Bumblebees don't push acceleration limits

Abstract

Stuck in our concrete box cities, we can sometimes forget just how complex the natural world is, but from the perspective of a foraging bumblebee, it's a convoluted three-dimensional maze. Whether negotiating a tree's foliage or waving grass in a meadow, bumblebees constantly have to deal with clutter in their surroundings, ‘[But] we still know relatively little about how insects and flying animals generally deal with complex natural environments’, says James Crall from Harvard University, USA. Intrigued by the challenges that these intrepid insects encounter on a daily basis, Crall and his colleagues, Sridhar Ravi, Andrew Mountcastle and Stacey Combes decided to find out which factors restrict bumble bees as they manoeuvre past obstacles and how much their size affected their manoeuvrability.‘During previous research we found that… bees tend to move more laterally than vertically,’ explains Crall, so the team designed two flight tunnels – one with horizontal obstacles that forced the insects to fly a wavy up-and-down path through the obstructions and a second with vertical structures that forced them to dodge from side to side – to test their horizontal versus their vertical manoeuvrability. Although Crall admits that wrangling the bees was even trickier: ‘It's always a challenge to get regular, natural flight behaviour out of insects in laboratory environments’, he shrugs. But then the team placed the tunnel between the bee's hive and the outside world, forcing them to negotiate the obstacles every time they went foraging.Once everything was in place, Crall began filming the insects’ manoeuvres in three dimensions with a pair of high-speed video cameras before analysing the complex flight paths. However, the team was surprised when their theory that the bees were more agile dodging sideways than vertically did not hold out. Explaining that the bees’ main goal is to use as little energy as possible while foraging, Crall used the length of time that it took the insects to complete the route as a measure of each individual's performance: ‘as they would say, “time is honey”’, he chuckles. However the bees were no more speedy negotiating obstacles when they had to swerve from side to side, than when weaving up and down.Crall also recorded the insects’ maximum acceleration, the curviness of the path that they followed and the number of times that they had to back-up and take another run past an obstruction to avoid a collision as they swerved along the tunnel. However, the bee's ability to accelerate hard didn't improve their transit time and larger bees seemed to bumble along more slowly than their smaller nest-mates; they chose a more cautious approach, taking pains to avoid collisions by backing away from objects before negotiating them and giving obstacles a wider berth.‘Bumblebees are probably performing well below their mechanical and physiological limits in order to minimize collision risk’, says Crall, adding, ‘This probably makes sense, since in natural environments flying animals are balancing a lot of constraints, such as risk of collision, predation, navigational and sensory challenges’. And now Crall is keen to find out whether the bumblebee's performance improves with familiarity. ‘Maybe as bees learn an environment better, they fly through it faster and faster until they eventually become limited by acceleration’, he says.