A small whiskered world: Facial hair across the animal kingdom

Which animals have facial hair, like whiskers or a mustache, and what do they do with it?

Cheerio the cat died in 2021 after living his life in a loving home, a fate millions of cats do not get to experience. (photo credit: Gadi Zaig)
Cheerio the cat died in 2021 after living his life in a loving home, a fate millions of cats do not get to experience.
(photo credit: Gadi Zaig)

Research provided by the Davidson Institute of Science Education

The mustache on our faces is simply a collection of hairs, though one that can grow to rather impressive lengths.

According to the Guinness Book of Records, the length of the longest ever human mustache was 4.29 meters. In other animals, however, it can grow to much longer lengths.

To mark the passing of the month of November, which is also known as Mustache Month, we will review a surprising variety of mustachioed animals, and find out what they are able to do with it. 

Many mammals, such as cats, utilize their whiskers (more generally termed vibrissae) as antennae, and are able to move them voluntarily to explore their surroundings. This ability is made possible by the special hair follicles from which whiskers grow.

In contrast to regular hair follicles, whisker follicles are deeper and have a richer blood supply, incorporating at their base tiny blood capsules, called blood sinuses, which are surrounded by multiple sensory nerve cells. These nerve cells allow their owners to sense subtle deflections of the whisker by sensing the smallest movements via the blood-filled sinuses.

When a whisker bends, the follicle at its base reacts both to the direction and to the speed of deflection. In this manner, whiskered mammals can navigate through complex surroundings and even distinguish between different textures that exert different types of pressure on the hairs.

This ability confers a special advantage to nocturnal mammals, who cannot rely on their eyesight. For this reason, nocturnal mammals, especially those adapted to tree-top living, usually have long and dense whiskers. Marine mammals, such as seals, who spend at least part of their time in the water, are also equipped with highly sensitive whiskers.  

These whisker follicles grow around the nostrils, on top of the lips, as well as in other areas of the face in most mammals. Unfortunately for the men among us, the human mustache has regular hair follicles and cannot be used therefore in a similar manner. Instead, we rely on the well-developed tactile sense of the palms of our hands, and, naturally, on our senses of sight and hearing.

Surprisingly, research on whiskers was scarcely conducted for decades, with renewed scientific interest in whiskers and their contribution to tactile sensing having emerged only recently. Researchers today are attempting to understand how animals use their whiskers in nature, and how their sensory systems represent features of the surrounding world in their brains.

Most of the data is taken from a small selection of mammals: rodents (mainly rats and shrews), marine mammals and marsupials.

Guidance whiskers

Rats (Rattus) use their whiskers to navigate in dark burrows and to explore their surroundings in general.

They are equipped with long whiskers, which grow at the sides of their noses, and also extend from their eyebrows and cheeks, as well as with very short whiskers, which grow around their mouths.

They tend to move their whiskers quickly - at a rate of eight motions per second - and thus obtain information regarding their surroundings. A simple back and forth motion allows them to acquire a great amount of information, navigate easily, locate food, explore objects they encounter, and examine the structure and texture of surfaces.

When in the dark, they rely on their whiskers more than on their sight for accurate depth perception - by testing whether their whiskers can reach the bottom of a hole or the other side of a cavity.

At times they strike the same object with their whiskers multiple times from different directions, acquiring thus a three-dimensional image of their surroundings in their brains. 

 White rat (credit: Wikimedia Commons) White rat (credit: Wikimedia Commons)

Recently, researchers from the Weizmann Institute of Science found that behavioral context has a significant effect on the manner in which rats move their whiskers.

For example, when a rat runs freely it will use its whiskers differently compared to a situation where its head or body are fixed in a single location. The range of motion of a free rat’s whiskers is relatively small and a certain emphasis on a slow studying forward motion is apparent.

The researchers speculate that this motion helps the rat to avoid objects in its path. Additionally, the researchers report that the rats appeared to vibrate their whiskers while examining objects. Observations of these vibrations allow researchers to identify both the object on which the rat is focusing its attention as well as the direction in which it is about to advance.

