The relatively low degree of complexity of behaviour in newborn animals makes them a very attractive model for experimental analysis (Campbell and Marbey, 1972; Groothuis, 1993; Eilam, 1994 b). Altricial rodents are typically born furless, have closed eyes and ears, are relatively immobile and are poorly supported by their limbs. Within 2–3 weeks, fur covers the body, the eyes and ears open and well-developed legs support the body. At this time, the pups exhibit marked hyperactivity to a degree that is rarely matched later in life (Spear and Barke, 1983). In the present study, we used this period of extensive changes to determine how gerbils develop their specialized mode of locomotion, in what order the movement types emerge and how the synchronous stepping typical of gerbils evolves in ontogeny. The morphology of an animal limits the range of movements that it may execute, thus providing a constraint to certain locomotor activities (Taylor, 1989). Since the morphological, physiological and behavioural demands imposed by walking and running increase markedly as speed, endurance and body size increase (Garland, 1983; Hildebrand, 1980, 1989), it is assumed that each animal will utilize its specific morphology to optimize locomotion (Alexander, 1989, 1992; Dagg, 1973; Eilam, 1994a). Rodents are an excellent group for assessing functional morphology, as they display a variety of body morphologies (Mendelssohn and Yom-Tov, 1987) and modes of locomotion (Djawdan and Garland, 1988). Since body mass and body morphology undergo extensive changes during the postnatal development of rodents, it is intriguing to determine whether motor performance is also transformed in the course of ontogeny. Consequently, the present work was aimed at studying the postnatal development of quadrupedal activity in Wagner’s gerbil (Gerbillus dasyurus). Gerbils, like other small cursorial mammals, run using a specialized mode of saltatory locomotion termed the ‘bound’ (Dagg, 1973; Gambaryan, 1974). In bounding, gerbils leap with their relatively long hindlegs and land on their relatively short forelegs, with each pair of legs acting synchronously. Bounders typically store elastic energy in their trunk, by arching it dorsoventrally when landing on the forelegs and unarching it when thrusting with the hindlegs in order to leap (Biewener, 1989). In accordance with this mode of locomotion, bounders share a typical body morphology: their hindlegs are longer than their forelegs and their trunk is arched in order to position the centre of body mass above the hindlegs, thus reducing yaw, pitch or roll torque (Dagg, 1973; Gabriel, 1984). While previous studies of postnatal behaviour in gerbils concentrated on the emergence of reflexes (Fox, 1964; Kaplan and Hyland, 1972) or on general biological perspectives (Valentine and Baudoin, 1980; Cabana et al. 1990), the present work focuses on describing the development of the bound and 363 The Journal of Experimental Biology 198, 363–372 (1995) Printed in Great Britain © The Company of Biologists Limited 1995 During the first two postnatal weeks, Wagner’s gerbil ( Gerbillus dasyurus) pups employed stepping sequences considered to be more basic than in the later stages of development and displayed alternate stepping of the legs in each girdle. In the third postnatal week, the adult mode of locomotion, the bound, became prominent and synchronous stepping dominated locomotion, gradually replacing alternate stepping. Motor performance of the gerbil pups corroborates previous studies in following a developmental motor gestalt termed ‘warm-up’, which involves an ordered incorporation of movements along three distinct spatial dimensions. In the present results, the validity of warm-up has been extended to a quantitative perspective: the order in which movement types reached their peak performance was identical to the order of their emergence in ontogeny. Transient modes of locomotion were also employed by gerbil pups during postnatal development, in accordance with changes in body morphology, indicating that there exists a causal link between body design and specific modes of locomotion. Key words: locomotion, walking, running, bound, stride, warm-up, rodent, gerbil, Gerbillus dasyurus . Introduction Summary POSTNATAL DEVELOPMENT OF SYNCHRONOUS STEPPING IN THE GERBIL (GERBILLUS DASYURUS) HAGIT BLUMBERG-FELDMAN AND DAVID EILAM* Department of Zoology, George S. Wise Faculty of Life-Sciences, Tel-Aviv University, Ramat-Aviv 69 978, Israel Accepted 12 August 1994 *Author for correspondence.