Nematode worms, fruit flies, mice and other lab animals live longer, healthier lives when they eat less than they otherwise would if more food were available. Primates may also benefit, and perhaps humans—which is why research funds are pouring into this phenomenon. But all this raises a puzzling question: Why did creatures evolve such a mechanism in the first place? Researchers have declared the most popular theory doesn’t make evolutionary sense, and they’ve proposed a new explanation in its place.



The most prominent theory involves what happens physiologically during times of food scarcity. When the living is good, natural selection favors organisms that invest energy in reproduction. In times of hardship, however, animals have fewer offspring, diverting precious nutrients to cell repair and recycling so they can survive until the famine ends, when reproduction begins anew. Cell repair and recycling appear to be substantial antiaging and anticancer processes, which may explain why underfed lab animals live longer and rarely develop old-age pathologies like cancer and heart disease.



Margo Adler agrees with the basic cellular pathways, but she’s not so sure about the evolutionary logic. Adler, an evolutionary biologist at the University of New South Wales in Australia, says this popular idea relies on a big assumption: that natural selection favors this energy switch from reproduction to survival because animals will have more young in the long run—so long as they actually survive and reproduce. “This idea is repeated over and over again in the literature as if it’s true, but it just doesn’t make that much sense for evolutionary reasons,” she says.



The problem, Adler says, is that wild animals don’t have the long, secure lives of their laboratory cousins. Instead, they’re not only endangered by famine but by predators and pathogens, random accidents and rogue weather as well. They also face physiological threats from a restricted diet, including a suppressed immune system, difficulty with healing and greater cold sensitivity. For these reasons, delaying reproduction until food supplies are more plentiful is a huge risk for wild animals. Death could be waiting just around the corner.



Better to reproduce now, Adler says. The new hypothesis she proposes holds that during a famine animals escalate cellular repair and recycling, but they do so for the purpose of having as many progeny as possible during a famine, not afterward. They “make the best of a bad situation” to maximize their fitness in the present. “It’s an efficiency mode that the animal goes into,” she says. Adler and colleague Russell Bonduriansky published their reasoning in the March BioEssays.



Other researchers believe that Adler’s theory may make sense for species that are short-lived or pay a low cost for reproduction, such as flies and mice. But the theory is problematic for animals with different biology. Birds and other mammals put a huge amount of energy into reproduction and parental care, so it’s a bigger risk for them to have offspring during famine compared with, say, fruit flies. Some species require so much energy to reproduce that their bodies prevent conception during a food shortage. Certain long-lived species (including humans) also have a better chance of survival in general, so they could afford to wait and pass on their genes when food finally returns to their plates.



Adler plans to test her idea in the wild, but it will be a challenge, she admits. “No one has ever been able to dietarily restrict animals in the wild because it’s really hard to manipulate diet in the wild.” Whether or not her hypothesis makes it into biology textbooks, it may help researchers in this field find novel ways to prevent age-related diseases.