Sublingua

A sublingua ("under-tongue") is a muscular secondary tongue found below the primary tongue in primitive mammals, such as treeshrews and marsupials, but is most fully developed in prosimian primates, which includes lemurs, lorises, galagos, and tarsiers. This "second tongue" lacks taste buds, and in living strepsirrhine primates, it is thought to be used to remove hair and other debris from the toothcomb, a specialized dental structure used to comb the fur during oral grooming.

A rigid structure called the plica mediana runs from the front to the back, down the center of the sublingua to give it support. The plica mediana is usually made of cartilage and attaches the sublingua to the underside of the tongue. In living strepsirrhines, the sublingua mostly consists of two plicae fimbriatam (singular: plica fimbriata), which run along the sides of the plica mediana and end in comb-like serrated edges that are hardened with keratin. The plicae fimbriatam are free-moving over a limited range. The plica sublingualis, which is found in all primates, but is particularly small in living strepsirrhines, attaches the tongue and sublingua to the floor of the mouth. Tarsiers, the prosimian primate most closely related to monkeys and apes, have a large but highly generalized sublingua.

The sublingua is thought to have evolved from specialized folds of tissue below that tongue, which can be seen in some marsupials and other primitive mammals. Simians do not have a sublingua, but the fimbria linguae found on the underside of ape tongues may be a vestigial version of the prosimian sublingua.

Anatomical structure

The sublingua, or "under-tongue", is a secondary tongue located below the primary tongue in prosimian primates and some primitive mammals. This structure does not have taste buds or salivary glands. In lemurs and lorisiforms, the sublingua is relatively large and its front edge is usually lined with keratinized serrations (sometimes called "denticles").

Down the middle of the sublingua is a thick strengthening rod called the plica mediana, which connects the sublingua to the underside of the tongue, and is part of the lingual septum (septum of the tongue). The thickness and size of the plica mediana can vary between species, and except in treeshrews, it is cartilaginous and provides support for the sublingua.

Only the serrated and often keratinized tip of the sublingua is free to move small distances along the underside of the tongue, while the majority of its length adheres to the underside of the tongue. These free-moving folds or filaments are called the plica fimbriata and attach to the base of the sublingua while supported at the midline by the plica mediana. The plica fimbriata is greatly developed and highly specialized in lemurs, and makes up the majority of the sublingua.

The fold that connects the back of the sublingua and tongue to the rear floor of the mouth is called the plica sublingualis. In lemurs, this is an underdeveloped structure consisting of only a tiny outgrowth on the floor of the mouth. The point where the plica sublingualis attaches to the rear floor of the mouth marks the location of the submandibular salivary glands.

Differences between species

In the aye-aye, the sublingua is not shaped like a brush like it is in most lemurs. Instead, there is a thickened area along the plica mediana (called the lytta) which has a hook-shaped structure on the end. Within cheirogaleids, the sublingua lacks cartilage, and the sublingua of the gray mouse lemur has a distinct plica mediana and ends in two lobe-like projections that lack keratinized serrations, but have three keratinized ridges which make the sublingua rigid.

In tarsiers, the sublingua does not have serrations along its tip and is much simpler and generalized in structure, making it clearly distinguishable from that of the living strepsirrhine primates. The tarsier has a distinct plica median and its plicae fimbriatam are large and stick to the entire underside of the tongue. The plica sublingualis is also prominent.

Marsupials such as opossums and the common brushtail possum have also developed noticeable "sublingua" with a plica mediana and a less specialized, but conspicuous, plica fimbriata.

Function

The sublingua is used to remove hair and debris from the toothcomb of lemurs and other strepsirrhine primates.

Originally, the sublingua in lemurs was thought to be a vestigial organ inherited from primitive mammalian ancestors. In living strepsirrhine primates, the sublingua is used to remove hair and debris from the highly specialized toothcomb, an arrangement of four or six long, forward-facing teeth in the lower jaw used in oral grooming. The toothcomb of living strepsirrhines consists of both incisors and canine teeth (which reinforce the incisors), and together, these finely spaced teeth to act like teeth on a comb. Although the cleaning function has been suspected for nearly a century, there has been no clear confirmation of this function. However, a study from 1941 did show that the toothcomb accumulated a mat of hair during oral grooming, and the author did observe that lemurs extend and retract their tongue rapidly, possibly to use the sublingua to clean the toothcomb.

In the aye-aye, which has replaced the toothcomb by evolving continually growing, rodent-like incisors, the hook-shaped tip of the sublingua fits precisely within the gap between the two lower incisors and keeps the area clean. Tarsiers lack a toothcomb, which may explain why their sublingua lacks the serrations typically found on the sublingua of living strepsirrhines. Although colugos also have a toothcomb, consisting of serrated edges on the tips of their incisors instead of finely spaced, elongated teeth, they do not have a sublingua. Instead, their toothcomb is cleaned by the tongue, which has serrated edges at the front that match the serrations on the incisors.

Evolution and development

The sublingua in prosimian primates may have evolved from the specialized folds of tissue below that tongue, as seen in some marsupials (e.g. sugar gliders), as well as some adult and embryonic eutherian mammals, such as treeshrews. Treeshews, close relatives of primates, have a sublingua, but it is less developed than in prosimian primates. Despite the differences, this shared trait suggests a phylogenetic relationship.

The prosimian sublingua is fully developed and unique, particularly in lemurs and lorisiforms. Tarsiers, which are prosimians most closely related to monkeys and apes (simians), also have a well developed but non-specialized sublingua. Simians, however, do not have a sublingua, although some, such as titis have a highly specialized frenal lamella (plica sublingualis). All primates have a plica sublingualis, and the fimbria linguae (plica fimbriata) found under the tongue of apes may be a vestigial version of the prosimian sublingua, although that is still disputed.

In the species that have cartilage in the sublingua or lytta, that cartilage is not derived from the hyoid bone or hyoid arch (the bone and cartilage that supports the tongue). Instead, the cartilage of the sublingua is a newly developed structure specifically adapted to support the sublingua.

References

Literature cited

• Ankel-Simons, F. (2007). Primate Anatomy (3rd ed.). Academic Press. ISBN 0-12-372576-3. {{cite book}}: Invalid |ref=harv (help)
• Osman Hill, W.C. (1953). Primates Comparative Anatomy and Taxonomy I—Strepsirhini. Edinburgh Univ Pubs Science & Maths, No 3. Edinburgh University Press. OCLC 500576914. {{cite book}}: Invalid |ref=harv (help)
• Tattersall, I. (2006). "Chapter 1: Origin of the Malagasy strepsirhine primates". In Gould, L.; Sauther, M.L (eds.). Lemurs: Ecology and Adaptation. Springer. ISBN 038734585X. {{cite book}}: Invalid |ref=harv (help)