This must have been asked before but I couldn’t find it.
When adult people get older, we can tell their age by their appearance, and seperate the 20 and 30s and 40s and 50s, and 60s and so on. Eventually people shrivel up and get white hair and massive wrinkles. (Even before modern medicine, many people lived to be quite old).
Yet other animals like dogs look the same, for the most part, with the only signs of age being more personality differences like more lethargic, but not physically old.
Actually, other animals do show observable physical signs of aging. Dogs get gray around the muzzle and often get gray hairs elsewhere, like on their backs. The underside of their muzzles (Is that a chin? What do you call that?) get saggy and wrinkly. They get a sunken area beneath their eyes. Their whiskers turn white. Often, their backs sag. It varies from breed to breed, but the signs are there. I’m sure it’s the case with other species.
Well, I kinda disagree with your reasoning, in a number of ways. First off, the age of dogs is kinda noticeable, the fur on their face whitening is a dead giveaway that you’re looking at an older dog. And yeah, that usually matches with a different behavior pattern.
We’ve also had the problem before, Cecil hinted at it at some of his columns. There’s no simple correlation between the human age brackets you gave, and animal ages. No, its not “every dog year is 7 human years” and it’s not perfectly “the first dog year is 18 human years, add 2 for every dog year after that”, because they simply age differently than we do.
Add to that the selective bias – you’ve looked at hundreds of humans at various ages your whole life. Have you seen as many African lions, or Asian elephants up close at personal throughout their lifetime? And the whole idea becomes muddled quickly.
My dog has visibly aged over his 8 years. He is very grey in the face and it’s creepy up onto his head. Puppy breath is long gone. Why does a dog go through the stages of aging so fast is my question.
Animals WILL exhibit visible signs of aging, as people are noting concerning their dogs. In the wild, very, very few individuals are going to live that long. And for domesticated species other than pets, we slaughter them long before old age, when they are ready to be eaten, or are no longer producing well enough to justify their keep.
[WAG]Wrinkles are probably correlated with net unadjusted stretching and sun exposure. In other words, a 20 year old dog that is just as active as a 20 year old human should be just as wrinkled as a … 20 year old human. Old animals don’t show signs of wrinkles because they die of other causes first.[/WAG]
Actually, I didn’t mean to, but see what I did there? WAG? Hahahaha, get it? We’re talking about dogs.
We can tell the age of people easily because we’re people and used to observing people and thinking about their age. Poeple with dogs can see them age; vets can estimate the age of animals they see; birdwatchers can tell you if a bird is juvenile; and so on and so on. I’d guess that anyone sufficiently interested in a species to watch individuals over long periods of time, will probably eventually be able to see the signs of aging.
You just haven’t been studying other species as hard (so far).
Only with certain species and genders…also a few birds have 1st, 2nd and even 3rd year plumage…gulls are quite famous for this. So I can say 'this bird is a baby, but a 5 and 10 year swan look the same.
If you take hair color out of the equation, humans have a lot of difficulty determining the age of humans of other races, even, because most of us are conditioned to read signs of aging in features most similar to our own. I’m sure if you were a ocelot, you’d easily be able to distinguish the young, hot ocelots from the pack.
Or why do we age so slowly? Or why do organisms age at all?
The mechanics of aging are still debated and there appears to be many factors.
But there is a selective pressure on mammalian species to at least live to approximately twice the age of sexual maturity (so that you are around to help your offspring survive to sexual maturity also). So there isn’t the evolutionary pressure on dogs to endure as long as we do.
…Of course, this is far from the end of the story: arguably all organisms have a selective pressure to survive and be fertile indefinitely. But if you ignore this, and imagine the pressure is just to successfully reproduce another generation, the aging across species is consistent with that.
There’s actually little-to-no selection for late life reproduction. In wild populations, there’s just so few older animals, as most have died from predation, competition, or disease. So there aren’t many old animals around – definitely two few for evolution to strongly select for continued reproduction. Or, in the terms of population genetics models, if producing one offspring confers evolutionary fitness x, producing an offspring at a particular old age confers evolutionary fitness x * (likelihood of survival to that old age). And in many natural populations that likelihood plummets, so that the chance of surviving to twice the age of sexual maturity is vanishingly small.
(I’m being very vague here because there are all sorts of life history strategies, so getting specific requires knowledge about some particular species).
Humans live far longer than other mammals, even when you correct for things like metabolic rate. Most mammals, if they’re really lucky, can hope to live for a billion heartbeats. Humans reach that point somewhere in our 20s.
It’s interesting (to me at least) that the animals people have suggested that show reasonably visible signs of aging are all species that are social, and long lived, with a low predation rate- apes,elephants, dogs (does the same go for wolves?). Even in a captive situation, it can be very hard to give even a WAG as to the age of a lot of other creatures.
With regards to things like birds, after they’re fully mature, no-one can really age them. I’ve dealt with individual birds from hatching til death, and heard this from experts who have worked with particular species for decades. They also don’t visibly show signs of a lot of illnesses until they’re actually dying- I’d suspect that they ‘try’ to avoid showing any signs that they are at less than 100% fitness- because that would attract the attention of predators. Chimps, elephants, domesticated pets and especially humans can all ‘afford’ to show signs that they’re gettin’ on a bit.
The only other exception I’ve noticed with animals I’ve worked with is certain species of fish. Rainbowfish look old when they are- thisis a red rainbowfish in the prime of his life, this is an elderly one- they can look truly terrible by the time they finally die. A chamaeleon we had also looked noticably old in his last year, but I don’t have comparison photos.
