Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics
Last time I wrote about spiders using electrostatic repulsion to spin webs and go ballooning. That sent me searching and I found a fascinating (and rather beautifully written) paper form Daniel Robert’s group in Bristol about Bumblebee use of electrostatic charges. It turns out that the atmosphere has a potential gradient – a field of static charge that increases with height by about +100 volts per metre, reaching the lightning-triggering 30kV at stratospheric levels, and on to perhaps 300kV at the Ionosphere. It’s easy to generate a static charge by, for example, rubbing a balloon on your hair, or on a jumper and when charged the balloon will stick to the ceiling and cause hairs to stand on end when it is brought near a hairy arm or head. In an experiment that sounds entirely sensible (!) Clarke, Morley and Robert investigated the triboelectric rank of bumblebees by ‘rubbing the dorsal surface of freshly-killed bees against various surfaces, both synthetic and natural’. They found that bumblebees rank to the extreme upper end of the positive scale for triboelectrics. Rub a bumblebee and it gets a positive charge. When a positively charged bumblebee approaches a flower, the flower is relatively negatively charged, the interaction between positive and negative charges causes the bumblebees hairs to stand on end, and the bumblebee can detect that effect with the mechanosensors at the base of the hairs. When the bumblebee lands on the flower the charges discharge. Bumblebees use that fact to detect flowers that haven’t recently been visited and therefore have a stronger negative charge (are more hair-raising) and still have a full load of nectar and pollen.
Now, which came first – the hairs, or the electrosensing? Bumblebees are conspicuously furry, and also a palearctic species – they are at the southern end of their range in the UK. Bernd Heinrich in his beautiful book ‘Thermal Warriors’ described the experiments he did to measure the body temperature of bumblebees mid-flight (welding gloves and micro-thermocouples were involved), and it turns out that Bumblebees are warm-blooded. They shiver to warm up before they fly, and they can only generate enough power to fly once their flight muscles are up to operating temperature. The hairs of bumblebees (and many other insects) probably came first, to aid with insulating their tiny warm bodies.
Dominic Clarke, Erica Morley ·and Daniel Robert. The bee, the flower, and the electric field: electric ecology and aerial electroreception. J Comp Physiol A (2017) 203:737–748 DOI 10.1007/s00359-017-1176-6