Welcome to the second part of the series on the various pouches, sacs and pockets present in the heads, necks and chests of mammals. Last time we looked at the laryngeal sacs of primates (and, should you encounter unfamiliar anatomical terms in the following text, be sure to check out that first article for an anatomical primer). Comparatively speaking, the structures present in primates are well known… or, at least, their existence is comparatively well known. Less well known is the suggestion that elephants possess a sort of pouch in the throat. Or… do they?
Many people have reported elephants (both Asian and African) to reach into the mouth with the trunk and withdraw water. The elephants then spray this water over themselves (particularly their ears) to keep cool. The quantity of water produced in these instances is apparently quite large: J. Emerson Tennent, writing in his 1861 Sketches of the Natural History of Ceylon, referred to “gallons of water” being withdrawn. Richard Owen wrote about this phenomenon too, and most authors followed him in assuming or asserting that this water was either stored in a special compartment in the stomach, or was somehow regurgitated.
However, Watson (1873) described a large pharyngeal pouch, located both beneath and behind the base of the tongue. Its posterior border was, Watson described, formed by the thyroid cartilage of the larynx [Watson's anatomical drawing is show above: the pouch is the oval structure located in the middle: its opening is the large recess immediately above. The base of the tongue is shown at far left]. Watson suggested that this was where that stored water came from. His observations were later elaborated upon by Jeheskel Shoshani and colleagues, who described the pharyngeal pouch and its possible function in several articles (Shoshani 1998, Shoshani et al. 1997, Shoshani & Marchant 2001). They suggested that it could be used to carry as much as 1 gallon (3.8 kg) of water. I actually just went and looked at a gallon of liquid for a short while, just to remind myself how much water this actually is. It’s a lot. The excellent diagram below shows the location and inferred function of the elephant’s pouch; the diagram is by Gary H. Marchant and originally appeared in Shoshani (1998).
Shoshani et al. (1997) proposed that the pouch evolved in concert with the lowered larynx as a resonating chamber, and that its later use as an occasional site for water storage was an exaptation (an exaptation – or exapted structure – is a structure that evolved under selection for one role, and layer became co-opted for another. For previous discussions of exapted structures on Tet Zoo, go here on the gliding flaps of geckos, here on the jaw muscles of caecilians, and here on saurischian dinosaur tracheae and sterna).
The pouch is partially supported by the hyoid skeleton and its associated musculature. And elephant hyoids are particularly unusual: the thyrohyals are delicate and strongly twisted along their length, and the stylohyals are really slender, with really long posterior and inferior rami [hyoids of Elephas shown here, from Shoshani & Marchant (2001)]. This distinctive anatomy means that certain inferences can be made about the soft tissue anatomy of the throat in extinct elephants (and other proboscideans) if their hyoids are well preserved. Shoshani et al. (1997) and Shoshani & Tassy (2005) concluded that a pouch like that of living elephants was present in all members of Elephantida: the proboscidean clade that includes gomphotheres and elephantids, but not mastodonts or more stem-ward proboscideans (here’s that (much simplified) proboscidean cladogram again [previously used here]. It features artwork by Maurice Wilson, Zdeněk Burian and Mauricio Antón].
One interesting question: if the pouch originally functioned in vocalising (as a resonating chamber), and if it can be used to hold water, how might the vocalising function and the water-holding function interact? So far as I know, no one has studied this in detail. However, Garstang (2004) – in an article on long-distance communication in elephants (via infrasound) – noted that the presence of water in the pharyngeal pouch might make a difference to the acoustic properties of elephant noises.
Ok, so far so good. It would appear that the existence of the elephant pharyngeal pouch is well established and fully verified. However…
Those of you who recall my review article on the elephant episode of Inside Nature’s Giants series 1 may recall the mention there of the pharyngeal pouch, or, rather, the mention of the lack of mention. Joy Reidenberg noted in the comments that she failed to find the pouch during dissection, despite a deliberate search and prior knowledge of the Shoshani et al. research. She had to conclude that the so-called pouch was not a ‘special’ feature of elephants at all, but rather that it represented the space normally present in this position (the epiglottic valecula: the part of the mouth where chewed food is formed into a bolus for swallowing). So far as I know, Joy’s observation is novel and has yet to be reported in the literature. In other words, you heard it here first [the impressive complexity of elephant throat musculature is well depicted in this diagram by Gary H. Marchant, from Shoshani & Marchant (2001)].
Admittedly, the idea that any animal might be able to carry around an internal ‘flask’ of stored water, and rely on it in times of stress, is extremely bizarre. But similar abilities have been suggested for some other mammals, and the idea was first proposed back when camels were also thought to carry water in their stomachs. Given that there are published observations and data that do seem to indicate the presence of the pouch in elephants, however, this isn’t the end of the story…
Next: baleen whales, reindeer, takins and quolls.
For the previous article on pouches, pockets and sacs in mammal heads, necks and chests, see…
If you’re interested in tracheae and their role in respiration, vocalising and such, or on any of the associated structures in the neck, check out…
- Deer oh deer, this joke gets worse every time I use it
- Dissecting an emu
- Ridiculous super-elongate, coiled windpipes allow some birds to function like trombones – – or is it violins?
- Inside Nature’s Giants part IV: the incredible anatomy of the giraffe
- Dissecting lions and tigers: Inside Nature’s Giants series 2, part III
And for more on proboscideans modern and extinct, see…
- Of dragons, marsupial lions and the sixth digits of elephants: functional anatomy part II
- RIP Yeheskel Shoshani
- How do you masturbate an elephant?
- The tangled mammoths
- The domes of wisdom
- Stuffed megamammal week, day 5: of elephants and gorillas
Refs – –
Garstang, M. (2004). Long-distance, low-frequency elephant communication Journal of Comparative Physiology A, 190 (10), 791-805 DOI: 10.1007/s00359-004-0553-0
Shoshani, J. 1998. Understanding proboscidean evolution: a formidable task. Trends in Ecology & Evolution 13, 480-487.
– ., Agnew, D., Watson, G., Marchant, G. H. & Matsac, E. C. 1997. The pharyngeal pouch: a unique receptacle in the throat of an elephant. Proceedings of the National Conference of the American Association of Zoo Keepers 23, 74-25.
– . & Marchant, G. H. 2001. Hyoid apparatus: a little known complex of bones and its “contribution” to proboscidean evolution. In The World of Elephants – International Congress, Rome 2001, pp. 668-675.
Shoshani, J. & Tassy, P. 2005. Advances in proboscidean taxonomy and classification, anatomy and physiology, and ecology and behavior. Quaternary International 126-128, 5-20 [free pdf here].
Watson, M. 1873. Contributions to the anatomy of the Indian elephant (Elephas indicus). Part III. The head. Journal of Anatomy and Physiology 8, 85-94.