Trophy hunters are being urged to shoot only lions with dark noses so as to do the least damage to big cat numbers.
Conflict with farmers and sport hunting threaten the animals
University of Minnesota scientists say male lions are born with pink noses which become increasingly freckled and black the older the animals get.
Taking out just the older males allows prides to grow, claim the researchers in a report to the journal Nature.
But the study has been criticised as
"poor science" by campaign groups that are opposed to big game hunting.
The research combines observation of lions in Tanzania's Serengeti and Ngorongoro reserves with statistical analysis of lion population data.
Craig Packer and colleagues say their strategy would prevent prides from changing hands too frequently.
They explain that when new males take over a pride, they typically kill cubs less than nine months old to ensure that females devote themselves to raising new young.
In theory, excessive trophy hunting could cause male takeovers to become so common that they prevent cubs from reaching adulthood - and accelerating population decline.
By only removing males older than five or six, younger males have the opportunity to remain resident in a coalition long enough to rear their own cohort of young.
The Nature paper describes how the nose of male lions increases its blackness as the animal ages, to the point where at five years old the nose is about half-black (in Serengeti lions).
Packer and his team argue that this could be used as a rule of thumb for hunters - rather than simple quotas for animals shot - in order to ensure that they kill older males only.
"Shooting too many young males leads to an inevitable decline in population size, whereas quota size is irrelevant when hunting is restricted to older males," the team write in Nature.
"The strategy of only shooting males of five or more years old had the added bonus that in the long run there would actually be more males in the lion population to hunt," they added.
Kate Nicholls of the Okavango Lion Conservation Project, Lion Aid, said the idea that nose colour was related to age was disputed by biologists.
She said it was ridiculous to suggest that older male lions should be shot.
"It is irresponsible, poor science and has got nothing to do with conservation," she said. "There is no such thing as a spare lion."
A spokesman in Kenya for the conservation group Ifaw (International Fund for Animal Welfare) said the organisation was opposed to the idea of hunting wildlife on ethical and moral grounds.
Dr David MacDonald is director of WildCRU, Oxford University's Wildlife Conservation Research Unit, and recently completed a five-year study of lion conservation in Africa.
Commenting on the Minnesota work, he told BBC News Online: "I think it's an excellent and much needed development.
"You might think that [trophy hunting] is ethically attractive or ethically unattractive. What matters is that [it] is sustainable."
Most researchers agree the greatest threats to lion populations are sport hunting and conflict with farmers, because the big cats take their livestock.
Dr MacDonald has suggested there might be fewer than 20,000 lions left in Africa - down from about 200,000 in the early 1980s.
CHANGE IN LION NOSE COLOUR WITH AGE
Dark freckles on noses of male lions increase as they get older
By the time the lions are five years old, half their noses are black
Killing males above the age of five impacts lion populations less