Take no risks, ‘do all the right things’, and you’ll lead a comfortable, but dull, existence. ‘Living dangerously’, on the other hand, yields ‘highs’ of excitement usually followed, alas, by pain and regret. A paper appearing in Nature Ecology & Evolution claims that similar trade-offs govern the lives of animals. Its lead author is Kevin Healy of Trinity College Dublin and NUI Galway.
On the Origin of Species or the Preservation of Favoured Races in the Struggle for Life is the full title of Darwin’s most notorious book. Humans choose their lifestyles in the struggle for life but animals can’t; they are at the mercy of blind forces.
Every living thing fights to survive and pass its genes on to the next generation. All creatures, great and small, are driven, Healy’s paper notes, by ‘the same evolutionary principle of maximising fitness through differing rates of survival’.
The CVs of mammals and birds are well documented. Most reptile and amphibian species have been studied, but the lifestyles of many creepy-crawlies remain unknown. Are there general lifestyle rules which we can apply even to creatures as yet unknown to science? To what extent do animals, facing widely differing challenges, adopt similar strategies?
To answer such questions, Healy’s team selected 121 species, ranging from sponges to humans, and carried out detailed reviews of the literature on each of them, focusing on longevity and breeding success.
The study revealed ‘an extraordinary diversity of life history strategies’ throughout the animal kingdom. It also showed, however, that widely differing creatures deploy ‘surprisingly similar’ ones.
Two elements, which the team call ‘pace’ and ‘shape’, proved particularly important. About 71% of the variation in life-strategies can be explained, the authors say, by ‘traits associated with the fast-slow continuum (pace of life)’.
Fast-paced creatures reach maturity, and breed, quickly; an eat drink and make merry for tomorrow we die approach to life. The turquoise killfish, native to Zimbabwe and Mozambique, is a spectacular example; it can complete its lifecycle in just 14 days.
At the other extreme, we find creatures which take their time. Gannets, nesting on stacks off the Irish coast, take five or six years to reach maturity and then lay just one egg annually. Some of our fulmars don’t breed until they are 12 years old.
The Greenland shark is the supreme
exponent of the laid-back approach to life. It takes 156 years, on average, to reach breeding age and some sharks celebrate their 500th birthday.
‘Shape’, or ‘lifestyle’, is equally important. It determines a creature’s approach to birth marriage and death. An eel, for example, may spend three or four decades in an Irish river pond or ditch. Then, one fateful day, it heads downstream, never to return.
Having swum thousands of kilometres to the Sargasso Sea off the Gulf of Mexico, it spawns and dies. The tiny offspring drift with ocean currents to the mouths of Irish rivers.
‘Whether you are a sponge, a fish or a human’, ScienceDaily quotes Healy as saying, ‘your life cycle can, in general, be described by two things — how fast you live and how your reproduction and chance of dying are spread across your lifespan’.
Kevin Healy et al. Animal life history is shaped by the pace of life and the distribution of age-specific mortality and reproduction. Nature Ecology & Evolution. 2019.