Comparative Biochemistry and Physiology Part A 133 (2002) 203–212 1095-6433/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII: S1095-6433 Ž 02 . 00136-8 Running energetics of the North American river otter: do short legs necessarily reduce efficiency on land?  Terrie M. Williams *, M. Ben-David , S. Noren , M. Rutishauser , K. McDonald , a, b a a a W. Heyward a a Department of EE Biology, Center for Ocean Health – Long Marine Laboratory, 100 Shaffer Road, University of California, Santa Cruz, CA 95060, USA b Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA Received 11 June 2001; received in revised form 5 May 2002; accepted 7 May 2002 Abstract Semi-aquatic mammals move between two very different media (air and water), and are subject to a greater range of physical forces (gravity, buoyancy, drag) than obligate swimmers or runners. This versatility is associated with morphological compromises that often lead to elevated locomotor energetic costs when compared to fully aquatic or terrestrial species. To understand the basis of these differences in energy expenditure, this study examined the interrelationships between limb morphology, cost of transport and biomechanics of running in a semi-aquatic mammal, the North American river otter. Oxygen consumption, preferred locomotor speeds, and stride characteristics were measured for river otters (body masss11.1 kg, appendicularyaxial lengths29%) trained to run on a treadmill. To assess the effects of limb length on performance parameters, kinematic measurements were also made for a terrestrial specialist of comparable stature, the Welsh corgi dog (body masss12.0 kg, appendicularyaxial lengths37%). The results were compared to predicted values for long legged terrestrial specialists. As found for other semi-aquatic mammals, the net cost of transport of running river otters (6.63 J kg min at 1.43 m s ) was greater than predicted for primarily y1 y1 y1 terrestrial mammals. The otters also showed a marked reduction in gait transition speed and in the range of preferred running speeds in comparison to short dogs and semi-aquatic mammals. As evident from the corgi dogs, short legs did not necessarily compromise running performance. Rather, the ability to incorporate a period of suspension during high speed running was an important compensatory mechanism for short limbs in the dogs. Such an aerial period was not observed in river otters with the result that energetic costs during running were higher and gait transition speeds slower for this versatile mammal compared to locomotor specialists.  2002 Elsevier Science Inc. All rights reserved. Keywords: Cost of transport; Stride frequency; Corgi dog; River otter; Limb length; Energetics; Running  This paper was presented in the session, ‘Physiology and Biochemistry of Exercise’, at the Society for Experimental Biology, April 2–6, 2001, Canterbury, UK. *Corresponding author. Tel.: q1-831-459-5123; fax: q1-831-459-3383. E-mail address: williams@biology.ucsc.edu (T.M. Williams).