How do parasites in the blood or eating habits influence the metabolism of bats? To understand these questions, a team of biologists is conducting a vast study on the Daubenton’s bats living on the University of Lausanne campus. Read on and find out more about these mysterious creatures.
‘Look how fast they fly!’ Night has yet to fall, and bats are already swirling through the air. Meanwhile, standing barefoot in the water, doctoral student Laura Clément and Kevin McMillian, a master’s degree student in the Department of Ecology and Evolution, are rushing to set up a ‘harp trap’ across the river at the mouth of a tunnel. Fortunately, the evening is rather mild.
Just a few seconds later, a Daubenton’s bat hits one of the three rows of nylon threads and falls into the tarpaulin below. ‘Every evening, they leave their colony in the forest and fly along the waterway in search of aquatic insects on the surface of the lake,’ explains Laura Clément. ‘Which is why they have those big, hairy hind feet.’ Relatively speaking, of course. Because the creature only weighs about 7 grams! Squinting into the tunnel, you can make out another ten or so hanging upside down from the ceiling. Nothing seems to disturb them in their lethargy. Not even the comings and goings of a beaver.
Protection against malaria?
In the space of two hours, the biologists catch eight bats including three males. Crouching in the darkness on the bank, Kevin McMillian starts to weigh a female bat and measure its forearm. The tiny creature is calm and does not flinch. There is silence all around. Until Laura Clément, who can talk for hours about her protégés, points out the unusual appearance of the Daubenton’s bat. And you have to admit – their bare faces do make them look a bit puny. ‘But I love all the different species, just the same,’ she hastens to add.
Helped by the lamp on her forehead, the biologist counts the mites which live on the chiropterans’ wings. Then she blows on the light brown fur to tease out the tiny flies that serve as vectors for the malaria parasite. This parasite, which goes by the rather grand name of Polychromophilus murinus, attacks the red blood cells of bats. ‘Between 70 and 90% of Daubenton’s bats are infected,’ points out the doctoral student. ‘And I want to understand the effects these blood parasites have on their host.’ Since the project began in March 2016, individual bats have been captured and then released several times on the campus with a view to studying their energy metabolism.
To help me understand how this is measured, I arranged a meeting for the following day with Michel Genoud, a scientist at the Department of Ecology and Evolution, and the three males delivered to the laboratory the evening before. Having spent the night there, they are placed in a metabolic chamber – that looks a bit like a refrigerator – where the temperature is kept at 25°C. Inside, Michael Genoud carefully places each creature in a jar. ‘We know the precise properties of the air coming in and going out.’
After two hours, a first set of measurements has been taken. The line on the monitor shows that one of the bats, and certainly not the plumpest, has consumed a lot of oxygen and released almost as much carbon dioxide. ‘This shows its metabolism has been very active and it has expended a lot of energy.’ Maybe to protect itself against malaria? Laura Clément will be able to answer this question by comparing the data with blood parasite levels.
A mysterious diet
The biologists will also be studying the links between energy metabolism and diet (fatty acid levels, in this case). ‘Research involving other hibernating mammals has shown that a diet richer in omega-3 than omega-6 makes it harder for them to slip into a state of lethargy,’ explains Michel Genoud (see box). Daubenton’s bats feed on aquatic insects, which are naturally rich in omega-3 but contain little omega-6. ‘So in theory, their periods of lethargy should be shorter, less profound, and less frequent. But according to our preliminary results, this isn’t the case. It would seem they adopt some kind of compensatory system, which we are keen to understand.’
During the afternoon, new measurements are taken at 35°C. Then Laura Clément takes a tiny drop of blood from each bat with a view to doing some analysis. At nightfall, she will release these brave ‘guinea pigs’ close to where they fell into the nets.
Sleep and longevity
To save energy, bats are able to slow their metabolism dramatically. During the day, they reduce their body temperature, heart rate, and breathing. ‘When hibernating, and depending on external conditions, Daubenton’s bats can lower their body temperature to 5°C and go 20 minutes without breathing,’ points out Michel Genoud. These particular characteristics may contribute to their longevity. They can live for up to 20 years, whereas other small mammals like hamsters have a life expectancy of two to three years.