell? A study reported in Nature in December 2006 showed that the star-nosed mole blows bubbles from its nose, touches them to the surface of prey, and then inhales the bubbles back into its nose, repeating this motion 5 to 10 times per second [4]. The results suggest that bubbles may carry smell clues even in water.
According to reports, in experiments tracing smell paths, the mole could follow the correct path 75 to 100% of the time if the bubbles could pass through a mesh. However, with a mesh too fine for bubbles to pass, the correct answer rate dropped to chance levels [5]. This study also included the northern short-tailed shrew.
Touch with the star, and explore smell clues carried by bubbles. The small nose is used on both land and in water.
6. Try It Yourself: Measure Taps Per Second by Tapping a Desk for 10 Seconds
Try tapping a desk lightly with your fingers. Count the number of taps while the second hand moves for 10 seconds, then divide by 10 to get the times per second. The star-nosed mole does 10 to 15 times. What about your fingers? You can also try changing the number of fingers or comparing one hand with two hands.
Try one more thing. Close your eyes and touch buttons or an eraser in a bag separately with your fingertips and palm. Which one detects details better? The star-nosed mole uses sensory organs on the surface of its star to examine things in detail. Do not put small items in your mouth.
After trying, think about it. Which detected shapes in more detail, the fingertips or the palm? The mole examining prey with its small 11th pair of tentacles may be using an area capable of fine discrimination.