Sustained Swimming Performance in Crocodiles (Crocodylus porosus): Effects of Body Size and Temperature

—We examined effects of body size and temperature on swimming performance in juvenile estuarine crocodiles, Crocodylus porosus, over the size range of 30–110 cm total body length. Swimming performance, expressed as maximum sustainable swimming speed, was measured in a temperatureand flow-controlled swimming flume. Absolute sustainable swimming speed increased with body length, but length-specific swimming performance decreased as body length increased. Sustained swimming speed increased with temperature between 158C and 238C, remained constant between 238 and 338C, and decreased as temperature rose above 338C. Q10-values of swimming speed were 2.60 (6 0.091 SE) between 188C and 238C, and there were no differences in Q10 between crocodiles of different sizes. The broad plateau of thermal independence in swimming speed observed in C. porosus may be of adaptive significance by allowing dispersal of juvenile animals at suboptimal body temperatures. Locomotory performance is intrinsically linked to ecological performance and fitness, because it directly impacts an animal’s ability to capture prey, disperse, avoid predators, and, in the case of crocodilians, engage in social interactions linked to reproduction (Bennett, 1982; Jayne and Bennett, 1990; Seebacher and Grigg, 2001; Vliet, 2001). Sustained aquatic locomotion in fish is powered exclusively by red, aerobic muscle fibers, whereas sprint performance is determined by a combination of red and anaerobic white muscle fibres (Beddow et al., 1995; Gillies, 1998; Reidy et al., 2000). However, differences in swimming performance between species and between higher taxonomic categories may be determined not only by differential power output of muscle fibers but also by hydrodynamic forces associated with body shape, which can play a major role in limiting locomotor performance (Wolfgang et al., 1999; Drucker and Lauder, 2000). In cetaceans, for example, different morphologies result in different hydrodynamic drag, and a combination between drag and muscle power output determines locomotor performance in these animals (Fish, 1998). Hence, the energetic cost of swimming is directly related to body shape and swimming mode, and it may be particularly high in semiaquatic animals, which have to function both on land and in water (Frey and Salisbury, 2001). For example, the semiaquatic water rat Hydromys chrysogaster incurs a much greater metabolic cost during swimming than fully aquatic mammals do (Fish and Baudinette, 1999). Crocodiles, like water rats, are semiaquatic so it may be expected that swimming in crocodiles would be relatively expensive compared to wholly aquatic animals such as fish. As a consequence, it may be that sustained swimming performance, often measured as 3 Corresponding Author. E-mail: [email protected]. edu.au the maximum sustainable swimming speed (Ucrit; Holk and Lykkeboe, 1998; Plaut, 2000) is poorer in semiaquatic animals compared to wholly aquatic animals. Nonetheless, crocodilians are primarily aquatic, and most ecologically important behaviors, such as prey capture (Sah and Stuebing, 1996; FS pers. obs.), reproduction (Vliet, 2001), social interactions (Lang, 1987; Seebacher and Grigg, 1997; Seebacher and Grigg, 2001), and dispersal (Webb and Messel, 1978; Sah and Stuebing, 1996; Tucker et al., 1998; Munoz and Thorbjarnarson, 2000) occur in water. Hence, swimming performance is of greater ecological importance in crocodilians than terrestrial locomotion. During much of their activity, crocodilians perform only short bursts of locomotion which are almost exclusively powered by anaerobic metabolism (Bennett, 1990). However, sustained swimming, fuelled by aerobic metabolism, is important during dispersal (Tucker et al., 1997, 1998), and during long migrations which may be up to thousands of kilometers (Brazaitis, 1973; Allen, 1974). Moreover, crocodilians are the most social of all reptiles (Lang, 1987) and often show sustained activity when establishing social hierarchies (Grigg et al., 1998; Seebacher and Grigg, 2001) and during mating (Vliet, 1989, 2001). Dispersal, social interactions, and reproductive behavior are linked to life-history stage of these ectotherms, so that the interaction between body size and temperature in determining locomotory performance is likely to be ecologically important. It was the aim of this research to determine the effects of body size and temperature on the aerobic swimming performance, measured as the maximum sustainable, or critical swimming speed (Ucrit; Fish, 1984; Graham et al., 1990). MATERIALS AND METHODS Juvenile estuarine crocodiles, Crocodylus porosus (N 5 10), were purchased from a commercial crocodile farm (Cairns Crocodile Farm, Cairns, Australia). Cro364 SHORTER COMMUNICATIONS FIG. 1. Effect of body length on critical swimming speed at 188C (N 5 13) and 338C (N 5 15) for Crocodylus porosus. TABLE 1. Linear regression equations relating absolute (cm/sec) and length-specific (BL/sec) critical swimming speed to body length (BL) for Crocodylus porosus at five experimental temperatures.