B. Sensory Systems and Behaviour

Transcription

1 B. Sensory Systems and Behaviour Animals perceive their environment using a suite of senses: hearing, vision, smell, taste, touch, and, in some cases, possibly through magnetism. Marine mammals are no exception, but because they live part or all of their lives in an aquatic environment, some senses are developed over others. Water has a number of properties that influence the role that different senses may play. Sound travels better through water than air, whereas light penetrates less well through water. It is therefore not surprising to find that those marine mammals most dependent upon water for living have developed sound as the most sophisticated sense for the detection of cues from their environment, and for communication to others both in close proximity and over great distances. This is exemplified by the order Cetacea, which has also been the group where sound production and hearing have been best studied, and, in chapter 4, Jonathan Gordon and Peter Tyack explore the many facets of the use of sound by cetaceans. Marine mammals can hear sounds over a wide range of frequencies, with the smaller odontocete cetacean species being sensitive to sounds at the highest frequencies (ultrasounds up to khz), and the largest mysticete cetaceans apparently to sounds at the lowest frequencies (down to 20 Hz). Although audiograms have been established for several odontocete cetaceans, as well as pinnipeds, there are scarcely any data for mysticete whales, and so we have to infer their hearing sensitivities from the normal frequency range of sounds they produce. Both mysticetes and odontocetes have soft-tissue channels for sound conduction to the ear; sirenians, the other truly aquatic group, have similar hearing structures. Pinnipeds need to be able to hear sounds both in air and underwater. Above water they use their external air-filled canal for sound reception. However, it is not known whether they have specialised mechanisms underwater for maintaining the external canal as the point for sound reception or whether they too use softtissue channels for this function. Phocid seals appear to be better adapted to hearing underwater than otariids (with bigger differences in peak sensitivities underwater compared with in air) and this fits in with their more aquatic lifestyle (Wartzok and Ketten, 1999). Peak hearing sensitivity in pinnipeds is between 10 and 30 khz (with a high frequency limit of 60 khz in phocids, and 40 khz in otariids). Although little data exist for sirenians, manatees appear to have a hearing range of approximately 0.l-40 khz, with peak sensitivities in the West Indian manatee near 16 khz, and 5-10 khz in the Amazonian manatee, although their audiograms are very flat compared with odontocete cetaceans, and more similar to phocid seals (Richardson et at., 1995). 133

2 134 Marine Mammals: Biology and Conservation Sea otters have ears most similar to those of land mammals. There is no information on how well they hear underwater. The same applies to polar bears. Unlike terrestrial mammals, all marine mammals have middle ears that are strongly modified structurally to reduce the impact of rapid and extreme changes in pressure. This may possibly predispose them to some protection from injury by high intensity noise (Wartzok and Ketten, 1999). Of the other senses, vision appears to be the one that remains most important for marine mammals, and is probably used by most species even at depth. Cetaceans apparently have the most acute underwater vision, adapting the lens of their eye to accommodate for the loss of focusing power of the air-cornea interface. Sirenians are less well adapted to underwater vision, and pinnipeds, with their dual need to see in air and water, have poor visual acuity in air except in bright light. The sea otter has good visual acuity in both media, achieved by changing the radius of curvature of the lens with the use of the ciliary and iris muscles, although like others, its visual acuity is poorer in low light. The polar bear, living primarily on land, has a similar eye anatomy to terrestrial mammals. The sense of smell, or olfaction, has been reduced in many marine mammals. It is apparently absent in cetaceans (adult odontocetes do not possess olfactory bulbs or nerves, whilst these are greatly reduced in mysticete whales), and only rudimentary in sirenians. Pinnipeds, on the other hand, use olfaction on land particularly during activities related to breeding, as do sea otters and polar bears. Taste is also not unduly developed amongst marine mammals, although tests have shown that different chemicals can be distinguished. This applied particularly to those with a bitter or sour taste, whereas there was a less well-developed ability to detect salinity and an apparent inability to detect sugar. Most work has been conducted on captive odontocete cetaceans, although limited studies on pinnipeds gave broadly similar results. Sirenians possess more taste buds than cetaceans but they remain less than herbivorous land mammals. Amongst marine mammals, sirenians have the most developed sensory hairs. These cover the entire body, although particularly dense around the muzzle. The manatee is unique in being able to use its bristles or vibrissae in a prehensile manner; the bristles can actually be everted and used to grasp and manipulate objects, as when feeding. The vibrissae ofpinnipeds are also extensively developed, and well supplied with nerve endings. They provide seals particularly with information about form and texture and are often used in a social context (Wartzok and Ketten, 1999). Cetaceans appear to be most sensitive around the blowhole although some baleen whales also have hairs around the mouth.

