Folks, this discussion does have a serious theme at its core, but I think it is a bit too essentialistic. I doubt that the primary association of size/longevity is fundamental, even allowing for separating Murids from Meloids, Coliids from Colubrids, or Cephalopods from Psittacoids. You need to look at the evolutionary strategy and ecological constraints. There are arctic birches just centimetres high that live decades longer than tropical trees that last only a season or two.
There are too many considerations for me to go into, let alone cover properly. For a start, you must cover phylogeny as well as ontogeny. The former refers to how the creature evolved; if recently arose from a lineage that had a seasonal life cycle, then it is a good bet that it is not any sort of Methuselah. I suspect that this could have something to do with the short lives of say, large cuttlefish and octopuses, in spite of their high intelligence. The ontogeny is affected by how the creature grows. For example, most insects in warm temperate or tropical climes, such as many moths, that munch away at rich seasonal supplies of rich plant food, are multivoltine (several generations per year) whereas often species in seasonal climates are univoltine. In contrast, many insects that live in dry wood, such as Cerambycids, take several years to produce even a mature larva, and then may live for moths as an adult; dry wood is not a food that you bolt down by the mouthful, and there is precious little nutrition in a mouthful too. (I don’t even know offhand whether many of them have gut flora to assist them digest it!) But in any case, you can have a Deaths-head hawk moth caterpillar say, weighing in at tens of times the mass of a longicorn beetle larva, but living only a fraction as long. You can have a bat with less mass than a rat and a higher metabolic rate, outliving it by a factor of five to ten.
Now, the point is not that there are exceptions to the rule, but that we are looking at THE WRONG RULE! Again this is too big a subject for my available time, and also to big for me. But let's have a thunk...
By and large our strategy is: “Be conceived; be born; grow; reproduce; possibly enter diapause from time to time.”
That is pretty comprehensive, pretty simple, pretty conclusive.
Except for a few details...
With a hellll of a lot of devils in those details.
Be conceived? Be born? How? What is the probability of a successful conception? If it is low, very likely, either the nubile fraction of a creature’s life will be long, so that there is a decent chance of reproduction at all. Or the creature could disseminate gametes like crazy, in the hope that some out of millions might find a mate. But that is only one part of the creature’s life cycle. There are many strategies for dealing with the dilemmas. Mayflies for example typically have quite a long life cycle, most often about a year, in spite of their name: Ephemeroptera. It is only the adult stage that has a brief duration of a few days, in which they emerge in their teeming millions, of which only a fraction manage to mate and lay eggs (but LOTS of eggs!) Emperor moths of temperate zones similarly (tropical species commonly are multivoltine, temperate species univoltine), but they increase their chance of mating by assembling, females attracting males from sometimes considerable distances pheromonally (depending on how fussy your definition of pheromones might be). There are great hazards to the lives of most young stages, so there is a major attraction to producing them at long intervals so that specialist enemies have to keep in step with them. Consider the springtime flush of tender oak leaves, the annual day or two of termite or ant reproductive flight and so on. Then try arid-zone amphibians and crustaceans that only emerge from aestivation when it rains every several years. Consider 17-year cicadas.
That is one aspect that affects life spans in various ways. It does not have a really tight correlation with size, though it obviously affects size as well. You can’t expect an elephant or whale to grow to reproductive size in a season, but some antelope do so, even though they are a LOT bigger than foxes, scrub jays, or of course, cicadas. And they have to do well to live to reproduce more than once. And most of them reproduce almost in synchrony, so that the local carnivora must miss some of the youngsters. But as large animals go, they are ephemeral, nearly like cuttlefish (but not quite; they have only one young at a time, usually.
Grow? That means feeding. Most insects actually have a special phase of their ontogeny in which they specialise in growing to a necessary size and accumulating enough reserves for reproduction. Usually that is the larval stage. Compare that with frogs. And larval humans. Most larvae do not reproduce, and their rate of growth and length of survival is tuned to the necessary adult size and reproductive requirements for breeding resources. Elephants reproduce for decades each, eating all the time. Some small mammals reproduce once. So do most cephalopods, some of them about human-sized. Some animals change their diet from growth phase to reproductive phases, such as mosquitoes. Some feed when they reproduce, some before. All those factors affect or reflect their longevity.
One way in which size affects longevity is that the larger the animal, and in particular, the harder it comes by its food the longer it takes it to grow, so big animals TEND to be long-lived for that direct reason. There are partial exceptions, for example, some species of frigid-zone seals feed their young on milk so rich that they grow to a viable size within weeks.
Again, the more creatures are subject to predation, the shorter their lifespan as a rule. It is not worth tooling up for a long life if you probably won’t make it. Better invest in many offspring and quickly. Otherwise longevity is an attractive option, so we find that bats outlive mice grossly.
OK for now. Have I conveyed anything helpful? Look at any animals and work out how such principles hang together. It will make more sense as a rule than derivative variables such as counting heartbeats.
Cheers,
Jon
