Elephants Don't Have Tear Ducts - So Why Are They Always Crying?
It's fairly common for people looking at elephants to notice that they have liquid seeping from the corners of their eyes. Some advocacy groups say elephants are crying because they're sad, but scientists say that elephants don't have tear ducts. If the latter is true, where is that liquid coming from, and why?
Elephants Don't Have Tear Ducts - So Why Are They Always Crying?
6/9/2018 - Rachel Garner
It's fairly common for people looking at elephants - both in zoo or circus settings and those lucky enough to see them in the wild - to notice that they have liquid seeping from the corners of their eyes. Some advocacy groups say elephants are crying because they're sad, but scientists say that elephants don't have tear ducts. If the latter is true, where is that liquid coming from, and why? It turns out elephants have evolved an innovative method of keeping their eyes moist due to their semi-aquatic ancestry that comes with an unexpected side-effect - the constant dribbling of fluid from the corners of their eyes. While this may look superficially like emotional "crying", it occurs simply because elephants have lost the normal mammalian structures that drain excess moisture away from their eyes; without a true lacrimal structure, elephants are physically unable to produce emotional tears.
Normal Mammal Tear Anatomy
To understand why elephants always appear to be crying, it’s first important to understand the anatomical system that allows animals to produce tear fluid. While humans are the only species currently known to produce emotional tears, all terrestrial mammals must produce some sort of liquid that keeps their eyeballs lubricated and free of irritants. The set of glands and ducts around the eyes involved in this process is called the “lacrimal apparatus.”
The main gland that produces tear fluid is located between the eyeball and brow ridge and is called the lacrimal gland. (A small amount of of liquid is also produced by small glands on the inside of the eyelids.) The tear fluid is deposited onto the eye through ducts from the lacrimal gland and is spread across the surface of the eyeball by blinking. Excess fluid is forced into little channels at the inside corner of the eyes (called canaliculi), where it collects in a sac inside the nose before draining into the nasal cavity. (Both the secretory ducts of the lacrimal gland and the nasolacrimal duct seem to be colloquially referred to as 'tear ducts').
"Crying" occurs when too much tear fluid is produced for the canaliculi to be able to handle at one time, and so the liquid spills out from the eye. This excess fluid production can be due to the body's attempt to flush an irritant out (called reflex tears, triggered by the trigeminal nerve), or in humans, because of strong emotional arousal (called psychic tears, triggered by the parasympathetic branch of the autonomic nervous system). Interestingly enough, different types of tears have very unique chemical compositions in humans: basal tears (the normal tear film) and reflex tears are full of anti-bacterial enzymes and anti-oxidants; whereas emotionally-triggered tears contain high levels of protein-based hormones, including the precursor to cortisol and one that functions as a natural painkiller.
The liquid that covers the eye during the normal course of a day isn't just a simple layer of basal tears, however. The human eye's "tear film" is actually comprised of three different layers, each of which is produced by a unique gland in the eye area. The mucous layer sits directly on the surface of the eyeball and is made up of a gel-like proteins called mucins, which are secreted by little glands on the inside of the eyelids. Because it is hydrophillic (water-attracting) it helps promote even dispersal of the tear liquid across the eye.
The aqueous layer sits on top of the mucous layer , in the middle of the tear film, and is comprised of the basal tears discussed earlier. On the outer-most surface of the tear film is the lipid layer, which is made up of oils secreted by glands on the rim of the eyelids. The hydro-phobic secretions of the lipid layer, called meibum, coat the aqueous layer and keeps it from evaporating or spilling out before it reaches the drainage channels in the inner corner of the eye.
How Elephants Make Tear Film
The reason all of the above matters with regards to elephants is because their evolutionary ancestors threw out that entire system and developed something else instead. Not only do elephants not have tear ducts, they actually don't have any piece of the normal mammalian lacrimal apparatus: that means no lacrimal gland, no tear ducts, no canaliculi at the corners of their eyes, and no canals for drainage of tear fluid into their nasal cavities. This is currently thought to be an adaptation that developed during a period where the ancestors of modern-day elephants were semi-aquatic; this seems credible, since modern-day pinnipeds also lack a lacrimal apparatus. (The same physiological changes discussed in this section are also found in elephants' closest living relatives, hyrax and manatees.)
So how do elephants make a tear film? Effectively by re-purposing other glands from around the eye to produce approximately the same type of fluids needed to keep the eye moist. They still have a mucous layer that's produced by glands on the inside of the eyelids, but that's the only normal structure in the entire system. In absence of a normal lacrimal system that produces basal tears, two glands were effectively hijacked to produce serous (water-based) fluid: a gland on the third eyelid and accessory glands along the eyelids.
Like many mammals, elephants have a fluid-producing gland on the third eyelid (called the nictitating membrane). In most mammals, this gland is small and only contributes a small amount of moisture to the tear film; in elephants, it is extremely well developed and appears to produce the majority of the fluid that is their eyes' aqueous layer. Unlike in most mammals, however, the fluid produced by this gland also contains mucous, which makes the consistency of their tear film different from that of most mammals.