Rats that remained stationary used their whiskers in a different manner - while the time invested in moving the whiskers back and forth was nearly identical, as was their vibration, their range of motion was much greater. It appeared that in this manner rats can expand their sensory range to a larger area, when necessary. 

 Identifying Prey

The pygmy shrew (Suncus Etruscans) is one of the smallest mammals in the world, as well as one of the fastest, and these traits confer it with an amazing hunting ability. Shrews feed on insects such as crickets - who are nearly the shrew’s size - as well as on worms, larvae and spiders.

Due to its size and rapid metabolism, the shrew must consume daily an amount of food greater than its body weight. In order to achieve this, it must feed at least once per hour. 

Since shrews are in a constant pursuit of their prey, they must optimize their energetic investment as much as possible by hunting accurately, without missing. The problem is that shrews have poor eyesight and often live in crowded obstacle-laden spaces. 

How do they maneuver?

It seems that their whiskers play an important role. In a study published in 2011, researchers presented shrews with plastic crickets alongside plastic objects of the same size but of a different shape.  

The shrews were found to rely primarily on their long and sensitive whisker hairs to identify prey. They periodically moved their whiskers back and forth while hunting, at a rate of 14 motions per second, until their whiskers touched the prey. A substantial fraction of a shrew’s tiny brain (only 60mg) is dedicated to tactile sensing.

Thus, a representation of the potential prey’s shape forms in the shrew’s brain, based on the multiple contacts between its whiskers and the potential prey.

Using this information, shrews can react to the presence of prey within the blink of an eye, with a reaction time of 25-30 milliseconds - an attack which is likely to have high success rates. In fact, shrews are so fast, that at times they are able to attack an insect before it even begins evading them. 

 Pygmy shrew (credit: FLICKR) Pygmy shrew (credit: FLICKR)

Cats also use their whiskers to navigate through space and expand their tactile perception beyond their bodies, thereby avoiding obstacles in the dark. The use they make of their whiskers can affect their shape and development. For example, cats that are born blind tend to develop especially long whiskers, and the tactile regions in their brain cortex will be more developed compared to those in sighted cats. 

A cat’s whiskers are iconic to such an extent that a plant, Cat’s Whiskers (Orthosiphon aristatus), which is reminiscent of their shape, was named after them. The plant’s flowers are white and its pollen is located on columns that resemble a cats’ whiskers.  

Whiskers Making Waves

Whiskers are also an important sensory tool for marine predators.  Walruses (Odobenus rosmarus), Sea Lions (Zalophus californianus) and Seals (Phocidae) use their whiskers to detect water turbulences, including turbulence generated by marine animals, as well as for active sensing of objects underwater. 

The shape of whiskers of marine mammals is different from one organism to another. Sea Lions have smooth elliptical whiskers, Seals’ whiskers are wavy and elliptical, while Walruses, huge marine mammals that can weigh as much as a ton and a half and be up to 3.5 meters long, have smooth rounded whiskers.

Their shape has also provided inspiration for the design of a human mustache that was called a Walrus Moustache. 

 Walrus (credit: Wikimedia Commons) Walrus (credit: Wikimedia Commons)

The reason for the round or elliptic shape or the walruses’ whiskers remains unclear. We know however that marine mammals use their whiskers mainly to locate food. Walruses use their 600-700 rounded whiskers to detect tiny food objects, such as oyster shells on the ocean floor, through direct contact, in a manner that resembles the way a rat uses its whiskers. 

Seals use their whiskers to look for food in a slightly different manner. Harbor Seals (Phoca vitulina, also known as the common seal) for example, are large seals that possess a unique pattern of spots on their fur and are known to live in all oceans.

A Harbor Seal’s whiskers are the longest known whiskers in the animal kingdom, they are wavy shaped and the number of nerve innervations in their follicles is ten times greater than that in the whiskers of rats.

This structure appears to promote fast and efficient data processing: studies show that seals’ whisker follicles are able to efficiently detect tiny turbulences in water currents, including the disturbances left by other animals that move through the water, even while they themselves are in motion, and thus convey highly detailed information regarding their surroundings.

