Watching frenetic leaf-cutter ants wielding chunks of leaf, nothing seems to get in their way; but even industrious leaf-cutter ants may opt to not take routes that incur sizeable metabolic costs. ‘If you put an obstruction in front of an animal that increases the energetic costs, then it becomes a behaviour consideration as to which way the animal should go. Should it take the shortest route, which is straight over the hill but encounter steep gradients or should it go on longer routes and encounter shallower gradients?’ queries Graham Askew from the University of Leeds, UK. Intrigued by the energetics of animal locomotion, Askew decided to identify the factors that influence the decisions taken by leaf-cutter ants while negotiating obstacles. Teaming up with graduate student Natalie Holt, Askew decided to measure the metabolic cost of climbing and descending slopes for leaf-cutter ants in order to begin to understand how the animals select routes (p. 2545).
However, Askew admits that measuring the minute amounts of carbon dioxide exhaled by the tiny insects was technically challenging. Designing a 30 cm long tube that could be inclined at a range of angles for the insects to run up and down, Askew and Holt used a recirculating air system to measure the miniscule quantities of carbon dioxide exhaled in the tube by the ants during their exertions. Allowing the ants to run at their own speed, the duo also rotated the tube when the insect reached the end so that it could continue running at the same angle until they had a reliable reading of its metabolic rate. Inclining the tube at 30 deg increments ranging from a vertical ascent to a horizontal run and finally a vertical descent, the duo filmed the ants' progress and recorded their metabolic rates.
Not surprisingly, the ants' speed fell dramatically as the angle increased, dropping from a speedy 2 cm s–1 on the level to 0.7 cm s–1 when scaling a precipice. Yet, when the duo analysed the insects' metabolic rates over the entire range of ascents and descents, they found that the insects all worked at the same metabolic rate. No matter which angle the ants were ascending or descending they produced carbon dioxide at a rate of 1.7 ml g–1 h–1. ‘They are putting in the same effort regardless of gradient’, says Askew.
However, calculating the metabolic cost of moving a set distance along the tube, Askew and Holt realised that the insects that were ascending and descending the steepest slopes had the highest metabolic costs per unit distance. ‘They are trying to maintain a constant metabolic rate. They do that by modulating speed and the consequence of that is that the cost per unit distance increases at the steepest gradients’, says Askew.
Finally, the duo calculated the vertical cost of locomotion over the range of ascents and descents. Explaining that insects have to trade off taking a shallower gradient – where the cost of locomotion is less but it takes longer to complete because it is farther – against a shorter route with a steeper gradient – which takes less time but at a higher cost of locomotion – Askew says, ‘The vertical cost of locomotion tells you the optimal gradient at which the animals can gain height for the minimal energy expenditure’. Calculating the vertical cost of locomotion for ascent and descent, the duo found that the descending ants should select gradients between –45 and –51 deg, while climbing ants should select gradients ranging from 51 to 57 deg to minimise their time and energy expenditures. And Askew is keen to find which paths and gradients the ants select in practice and how their choices alter with time.