Ontogeny of locomotory behaviour in the American locust, Schistocerca americana: from marathoner to broad jumper SCOTT D. KIRKTON & JON F. HARRISON Section of Organismal, Integrative, and Systems Biology, School of Life Sciences, Arizona State University, Tempe (Received 24 May 2005; initial acceptance 14 July 2005; final acceptance 7 September 2005; published online 24 February 2006; MS. number: A10172) Developmental changes in locomotory ability may affect many aspects of organismal behaviour and eco- logical success. While juvenile vertebrates have lower power output and endurance than older animals, the effects of ontogeny on locomotory performance in invertebrates are unknown. Among insects, grasshop- pers serve as an ideal model to study ontogeny of locomotory behaviour. Ecological and behavioural stud- ies show that adult and juvenile grasshoppers have different predators and dispersal behaviours, suggesting different locomotory abilities. In addition, older American locust grasshoppers, Schistocerca americana, have improved oxygen delivery, which may enhance endurance. In this study, we forced American locust grass- hoppers of different ages to jump repeatedly to exhaustion and quantified jump frequencies and distances. We found that, in contrast to vertebrates, grasshoppers appear to develop from low-power, high-endurance juveniles to high-power, low-endurance adults. These developmental changes in physiology match the on- togeny of life-history behaviours. Adult grasshopper jumping legs produce the high power output required to initiate flight for escape from vertebrate predators and migration, whereas high-endurance juvenile grasshoppers repeatedly jump to escape invertebrate predators and may also be forced to disperse long dis- tances to find food. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Investigating behavioural changes during an organism’s development may provide insight into both the evolution of these behaviours and differential ecology of various life stages (Stamps 2003). Understanding the behavioural capacities of juvenile stages is particularly important, since juveniles have increased mortality rates compared to adults and individuals at this developmental stage may be most influenced by natural selection (insects: Agnew et al. 2002; crabs: McDonald et al. 2001; fish: Gust et al. 2002; birds: Frederiksen & Bregnballe 2000; mammals: Sendor & Simon 2003). One behavioural trait critical to an animal’s survival and subject to increased selection in juveniles is locomotory performance (Wassersug & Sperry 1977; Carrier 1996). Juvenile and smaller vertebrates gen- erally have reduced speeds and endurance compared to older, larger vertebrates (Garland 1984; Carrier 1996; Wakeling et al. 1999; Van Praagh & Dore ´ 2002), but this issue has hardly been addressed for invertebrates. In this study, we tested the hypothesis that developmental differ- ences in sustained locomotory performance in grasshop- pers reflect observed ontogenetic specific variation in predation pressures and migratory behaviour. Increased speed and endurance permit older vertebrates to better escape predators, migrate and engage in other energetically costly behaviours (such as territoriality, courtship and mating). Reduced speeds and endurance (stamina) in juvenile vertebrates have been attributed to both body size effects and developmental changes across various taxa including reptiles (Bennett et al. 1989; Jayne & Bennett 1990) and mammals (Carrier 1995; Horning & Trillmich 1997). Developmental changes that improve lo- comotory endurance in older vertebrates include a greater proportion of muscle mass (Garland 1984), an increased aerobic capacity (Garland & Else 1987) and an improved blood oxygen-carrying capacity (Pough 1977, 1978). Although it is unknown how ontogeny affects speed and endurance for invertebrates, physiological and eco- logical data provide different predictions for how de- velopment may affect grasshopper endurance. Similar to Correspondence and present address: S. D. Kirkton is at the Division of Physiology, University of California, San Diego, 9500 Gilman Drive, 0623A, La Jolla, CA 92093-0623, U.S.A. (email: skirkton@ucsd. edu). J. F. Harrison is at the Section of Organismal, Integrative, and Sys- tems Biology, School of Life Sciences, Arizona State University, PO Box 874601, Tempe, AZ 85287-4601, U.S.A. 925 0003–3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 71, 925–931 doi:10.1016/j.anbehav.2005.09.010