3 Sensory Systems and Behaviour 135 The presence of magnetite, which has been implicated in magnetic field detection, has been demonstrated in a wide variety of animal taxa including cetaceans. However, magnetite is a very common contaminant from industrial processes and to distinguish this from functional magnetite, researchers have identified those characteristics most likely to indicate its use in magnetic field detection. These include single-domain crystals, magnetite with few oxides (besides iron oxide), and location of magnetite consistently in certain parts of the body, notably associated with the cerebrum and cerebellum. However, increasing magnetite concentrations with age parallels the ossification of these structures, for which it has a known function, providing structural support by increasing the hardness of chitin in teeth. Other evidence that has been used relates to correlations between locations where geomagnetic lows intersected with the coast or islands, with live strandings of cetaceans. This was demonstrated for the UK and eastern United States, but no such correlation was found in New Zealand, possibly because that country lacks the consistent orientation of magnetic contours that the North Atlantic possesses (Klinowska, 1986; Kirschvink et al., 1986; Kirschvink, 1990; Brabyn and Frew, 1994). The difficulty of demonstrating in any experimental way the mechanism by which this potential sense could be used continues to limit our ability to determine its role with confidence. Two excellent reviews of sensory systems in marine mammals are Thomas et al. (1992), and Wartzok and Ketten (1999), whilst Thomas and Kastelein (1990) present evidence from laboratory and field studies specifically upon cetaceans. The ways that animals perceive their environment mould their behaviour within it. Information about the environment is first processed by the nervous system, and, in mammals, especially by the brain. This is termed cognition. We do not have a specific chapter on this subject, but Schusterman et al. (1986) have a book on cognition in dolphins; Tyack (1999) gives a brief review of the role of cognition in marine mammal communication; and it is explored further in Chapter 4 of this book, by Jonathan Gordon and Peter Tyack. Cetaceans in particular are often represented as big-brained, highly intelligent creatures displaying altruistic traits and a sophisticated vocal learning and communication system that invite comparison with humans. Given the very different evolutionary paths taken by these two taxa, and the fact that cetaceans live in an aquatic medium, and humans a terrestrial one, this comparative approach may not be very helpful. True, the sperm whale has the largest brain (7.8kg) of any mammal, though not relative to its body size (and by allometric growth, larger mammals have larger brains). Ifbody weight is taken into account, the bottlenose dolphin (and some other small

4 136 Marine Mammals: Biology and Conservation odontocetes like the white-beaked dolphin) has a brain nearly as large (c %) as a human ( %). Pinnipeds show relative brain sizes (termed encephalisation quotients, or EQs) close to the overall pattern for mammals, whilst sperm whales, mysticete whales, and sirenians have relatively small brains (Worthy and Rickie, 1986). Furthermore, relative brain size is not equivalent to intelligence. A more meaningful feature is probably the degree of folding of the cerebral cortex; that part of the brain most concerned with higher level neuronal processing. In this respect, odontocetes do actually exceed humans and other animals, and various theories have been proposed to explain these findings. They include the need to rapidly process complex, high frequency sounds as used in echolocation; historical redundancy of the large brain (which has been large in odontocetes over a long period of evolution); and the associated development in sociality with long term individual associations (Wood and Evans, 1980; Herman, 1980; Ridgway and Brownson, 1984; Ridgway, 1986; Worthy and Rickie, 1986; Marino, 1998). At present, we do not know which of these theories, if any of them, is correct, although there is increasing support for the latter explanation (see, for example, summaries by Evans, 1987: 202, and Tyack, 1999: ). The study of behavioural ecology attempts to answer questions about the functions of behaviour, particularly in the context of an individual's or species' environment (Krebs and Davies, 1997). In Chapter 5, James Boran, Peter Evans and Martin Rosen examine patterns of behaviour as exhibited by cetaceans in terms of two main survival strategies - foraging and mating. They review the roles of locating resources (food and mates) and avoiding predators in the development of sociality, and they show how recent advances in genetics have enabled us to have a better idea of the mating systems of species with various ecologies. Luis Cappozzo continues this theme in Chapter 6, when he reviews new perspectives on the behavioural ecology of pinnipeds, with particular reference to the evolution of pinniped social organisation and mating behaviour. REFERENCES Brabyn, M. and Frew, R.V.C. (1994) New Zealand herd stranding sites do not relate to geomagnetic topography. Marine Mammal Science, 10, Evans, P.G.H. (1987) The Natural History of Whales and Dolphins. Christopher Helm/Academic Press, London, and Facts on File, New York. 343pp. Hennan, L.M. (1980) Cognitive characteristics of dolphins. In: Cetacean Behavior (Ed. by L.M. Hennan), pp Wiley Interscience, New York, NY.

