No mid-day nap for Finnish flies

Date: March 15, 2017

Source: University of Würzburg

There are more than 2,000 species of fruit flies around the world. Some prefer warmer climates, whereas others are home in northerly latitudes. "We wanted to find out whether the circadian clock of the northern species differs from that of their southern relatives," explains Professor Charlotte Helfrich-Förster from the University of Würzburg's Biocenter. "For this purpose, we compared two fruit fly species from Finland with one from Tanzania."

Long siesta at the equator

In the laboratory, the scientists varied the length of light and dark cycles to which the insects were exposed. In their first experiment, twelve hours of day were followed by twelve hours of night. This rhythm is typical for the equator where day and night are about equally long throughout the year. The African flies exhibited a characteristic pattern of activity under these conditions: Their active phases were limited to the time around dusk and dawn; in the meantime they rested. In nature, such behaviour is highly advantageous as it allows the insects to better cope with the heat of the day.

Being slightly more relaxed in the morning, the Finnish dipterans in contrast had their activity peak in the early afternoon and stayed active until nightfall, mostly without taking a siesta. From a biological point of view, this behaviour makes sense, because even at the height of summer, the northern Scandinavian sun is rarely strong enough to harm the animals.

In the next step, the scientists extended the lab day: They left the lights on for 20 hours before turning them off again for four hours. In response to this, the Tanzanian flies did not extend their resting period, but rather started to bustle about long before dusk. Their activity now peaked at a time of day when it would normally still be scorching hot. If there were 20-hour days in Tanzania, such behaviour would probably be very risky.

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