This sense is so developed that the Harbor Seal is able to identify the water signature of fish even 35 seconds after it has passed by, and sense juicy prey even in murky water. Indeed, blind seals are occasionally found fully fed, suggesting that their sight is not essential for hunting. 

Pollinating Whiskers

Studies have shown that in some mammals, such as lions (Panthera leo), Australian sea lions (Neophoca cinerea) and polar bears (Ursus maritimus), the position and pattern of whiskers on the muzzle is unique for each individual. Thus, using image analysis softwares, researchers can identify specific individuals in a population.

The factors that affect the pattern of whisker hairs are not entirely clear, but researchers speculate that it is probably a combination of factors, including environmental, nutritional, genetic and age-related factors. 

Individual identification is probably the main reason for one of the most glorious animal “mustaches” -  that of the Emperor tamarin (Saguinus imperator).

It is told that when Swiss zoologist Émil Goeldi reported that he has identified a new species of small monkeys in South America, he jokingly referred to them as “Emperor'' due to the strong resemblance of their “mustache” to that of the German emperor Wilhelm II. The name, which was given as a joke, stuck with the money and was eventually adopted as its official name. 

Emperor tamarin’s magnificent whiskers consist of regular hairs, like the human mustache, and are not used for tactile sensing.

Aside from their role in identifying different members in a group, it may also play an additional significant role: plant pollination. The Emperor tamarin feeds mainly on fruit, flowers and plant nectar, while at certain seasons their menu focuses differentially on one specific plant species or another.

As the Tamarins move between branches in order to reach for nectar, their mustache gets filled with pollen. Some researchers claim that they pollinate the flowers using their long whiskers whilst moving between them, just like insects. 

Not Only Mammals

Typical whiskers are composed of hairs and therefore exist only in mammals, since the ability to grow hair is uniquely mammalian. However, many other animals are equipped with whiskers-like tentacles around their mouths and heads. One of these is the Catfish (Clarias gariepinus), whose name even alludes to a similarity with the whiskered animal.  

Catfish (Siluriformes) are a large order of fish, numbering almost 3000 species worldwide. The Sharptooth Catfish (Clarias gariepinus) is the largest sweetwater fish living in Israel. Catfish are equipped with four long whisker-like sensory organs, termed barbels, around their mouths. Catfish barbels have taste buds that allow them to identify the presence of different chemical substances in their surroundings, and thus to locate food sources. 

A research that focused on the Japanese sea catfish (Plotosus japonicus) found that they use their barbels to detect changes in water pH (acidity) levels, which result from changes in the levels of carbon dioxide in them.

When the researchers artificially increased water pH levels, they found that the general activity level of the fish increased and that they were more actively searching for food under these circumstances.

Every living organism, including worms, fish and small amphibians - all of which serve as Catfish prey - emits carbon dioxide generated during the process of respiration. When a marine organism emits carbon dioxide into the water this slightly increases the water's acidity.

Therefore, a sensitive acidity sensor can detect the presence of living organisms in the Catfish’s vicinity. Researchers have found that the sensors in the Catfish barbels are sensitive enough to be able to detect the slight changes in water pH caused by the respiration of worms. 

In this manner the barbels of Catfish enable them to detect their prey in a muddy environment, when worms hide in the ground and even in a state of complete darkness. Catfish were the first fish to be found to be equipped with pH sensors.

One of the implications of this study is that Catfish will be very vulnerable to the changes in water pH levels that are caused by the constant increase in the amounts of carbon dioxide, which mankind emits into the atmosphere and which eventually partially dissolve in the oceans. 

Whiskers for Birds - and for Butterflies 

Certain birds use whiskers-like feathers, but not necessarily for sensing. For example, Inca Terns (Larosterna inca) of South America have white feather whisker-like structures that resemble the curly mustache of the Spanish painter Salvador Dali.

The length of their whiskers is a testament to their health and physical condition, and thereby, the individuals with the longest whiskers are usually the healthiest and are more frequently selected by the females for mating. 

 Inca Terns (credit: Wikimedia Commons) Inca Terns (credit: Wikimedia Commons)

 In 2009, the curator of the Butterfly Collection at the Museum of Natural History in London, Blanca Huertas, discovered a new species of butterflies, initially collected in Columbia, which she called Splendeuptychia ackeryi.

