Is the key to a long life a big heart?
People in our part of the world who eat too much meat and not enough fruit and vegetables die younger as a result but is it the same for other animals?
In his Mammals of Britain and Europe, David MacDonald gives life-span estimates for the furry creatures around us.
At first sight, his figures might suggest that herbivores live longer. The European bison, for example, may reach the age of 40 and another plant-eater, the red deer, might celebrate its 25th birthday.
Europe’s leading carnivore the wolf, however, lasts only 15 to 20 years. A badger, eating earthworms and small mammals, survives to 14, and the fish-eating otter to 15. Few rats, consuming just about everything, see their third birthday.
So would opting for herbivory in the exam get you full marks? Not necessarily, because this is not a black-and-white issue.
Big creatures, irrespective of diet, tend to live longer than small ones. The average heart, according to one rule of thumb, beats about eight hundred million times. A blue whale’s ticker takes a century to do so but a shrew’s heart, pumping a thousand times every minute, is worn out in just over a year.
It’s only to be expected, therefore, that a bison lives longer than a red deer; it’s four times heavier. Elephants and Galapagos giant tortoises last well into their 80s. When it comes to longevity, it’s size, rather than diet, that matters.
So how do vegetarians and meat-eaters of similar weight compare? The hare, a vegetarian tipping the scales at 5-6kg, can live to be nine. A meat-eating fox, weighing 5-9kg, has roughly the same life expectancy. Sika deer and wolves, with roughly similar weights, have comparable life-spans.
So, would candidates who claimed that herbivores and carnivores have similar life-expectancies win the day? Not necessarily; it could be argued that size, the crucial factor, depends on diet.
People who tuck into steaks tend to be fatter than their vegetarian equivalents, but the opposite is true for animals. Weight slows a hunter down, making prey harder to catch; meat-eaters must be slim and trim.
Plants, on the other hand, can’t flee, so creatures which eat them can take their time. But plants have other defences. Their cells, unlike those of animals, are encased in protective shells of cellulose, difficult to break down and a challenge to herbivores. It takes longer to digest plant cells than animal ones. The nutrient value of plant food tends to be low, so more must be consumed. Keeping large quantities of food on board requires a big stomach and a long digestive tract. It’s little wonder that elephants, rhinos and hippos, which take several days to break down the huge quantities of coarse vegetation they eat, are big.
And there’s a bonus; being big and thick-skinned makes you less vulnerable to predators. Hares and rabbits haven’t got the size option. To outrun their enemies, they must be small; retaining bulky food in the stomach is not on. They solve the problem by eating their own excrement, passing the food through their systems twice over.
But the claim that herbivory equates with longevity falls apart when it comes to birds. The oldest known yellowhammer, a typical seed-eater, lived for nine-and-a-half years, whereas the oldest robin, a flesh eater, reached 13.
Nor does size matter much where the life-expectancy of a bird is concerned. A flying creature has to be light; carrying bulky food in the gut isn’t on. A large swan is 700 times heavier than a robin but the oldest swan, at 26, lived only twice as long as the robin. Seabirds, all carnivores, would still be in their prime at 26. The oldest known, a Manx Shearwater ringed in Copeland, Co Down, was 54 when re-trapped.
So what should the exam candidate’s answer be? That the jury is still out? That, for mammals and reptiles, but not birds, life in the meat-eating fast lane favours smaller size and a faster metabolism and that this, in turn, reduces longevity?
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