AN OBSERVATION ON THE PAPILLARY PROJECTIONS AT THE LINGUAL MARGIN IN THE STRIPED DOLPHIN

Papillary projections were observed along the anterolateral margins of the tongue in fetal and young stages of the striped dolphin, Stenella coeruleoalba. The projections appear in the fetal stage and attain maximum development in the early postnatal period. They disappear almost completely by weaning, although small vestigial eminences remain at the corresponding region in the adult. The projections may have a close relation to suck milk. There are few descriptions of the anatomy of the tongue in Cetacea, and many uncertainties remain concerning lingual structure and function in many species of this order of mammals. One of the most remarkable features of the tongue in Cetacea is the completely absence of lingual papillae. We have found papilla-like projections at the anterolateral margin of the tongue in fetal and young individuals of some species of dolphins. Growth and involution of these projections were demonstrated in the striped dolphin, Stenella coeruleoalba. The fact that the projections appear temporarily, and do not persist throughout life, has not been reported previously. As shown in Fig. 1, small projections, approximately 0.5 mm in length and about twenty in number on each side, are present at the lateral margins of the anterior one-third of the body of the tongue in a 27 cm fetus. The projections are distinct (1 to 2 mm long) occupying the entire anterior margin of the tongue in an 80 cm fetus. In a 98 cm fetus, they have grown longer (2 to 5 mm) and wider than those in the 80 cm fetus (Fig. 2) and some of them are subdivided at their tips (arrows in Fig. 2). In a young Stenella (body length 150 cm), projections have become somewhat thinner and broken, and some of them have disappeared in the posterior part (Fig. 3). In an adult animal, several distinct Sci. Rep. Whales Res. Inst., No. 28, 1976, 137-140. 138 YAMASAKI, SA TOMI AND KA:\1IY A eminences are present near the apex of the tongue (Fig. 4). In another larger adult one, the eminences have reduced in size and the lateral margins are slightly uneven in appearance. The entire dorsum of each tongue we observed was quite smooth and no lingual papillae of any type were present. Since the body length of Stenella coeriileoalba is about 100 cm at the time of birth (Kasuya, 1972), the appearance of the tongue in the newborn animal is probably similar to that in the 98 cm fetus we observed. Thus, it is certain that the papillary projections in the Stenella appear during intrauterine life and gradually develop towards parturition. The projections may be most numerous and well developed in the early postnatal period and they may begin to degenerate and disappear at about the 150 cm stage, which roughly corresponds to weaning in Stenella coeritleoalba (Kasuya, 1972). How the number of the projections increase and how the projections disappear are interesting points. It can be assumed that the former increase due to subdivision of the projections that arc present in the 98 cm fetus as shown with arrows in Fig. 2, and the latter vanish through repeated narrowings in several parts of each projection (arrows in Fig. 3) and finally shed off from the tongue proper. In the adult Franciscana dolphin, Pontoporia blainvillei, no papillae were present on the dorsum of the tongue but small eminences were observed at its lateral margins. Distinct papillary projections (2 to 3 mm long) were found at the anterior free margin of the tongue in a young, 98.5 cm, Pontoporia (Yamasaki, et al., 1976), and the same region was dotted with warts and fringes in a young animal (Burmeister, 1869). The small eminences at the anterolateral margins of the tongue of the adult Stenella and also of the adult Pontoporia appear to be the remnants of the papillary projections seen in the young animals. We also observed well developed projections (2 to 4 mm long) at the anterior margin of the tongue in a newborn finless porpoise, Neophocaena phocaenoides (71.5 cm). And foliate projections (3 to 6 mm), which decrease in their number with age, were observed in young bottlenose dolphin, Tursiopus truncatus, and Risso's dolphins, Grampus griseus (Nakajima, et aL, 1963). Thus, the papillary projections will be found in the fetuses and the young of some other dolphins or toothed whales. Although Anderson (1879) and Sonntag (1922) described filiform and fungiform papillae on the tongue of Ganges dolphin, Platanista gangetica, the dorsum of the tongues of the adults was completely smooth and no type of lingual papillae could be observed (Arvy and Pilleri, 1970, Yamasaki, et al., 1976). As they only observed the tongue in the young stage without observation in the adult, projections were propably described as the lingual papillae mentioned above. The projection is composed of a core of the lamina propria mucosa and a thick stratified squamous epithelium without taste buds. Both are continuous with those of the tongue proper. This structure, as a whole, is similar to that of filiform or fungiform papillae. However, the projections differ markedly from the lingual papillae of the general mammalian tongue in that they are temporary, localized at the anterolateral margin in a radial manner and large Sci. Rep. Whales Res. Inst., No. 28, 1976. LINGUAL PAPILLARY PROJECTION OF DOLPHIN 139 m size. Sonntag (1922) stated that the apex of the tongue in toothed whales may or may not have processes and warty growths, however, we suppose that he observed the tongues in variable stages of involution of the papillary projections. Since the projections only appear in a specific period during life, more care is neccessary as to the estimation of the age when observing the appearance of the cetacean tongue. The function of the lingual papillary projections described above is not known. Because they are well developed during the suckling period they may have an important mechanical functions during suckling in these aquatic mammals. As there as yet seem to be several unclear and interesting problems concerning the papillary projections in dolphins, further observations will be done on the relationship between the existence of the projections and the suckling period as well as on food habits and on the process of their development and degeneration, macroand microscopically. In addition, comparative functional anatomical studies with other cetaceans will be neccessary to clarify this problem.