Burning out or fading away might not be a matter of choice

Burning out or fading away might not be a matter of choice
Kirk Douglas, left, and Brendan Behan: Differing lifestyles would see the actor live past a century, while the playwright’s hard living caught up with him at a relatively young age.

Pace and shape of life may be two of the key indicators to how different species get by in the world according to a new study, writes Dan Buckley.

Living life in the fast lane can be hectic, frenetic — and short.Taking things slowly, on the other hand, may aid longevity.

Think writer Brendan Behan, who lived fast and furious and died at the age of 41. Or Hollywood star James Dean, who sped to his death on the way to a sports car race. He was only 24.

Actor Kirk Douglas, on the other hand, is still with us at the age of 100,almost as long-living as Herman Wouk, the American author best known for historical fiction such as The CaineMutiny, who died in May at the age of 103.

But, as a group of international scientists have found, being a sprinter or a marathon runner — living fast and dying young or playing the long game — is not always a matter of choice, whether you are a human or a freshwater crocodile.

Scientists, from the NUI Galway, Trinity College Dublin, Oxford University, the University of Southampton, and the University of Southern Denmark, have pinpointed the ‘pace’ and ‘shape’ of life as the two key elements in animal life cycles that affect howdifferent species get by in the world.

Their findings, which come from a detailed assessment of 121 species ranging from humans to sponges, may have important implications for conservation strategies and for predicting which species will be the winners and losers from the global environment crisis.

‘Pace of life’ relates to how fast animals reach maturity, how long they are expected to live, and the rate at which they can replenish a population with offspring. ‘Shape of life’ relates to how an animal’s chance of breeding or dying is spread out across its lifespan.

Why animal life cycles vary so much has long been an important puzzle for scientists to solve because understanding why animals age, reproduce, and grow at different rates may help shed light on the evolution of aging itself, and help identify how species will respond to global environmental change.

In their study, the scientists used population data to compare detailed life cycles for species ranging from sponges to corals, salmon to turtles, and vultures to humans. By mapping 121 life cycles, the scientists noticed that certain animal ecologies and physiologies were associated with certain life cycles.

“When we mapped out the range of life cycles in the animal kingdom we saw that they follow general patterns,” says Kevin Healy, lead author of the study who conducted the research at Trinity and is now Lecturer of Zoology at the NUI Galway.

“Whether you are a sponge, a fish or a human, your life cycle can, in general, be described by two things — how fast you live and how your reproduction and chance of dying is spread out across your lifespan.

As we expected, species with low metabolic rates and slow modes-of-life were associated with slower life cycles. This makes sense; if you don’t burn much energy per second, you are restricted in how fast you can grow.

"Similarly, if you are an animal that doesn’t move around a lot, such as a sponge or a fish that lives on the sea bed, playing a longer game in terms of your pace of life makes sense as you may need to wait for food to come to you.”

The work, which appears in the leading journal Nature Ecology & Evolution, shows that animal life cycles vary to a staggering degree. Some animals, such as the turquoise killifish (a small fish that can complete its life cycle in 14 days) grow fast and die young, while others, like the Greenland shark, (a fish that glides around for up to 500 years), grow slowly and have extraordinarily long lifespans.

Similarly, the spread of death and reproduction across animal life cycles also varies greatly. Salmon, for example, spawn over a short period of time, with the probability of dying being particularly high both at the start of their life cycle and when they reproduce.

Fulmars and some other sea birds, on the other hand, have wider time periods of reproduction and face relatively similar chances of dying throughout their lives.

Humans and Asian elephants have long lifespans and face a relatively low risk of mortality until their later years but have a fairly narrow age range for reproduction because they have longjuvenile periods and live a long time after the reproductive part of their life-cycles.

Both species share a similar lifespan with the Australian freshwater crocodile, but the crocodile has a completely different reproductive strategy — its reproduction is spread relatively evenly throughout its lifespan but its young have a low chance of reaching adulthood and reproducing.

The scientists also investigated whether certain life cycles made animals more susceptible to ecological threats, by looking for associations between an animal’s life cycle and its position on the ‘red list’ of threatened and endangered species compiled by the International Union for Conservation of Nature, the global authority on the status of the natural world and the measures needed to safeguard it.

“We found that extinction risks were not confined to particular types of life history for the 121 species,” says Yvonne Buckley, co-senior author of the research and Head of the Zoology Department at Trinity.

“Despite these animals having very different ways of maintaining their populations, they faced similar levels of threat.

Populations of a particular species, like the Chinook salmon or freshwater crocodile, vary more in how mortality and reproduction are spread across their life-spans than they vary in their pace of life.

"This is important for the animal populations that we need to conserve as it suggests it may be wiser to consider actions that boost reproduction and/or impart bigger effects on the periods of the life cycles when mortality and reproduction are more likely —rather than simply aiming to extend the lifespans of these animals.”

The research was funded by Science Foundation Ireland, the Natural Environment Research Council, Australian Research Council, Danish Council for Independent Research, and the Max PlanckInstitute for Demographic Research.

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