Sensory biology of aquatic mammals
Mammals that exclusively or partly live in the water have to cope with many physiological challenges, including challenges to their sensory systems. With the transition from the terrestrial to the aquatic environment, the different physical properties of air and water required new adaptations. Many mammalian species still live in both worlds, and we include these semi-aquatic mammals in our present definition of aquatic mammals. This special issue on the ‘‘Sensory biology of aquatic mammals’’ presents reviews and original studies on most major taxa that have been studied so far. The authors of original studies were encouraged to extend the discussion part to aid the nonspecialist reader in gaining an overview. This special issue is closely related to, but not exactly congruent with, an international conference with the same title that was held at the Marine Science Center in Rostock, Germany, in October 2012. Marine mammals are the most extensively studied group of aquatic mammals. This is partly due to the fact that they represent the majority of species of aquatic mammals. Furthermore, marine mammals have often been more relevant economically to humans than freshwater species, e.g. in fisheries or whaling. And last but not least, people have a deeply rooted fascination for marine mammals. Maybe this is founded in the awareness that in the animal kingdom marine mammals are our close relatives, but at the same time they easily roam the last unexplored areas of the earth, where humans have not really managed to follow them even with modern technology. The three major groups of marine mammals are the pinnipeds, the cetaceans and the sirenians (dugong and manatees). They are covered by original work and a review in this special issue. In addition, the polar bear (Ursus maritimus) and the sea otter (Enhydra lutris) are counted among the marine mammals. For information on the sensory biology of the polar bear, the reader is referred to Nachtigall et al. (2007); for the sea otter, to Murphy et al. (1990) and Ghoul and Reichmuth (2012); for marine mammals in general, to Supin et al. (2001), Dehnhardt (2002) and Thewissen and Nummela (2008). All major clades of marine mammals also include species that live fully or partly in fresh water, such as the Baikal seal (Pusa sibirica), two families of river dolphins (Iniidae and Platinistidae), dolphins of the genera Sotalia, Sousa and Orcaella, and the manatees (Trichechus). Aquatic mammals originally living in limnic waters have also attracted a lot of attention. In the present special issue, they are represented by an insectivore and a monotreme, the American water shrew (see article by K. C. Catania) and the duck-billed platypus (see article by N. Czech-Damal et al.). The anatomy of some sensory systems was also studied in other originally limnic mammals: the otters of the genus Lutra, the hippopotamus (Hippopotamus amphibius), the beavers (Castor spp.) and the Australian water rat (Hydromys chrysogaster). A particularly large body of scientific literature has dealt with the sense of hearing. This focus is partly due to the For special issue ‘‘Sensory biology of aquatic mammals’’.