As animals that require substantial parental care, we cannot fathom the notion of being on our own from day one. Yet as spring arrives, this is the reality faced by many newborns. In such young, the selective pressure to escape predators is probably high from the moment they leave their mother or egg. However, the study of the ontogeny of locomotor performance in animals remains in its infancy; only a few systems have been studied in any detail. In general, vertebrate studies have shown that maximum speeds and levels of endurance increase as animals develop and grow and that these `improvements'are linked to physiological changes in the underlying musculoskeletal and oxygen transport systems. In a recent paper published in Animal Behaviour, Scott Kirkton and Jon Harrison use the grasshopper, Schistocerca americana, as a model for studying shifts in locomotor capacity through ontogeny in an insect. It appears that the results from vertebrates don't really apply; instead, an understanding of the grasshopper's life history seems to make better sense of their ontogenetic performance trajectory.
Life history patterns in S. americana suggest a locomotor trade-off as animals mature. Specifically, juvenile grasshoppers must evade predators and travel over long distances, but do so using only their legs. In contrast, adults gain the use of wings and fly to cover significant ground. The authors chose to study jumping over a range of developmental stages to test the hypothesis that juveniles exhibit relatively high levels of performance using this locomotor mode, since it is their sole means of transport. Because there is likely stronger selective pressure on jumping performance in juveniles, compared to adults, the authors predicted that both jumping speed and jumping endurance would be higher in younger grasshoppers.
To test their prediction, Kirkton and Harrison studied the jump performance of grasshoppers in four developmental stages: second, fourth and sixth instars and adults. Size across these animals ranged 30-fold from a mean of 0.06 g in the second instar to 1.78 g in the adults. For performance trials, individuals were placed in a large gridded jumping arena and prodded to jump for 20 minutes or until fatigued (defined as 30 s of prodding without a jump). Adults had their wings clipped prior to these trials to prevent them from flying. Jumping frequency and distance, as well as jump energy and power were determined for each minute of a jumping trial.
All results indicated that adults had relatively low endurance. Adult jump frequencies plummeted within 5 minutes of activity and only about one third of adults sustained jumping for the full 20 minutes. For comparison, 2nd and 4th instars exhibited little decrement in frequency over time and approximately 90% continued jumping the entire 20 minutes, agreeing with the team's prediction regarding juvenile endurance. However, despite their poor endurance, adults consistently jumped further and faster early in trials and generated more mass-specific power throughout, suggesting they might rely more heavily on anaerobic metabolism for jumping (older grasshoppers also produce more lactic acid when jumping). So, adult grasshoppers have poor stamina but can jump powerfully, which seems quite reasonable given that they can fly and need only one good jump to get airborne. In contrast, younger instars don't have as much power, but have high endurance, enabling long distance movements over time, for escape or dispersal. In short, neither adults nor juveniles are great in all aspects of jumping performance, but they're good at what they need to do.
