What determines whether a wild animal ages gracefully? New research suggests environmental conditions during an animal's formative years can affect the animal's aging process.

To better understand the link between a wild animal's adolescence and twilight years, researchers at the Australian National University gathered data on 14 different bird and mammal species, including swallows, storks and kestrels, as well as deer, sheep, mountain goats, squirrels and banded mongoose.

"We investigated the effect in two different types of senescence: reproductive senescence, measured as declines in reproductive output in late life, and survival senescence, measured as the decline in survival probability in late life," Eve Cooper, a Ph.D. student in the biology department at ANU, said in a news release.

While environmental conditions during an animal's early years had no impact on survival rates later in life, the data showed animals who enjoyed cushier conditions during adolescence had greater reproductive success during their later years.

For almost all animals, reproductive rates decline as they age. But for animals who enjoyed better environmental conditions early, the drop-off was less severe.

Researchers said it's possible early life conditions effect survival, too, but that's too difficult to measure. It's likely many animals born into poor conditions die before they reach old age, skewing the statistics.

Until recently, many ecologists assumed old age was rare among wild animals.

"Because we now have a better basis of data on wild animals and what happens to them from birth to death, it's been realized that senescence is quite common in wild populations." Cooper said. "What we've also learnt from these long-term animal studies is that there's a lot of variation between individuals, so two animals living in the same population can have dramatically difference rates of senescence."

Researchers also used to believe old age was rare among humans, but recent studies have shown the average age of death was 70 years among many ancient human populations.

Scientists still aren't sure what accounts for the variability of senescence. Many studies have looked to animals for ways to slow the aging process. But authors of the latest research -- published this week in the journal Evolution Letters -- think it's equally important to understand aging from an evolutionary perspective.

"Understanding the evolution of aging in the natural world can actually have pretty broad implications to our understanding of aging in humans," Cooper said.

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