Life is tough for young worker bees, who spend their short adult lives diligently rearing their queen's offspring. First, they lavish their queen's progeny with nourishing jelly, before switching to a foraging role, where they quickly age and die after just 2 weeks. In total, while queens can live for up to 5 years, most workers will live for a mere 2 months. However, during winter, workers can live longer, with bees born in autumn living for 6–7 months before spring arrives. These dramatic differences in lifespan in otherwise genetically related bees offer scientists crucial insights into how ageing and its unwanted symptoms, such as cognitive decline, develop. Many researchers have focused on summer workers, but Daniel Münch from the Norwegian University of Life Sciences, Norway, was more interested in the long-lived winter bees: ‘We know that they live quite long, but the big question is, do they show functional decline? Do they stay healthy over time? Could we transfer a long-lived phenotype that showed negligible ageing into a short-lived phenotype?’ Working in a country with cold, snowy winters, Münch had the perfect conditions to answer these questions and so, with the help of his mentor Gro Amdam and the lab's excellent beekeeper, Claus Kreibich, he began his study (p. 1638).
First, the team collected bees from their hives during early, mid- and late winter to test for functional learning decline over time – could they learn to associate a carnation oil aroma with a sugary reward? To do this, Münch took advantage of the characteristic bee feeding mechanism, where the proboscis (the insect's tongue) automatically extends when it's presented with something tasty. Münch presented each bee with the flowery perfume for 3 s before rewarding it with a sweet treat. In subsequent trials he looked for bees that extended their proboscis following scent release but before the tasty treat appeared – a sign they'd learnt that the reward was on its way. Despite increasing in chronological age, Münch found that the winter bees' learning capacity did not decline as winter went on. In fact, their ability for associative learning was on par with that of a young, 9 day old bee reared indoors; most bees learnt to extend their proboscis in response to the perfume after just two rounds of scent presentations.
Münch then wonder whether he could speed up ageing by transferring some of the winter hives indoors, where he mimicked summery conditions. Sure enough, the hives began brood rearing and after just 1 or 2 weeks of foraging these ex-winter bees aged rapidly and showed a decline in learning ability, unlike their compatriots of similar chronological age that had remained outside. Furthermore, when the team looked at the pattern of lipofuscin – aggregates of lipids and proteins that accumulate with age in all animals – in the brain, they found that lipofuscin accumulation was greater in specific areas in foragers.
As a shortening in life expectancy coincided with brood rearing, Münch wondered what would happen if the brood were taken away. ‘What we observed was surprising; after 2 weeks there were still a lot of foragers left and they didn't show any functional [learning] decline. So we waited for a period of 70 days and we still couldn't detect any functional decline, any ageing, in these colonies’, recalls Münch. So despite having adopted forager status they lived longer and, again, chronological age didn't automatically correspond with functional decline. By speeding up or slowly down ageing in bees, Münch and his colleagues hope to identify what causes functional decline, and suggest that lipofuscin accumulation in specific brain regions may be partially responsible.