5 Sensory Systems and Behaviour 137 Kirschvink, ll. (1990) Geomagnetic sensitivity in cetaceans: An update with live stranding records in the United States. In: Sensory Abilities of Cetaceans: Laboratory and Field Evidence (Ed. by J.A. Thomas and R.A. Kastelein), pp Plenum, New York, NY. Kirschvink, ll., Dizon, A.E., and Westphal, la. (1986) Evidence from strandings for geomagetic sensitivity in cetaceans. Journal of Experimental Biology, 120, Klinowska, M. (1986) The cetacean magnetic sense - evidence from strandings. In: Research on Dolphins (Ed. by M.M. Bryden and R. Harrison), pp Clarendon Press, Oxford. Krebs, lr. and Davies, N.B. (1997) Behavioural Ecology: An Evolutionary Approach. Blackwell Scientific Publications, Oxford. Marino, L. (1998) A comparison of encephalization between odontocete cetaceans and anthropoid primates. Evolutionary Anthropology, 5, Richardson, W.J., Greene, C.R. Jr., Malme, c.r. and Thomson, D.H. (1995) Marine Mammals and Noise. Academic Press, San Diego. 576pp. Ridgway, S. (1986) Physiological observations of dolphin brains. In: Dolphin Cognition and Behavior: A Comparative Approach (Ed. by R.J. Schusterman, la. Thomas, and F.G. Wood, F.G.), pp. 3 I-59. Lawrence Erlbaum, Hillsdale, Nl Ridgway, S.H. and Brownson, R. H. (1984) Relative brain sizes and cortical areas of odontocetes. Acta Zoologica Fennica, 172, Schusterman, R.J., Thomas, la., and Wood, F.G. (Eds.) (1986) Dolphin Cognition and Behavior: A Comparative Approach. Lawrence Erlbaum, Hillsdale, Nl Thomas, la. and Kastelein, R.A. (Eds.) (I990) Sensory Abilities of Cetaceans: Laboratory and Field Evidence. Plenum, New York, NY. Thomas, la., Kastelein, R.A., and Supin, A.Y. (Eds.) (1992) Marine Mammal Sensory Systems. Plenum, New York, NY. Tyack, P. (1999) Communication and Cognition. In: Biology of Marine Mammals (Ed. by J.E. Reynolds III and S.A. Rommel), pp Smithsonian Institution Press, Washington DC. 578pp. Wartzok, D. and Ketten, D.R. (1999) Marine Mammal Sensory Systems. In: Biology of Marine Mammals (Ed. by J.E. Reynolds III and S.A. Rommel), pp Smithsonian Institution Press, Washington DC. 578pp. Wood, F.G. and Evans, W.E. (1980) Adaptiveness and ecology of echolocation in toothed whales. In: Animal Sonar Systems (Ed. by R.-G. Busnel and J. Fish), pp Plenum, New York, NY. Worthy, G.A.J. and Hickie, J.P. (1986) Relative brain size in marine mammals. American Naturalist, 128,