Let’s face it, growing old isn’t appealing. Stiffer joints, wrinkly skin, senior moments; it’s all just a matter of time. But, to add insult to injury, it seems a dwindling sex life is unavoidable too – at least if you’re a fruit fly. That’s the inescapable conclusion of work by Tsung-Han Kuo and Scott Pletcher of the University of Michigan and Baylor College of Medicine and their colleagues (p. 814).
For any animal, reproduction is top of the to-do list. But since only the fittest attract a mate, it seems reasonable to assume that animals become less attractive as they grow older. Fruit flies use pheromones to entice the opposite sex, so Kuo, Pletcher and colleagues wondered how flies’ pheromone profiles and sex appeal alter with age. To examine the link between attractiveness and age, the team used mass spectroscopy to take a closer look at pheromones on the cuticles of flies aged between 7 and 65 days. Males and females differed in their specific pheromone profiles, but as the team had expected, they saw that profiles shifted with age in both sexes. Elderly males and females produced a higher proportion of heavier pheromones compared with younger flies. ‘The results were remarkably consistent across different strains of flies’, says Pletcher, which suggests that pheromone profile changes are strongly regulated throughout the lifespan and unlikely to be a random by-product of ageing.
Next, the team tested whether these changes affect how desirable the flies are. When the team presented males with a choice between young and old females, the males courted the younger specimens more vigorously, suggesting that they were more attractive to the males. This was backed up by video analysis, which showed that males spent more time close to younger females.
But were age-related changes in the females’ pheromone profiles responsible for the males’ waning interest in elderly ladies? To rule out the possibility that beauty was in the eye of the beholder, the team tested males’ preferences in the dark – but males still ardently pursued younger females, so they weren’t relying on visual cues to pick a partner. Next, the team washed females’ pheromones off their bodies and retested their sex appeal. Sure enough, males could no longer tell the difference between young and old females. As a final test, they sprayed pheromones from young and old females onto genetically modified flies who couldn’t produce pheromones of their own, and saw that the males preferred flies covered with young females’ pheromones. And it wasn’t only males who favoured youthfulness; when the team tested female flies’ preferences, they saw that females fancied younger males.
The team concludes that both male and female flies become less alluring with age, and this is caused by changing pheromone profiles. Speculating about the reasons for this, Pletcher points to the possibility that heavier hydrocarbons help prevent older flies from drying out. ‘If these changes are simply reflective of an ageing physiology,’ he notes, ‘flies may have evolved the ability to read these signals as honest indicators of health.’ These findings pave the way for an exploration of the molecular nature of attractiveness, says Pletcher. ‘We know that ageing is conserved across species, so we want to examine the exciting possibility that the mechanisms underlying attractiveness are also conserved across species.’