Despite their nocturnal and aerial lifestyle, bats are still at risk from predators. Weasels and stoats can scale the walls of bat roosts and young and old bats are in danger from foxes if they fall. Tess Driessens from Vrije Universiteit Brussels, Belgium, and Björn Siemers from the Max Planck Institute for Ornithology, Germany, wanted to know how bats recognise predators. ‘It might be important for bats to assess whether or not predators are there when they inspect new roosts,’ explains Driessens. While sound and visual cues could be helpful warnings when predators are in residence, they are of little help if a predator is absent when a bat investigates a new roost. However, odours can linger long after a predator has departed. Curious to find out whether bats react to odours left by potential predators, Driessens and Siemers decided to find out whether bats fear odours left by foxes, weasels and stoats (p. ).
‘Synthetic predator odours such as TMT [found in fox faeces] and 2-PT [found in weasel and stoat odours] induce innate fear responses in rodents so we decided to use these synthetic olfactory cues and the odour of a natural least weasel to compare bat responses,’ explains Driessens. Travelling to the Max Planck Institute's Tabachka Bat Research Station in Bulgaria, Driessens and Siemers collected greater mouse-eared bats as they returned to their cave after a night of foraging. The duo then took the animals to the lab to test their sense of smell before releasing the animals back at their roost.
Putting individual bats in a Y-shaped maze, Driessens placed a cotton pad that carried the scent of either a least weasel, 1.8×10−2 mol l−1 TMT or 1.8×10−4 mol l−1 2-PT in one arm of the maze and a cotton pad soaked with the odourless solvent (DEP – used to dissolve TMT and 2-PT) in the other arm. Then she filmed the bat's behaviour for 8 min, recording and scoring the animal's activity levels and whether it avoided the predator's odour. Cleaning the maze with ethanol so that no trace of the smell was left, Driessens then tested the bat's response to an equally unpleasant odour, either basil extract or goat smell, which does not terrify rodents, to see if the bats were just avoiding the smell because they didn't like it, or they avoided it because it terrified them. If the bat was frightened by the fox and weasel scents, Driessens expected it to become inactive and avoid the TMT or 2-PT arm of the maze, while remaining active in the maze when the acrid odour was around.
But after testing the bats, Driessens found that they did not respond differently to the two types of odour. They remained equally active in both experiments and were happy to visit both arms of the predator maze. The bats weren't bothered by the predators' smells.
So why didn't the greater mouse-eared bats avoid fox and weasel odours when encounters with either animal could prove fatal? Initially the duo was concerned that the bats couldn't smell the predator odours in the maze. However, when they considered the bat's olfactory threshold, which is similar to that of humans, and tested the smells on colleagues – who regularly work in smelly bat caves and had no problem picking up the stench – they were convinced that the bats must have been able to smell the odours.
Driessens suspects that the bats may be ignoring the odours because they have other more pressing concerns than predation when choosing a roost. Alternatively, bats could be so familiar with the odours of cohabiting weasels and foxes that they no longer perceived the odours as a threat.