Nictiating membranes are highly mobile pieces of tissue that are hinged at one corner of the eye - when engaged, they sweep across the surface of the cornea, removing debris and refreshing the tear film as they go. Unlike in many species, where the nictitating membrane can close independetly of the other two eyelids, the movement of the third eyelid in elephants appears to be involuntary and directly related to the closing of the eye.
In addition to the gland on the nictitating membrane, small accessory glands on the rim of the eyelid appear to contribute moisture to the tear film of an elephant's eye as well - but again, the composition of the liquid is unusual. As covered earlier in this article, the glands on the rim of the eyelids (called tarsal glands) in most mammals produce the outermost layer of the tear film: the oily lipid layer that helps prevent the aqueous layer from evaporating. However, elephants don't have tarsal glands at all. Instead, accessory glands that produce another combination of mucous and serous (water-based) fluid have taken their place; it's assumed that these secretions also contribute to the aqueous layer of the tear film, but more research is needed.
In order for the aqueous layer to provide lasting moisture, it has to have some sort of hydrophobic coating - but elephants don't have the glands that normally produce that lipid layer. It appears that elephants adapted to the loss of these glands by evolving incredibly productive sebaceous glands at the base of their "eyelashes", and using sebum to provide that barrier instead of the meibum of most mammals. (Elephants do not have true eyelashes. Eyelashes are a type of hair embedded in the muscle of the eyelids, and which trigger a reflex to close the eye when touched. Elephants have, instead, a large number of accessory hairs all over their eyelids: they look like eyelashes, but are not in a row along the tarsal rim and do not have the same function.)
So Why Do They Cry?
The main reason elephants look like they're"crying" is simply because they lack the drainage canals that most mammals have to wick the moisture away. With nowhere to go, the tear fluid accumulates at the medial canthus (the inner corner of the eye) and then spills out from there down the face. There is actually a diagonal groove in the skin next to the inner corner of the eye in both species of elephants that serves to draw the liquid down and away from the eye where it can evaporate. In all living elephants, this groove and the skin below the end of it are constantly moist with shed tear film. Sometimes a "white goop" or "foam" is visible at the medial canthus of an elephant's eyes; while there does not appear to be scientific literature directly address the cause of it, it is presumed to be an accumulation of sebum and mucous from the tear film, as well as potentially a deposit for bacteria and physical irritants that have been cleared out of the eye. When it is windy or there is particulate matter in the air (such as smoke) it is presumed that the nictitating membrane works overtime, both in producing more tear film and in sweeping debris away; as a result, elephants are often seen with more fluid below their eyes and more foam in the medial canthus during adverse weather conditions.
What About The Liquid Behind Their Eyes?
Sometimes, fluid secreted by the temporal ducts on the sides of the head is also mistaken for tears by observers; while most commonly noticed when bull elephants are in musth during the breeding season, African elephants of both sexes will also drain from their temporal glands during periods of high emotional arousal. While this substance (called often temporin) may look similar to tear film at a distance, the liquids do not have similar chemical compositions. Bulls in musth secrete temporin that is tarry in texture and highly oderous; secretions produced at other times appear to contain a complex and constantly fluctuating bouquet of chemicals thought to be communicative in nature.
Even though elephants do drain from their temporal glands during high arousal periods, it is not analogous to the situations in which emotional tears are produced by humans. Emotional tear production, as covered earlier, is a product of the parasympathetic nervous system: crying serves to help relax the cryer and bring the down down from a reactive fight-or-flight state. The temporal glands that elephants have are actually modified apocrine sweat glands, which are activated by the sympathetic nervous system: they engage when an animal needs to be primed and ready for action, whether that be due to stress, fear, excitement, or sexual arousal. So while tears in a human indicate that the cryer is in the process of recovering from an intense stress to the system, drainage from an elephant's temporal glands outside of musth indicates the animal is getting amped up for whatever is about to occur.
Lots of Liquid, But No Emotional Tears
Neither the drainage of tear film from the corner of an elephant's eyes, nor the draining of fluid from an elephant's temporal gland can be considered analagous to the production of emotional tears. Even if the differences in physical structures involved in fluid production are set aside, psychic tears by definition are triggered by the parasympathetic nervous system and involve the release of multiple protein-based hormones; excess tear fluid in an elephant's eyes is simply produced as a response to the presence of physical irritants and does not contain the same molecules as emotional tears, and temporal gland drainage is the result of the activation of the sympathetic nervous system and is comprised of an entirely different type of chemical than emotional tears. There is no evidence that elephants release any kind of fluid from their eyes (or other orifice) during the activation of the parasympathetic nervous system after a stress to the animal's system occurs - therefore, while elephants may appear to a casual observer to be crying, they are not physically capable of producing emotional tears.
Curious to Know How Human Tear Ducts Work? Find Out Here. (2017, December 21). Retrieved September 2, 2018, from https://bodytomy.com/how-human-tear-ducts-work
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Rachel is an educator and animal science writer. With prior professional experience in zookeeping, visitor education, shelter behavior management, and more, she works to translate pertinent field-specific knowledge into comprehensive explanations about current animal related topics.