'est Indian 

 Tuskers 



by Daryl R Domning 



Amid the contemporary traffic of the 

 Horida and Caribbean coasts, the rotund 

 marine mammals loiown as West Indian 

 manatees attempt to live the slow, deliber- 

 ate life of aquatic grazers. Found in both 

 tropical salt water and the fresh waters of 

 inland springs, these sirenians, or sea 

 cows, placidly paddle through warm wa- 

 ters, grazing on a wide assortment of fi- 

 brous-leaved water plants, including the 

 introduced water hyacinth. Intermittently, 

 a manatee snout breaks the surface; after a 

 breath of air, the animal closes its nostrils 

 and silently submerges. Half a world 

 away, the manatees' look-alike but strictly 

 saltwater cousins, the dugongs, quietly ply 

 warm shallows of the Indian and south- 

 western Pacific oceans. While manatees 

 have an ever-growing series of teeth 

 adapted to the abrasive grasses that grow 

 in fresh water, dugongs specialize in eat- 



ing softer, less abrasive sea grasses that 

 they uproot with a pair of tusks in their 

 upper jaws. 



Sirenians have a long history, first ap- 

 pearing on earth some fifty million years 

 ago, and their family tree has included 

 denizens of cold as well as warm waters. 

 The huge Steller's sea cow, for example, 

 inhabited the waters of the North Pacific 

 and Bering Sea, until it was hunted to ex- 

 tinction in 1768, just twenty-seven years 

 after its discovery {see "A Sea Cow Fam- 

 ily Reunion," Natural History, April 

 1987). Nor have dugongs and manatees 

 always so neatly divided their tropical 

 realms between the Atlantic and Indopa- 

 cific oceans. West Indian manatees are, ge- 

 ologically speaking, relative newcomers to 

 the Caribbean; for millions of years, their 

 cousins the dugongs dominated the tropi- 

 cal Western Hemisphere. Not only were 



these ancient dugongs abundant, they 

 were diverse. From the Oligocene to the 

 Pliocene — that is, from more than thirty to 

 less than five million years ago — at least 

 three, probably more, kinds of dugongs 

 lived together in the Caribbean. 



This newly discovered diversity raises 

 the question of how these different spe- 

 cies, which had such similar diets, could 

 have coexisted in the same environment. 

 Today, no place in the world supports 

 more than a single species of sirenian. 

 What, if anything, was different about the 

 Caribbean during much of the Age of 

 Mammals that promoted a degree of sea 

 cow diversity unknown today? And what 

 caused these animals to later die out? 

 Much of my work with fossil sirenians has 

 focused on how various combinations of 

 anatomy and behavior might have allowed 

 these separate species to share the avail- 

 able marine plant foods. 



Most of the extinct Caribbean dugongs, 

 like their living Indopacific relatives, 

 wielded impressive tusks. Some were 

 more than a foot long and were shaped like 

 knives or chisels, with self-sharpening 

 enamel edges. These were not carried for 

 show; lodged solidly in deep sockets in the 

 upper jaw, with only a few inches of tip 

 exposed, they were powerful tools that 

 could have been used in combat, as are the 

 tusks of modem male dugongs. But while 



72 Natural History 4/94 