The new butterfly had an interesting feature: the main distinction between it and other members of the same family was an unusual and unexplained amount of hairs in their mouthpart, reminiscent of a mustache. It is still unknown what, if any, is the physiological role of this “mustache”.

There are other marvelous, strange and fascinating “mustaches'' in nature: The Tentacled Snake (Erpeton teltaclatum) has long and flexible whisker-like sensory organs on its snout, which enable it to detect vibrations that are generated by its prey; The Whiskered screech owl (Megascops trichopsis) possesses whisker-like feathers around its beak; and males of the Emei moustache toad (Leptobrachium boringii) grow a thorn “mustache” during the mating season, to be used for combat and for courtship of females.

There is even a type of bat called Whiskered Bat (Myotis mystacinus), but despite its name, it does not have especially prominent whiskers compared to other bats.

The Rise and Fall of the Human Mustache

Throughout human history, facial hair went in and out of style, influenced by aesthetic, cultural and even psychological reasons.

The mustache in human society is one of the symbols of masculinity: it starts growing in adolescent males as part of the hormonal development during puberty, concomitant with an increase in the levels of the male hormone testosterone.

Charles Darwin, who developed the theory of evolution by natural selection, argued, at a certain point, that male facial hair evolved in human ancestors via female choice, as a symbol of vigor and dominance, for the attraction of females.

He even tried to prove that bearded men reproduce more than men with clean shaven faces. However, to this day it remains unclear whether or not human facial hair grown above the upper lip indeed helps to attract more sexual parteners, or whether it ever did so in the past.

In recent years, certain studies proposed that facial hair could provide different evolutionary advantages, such as protection from UV radiation, or even partial absorption of the energy inflicted by a blunt blow  to the face during combat. 

In a study from 2019 it was found that young children associated a mustache with dominance, but were less inclined to associate it with traits such as attractiveness or parental quality. Children whose father had a mustache had a generally more positive stance towards mustachioed faces and were more inclined to associate them with positive social qualities. 

In contrast to other animal species, in humans mustaches are strictly associated with the male gender. This is especially true for Western culture, in which a female mustache is considered an aesthetic defect and in which mustachioed women usually make efforts to get rid of their facial hair.

It is important to remember that facial hair is a result of a hormonal biological cause and that different women have different levels of testosterone in their bodies, which may, among other things, promote facial hair growth. This phenomenon becomes especially dominant with the decrease in the production of the female hormones estrogen and progesterone during menopause.

A female mustache is no less natural than a male mustache, and the difference in the way they are perceived is strictly cultural.  

The month of November that recently ended has lately been referred to as Movember, a combination of the words mustache and November, meaning “November, the month of the mustache”. The name was conceived as part of a social movement for raising awareness to men’s health, mainly with respect to mental health and prostate and testicular cancer. 

The movement began in 2003 at a meeting of friends in a bar in Melbourne, Australia, where they were jokingly discussing the idea of bringing the mustache back into fashion.

Inspired by the mother of one of the group members, who raised money for a foundation committed to fighting breast cancer, they decided (seriously this time) to launch a campaign that will promote awareness to men’s health, and will be symbolized by the growing of moustaches during the month of November.

The mustache is a strong visual symbol, which participants of the campaign grow on their faces in order to promote awareness and raise money for their cause. The initiative rapidly spread out of the borders of Australia and has reached 21 countries so far.

The organizers report that, by 2020 “Movember” had over 6.5 participants, who raised over 730 million Australian Dollars for 1,250 medical research programs to promote mens’ health, and made a significant contribution to the encouragement of public discourse on the issue of male health.    

Thirty four new programs, dedicated to the health of men and boys who suffer from isolation due to COVID-related lockdowns, or from prolonged physiological conditions following recovery from COVID-19 (Long-COVID), were added to the initiative in 2020.

The purpose of the founders from the Melbourn bar was to change the face of society - and they succeed at doing so every Movember.

This research was first published at the Davidson Institute of Science Education.