When a honey bee embarks on its foraging career, its days are numbered. The relentless schedule takes its toll and most of the insects die within 3 weeks. ‘Foraging has really profound effects on longevity, flight ability and physiological performance’, says Stephen Roberts from Central Michigan University, USA.
Intrigued by the ageing process – known as senescence – in insects, Roberts and his colleague Michelle Elekonich are keen to understand how flight, the most energetically expensive behaviour known, affects senescence. However, when it comes to answering complex questions about the mechanisms that govern ageing, Drosophila are better insects than bees for experimentally disentangling the effects of age and flight on senescence. Yet, little was known about the effects of a lifetime of activity on these ageing flies, so Roberts decided to get to grips with how different flight histories impact on geriatric fruit flies (p. ).
Teaming up with master's student Steven Lane, Roberts monitored how three populations of flies that had experienced different amounts of flight activity through their lives fared as they aged. The team allowed the first population to fly whenever they wished, but imposed a complete flight ban on the second group by loosely stuffing the insects' jar home with light wedding-veil gauze, only allowing the insects to walk around the interior. The third population was also free to fly, but their jar was strapped to a vortex shaker that was programmed to give the insects a brief nudge at random times over the day, frequently forcing them to take flight. Then, Lane analysed the insects' flight performance, weight and metabolism as they aged – starting with 15 day old youngsters, moving up to 35 day old middle-aged insects and concluding with 65 day old geriatrics – to find out how they had senesced.
Comparing the three populations, the team could see that insects that had been allowed to fly whenever they wished fared reasonably well. Testing the insects' ability to take to the air in low density air (where half of the nitrogen had been replaced with helium), they found the majority flew well in middle age and 30% were still able to take to the thin air as geriatrics. However, when the team tested the flight performance of flies that had been forced to fly throughout their lives, they had essentially burned out. By 35 days their metabolic rate had plummeted by 57% and by 65 days none of them were able to get off the ground in the helium-supplemented air. Roberts explains that this dramatic decline was predictable because of wear and tear and the insects' increased exposure to toxic oxygen by-products produced by their hectic lifestyles.
However, the big surprise came when the team investigated the insects that had been prevented from flying. Instead of benefiting from their leisure, the couch potatoes paid a high metabolic price for their sloth. ‘Their flight ability was compromised the earliest and the most out of all of the treatment groups’, says Roberts. By 65 days, none of the flies could get off the ground in the test atmosphere. However, unlike immobile humans, they had not gained weight.
‘We didn't know what we were going to see with the no-flight group’, says Roberts, who explains that it was possible that inactivity might have slowed the ageing process, but evidently it had not. In fact, it exacerbated the insect's decline. He says, ‘Behaviour can have profound effects on an organism's future; what it has done in the past has profound effects on how good its life may be in the future.’
