p.s.Big cats and humans have similar body weights, if not higher.
In: Biology
This isn’t true for dogs specifically. Smaller dogs tend to have longer lifespans than larger ones. There isn’t a ELI5 answer for this because it is a faulty speculation.
Animals need to die to let newer generations take over, and how long their lifespan is depends on the pressures the species face (predation, toxins, cumulative damage, etc.)
Animals like mice that face a lot of predators don’t evolve longer lifespans because it’s not beneficial, they’ll still get eaten early on and evolution won’t have a chance to weed out the short life genes and favor the longevity genes.
Animals like lions don’t evolve human-like lifespans because their harsh life means they get a lot of injuries and illnesses that wear them down over time. Longevity genes can’t protect you against being gored by a water buffalo, or having your bones broken by a giraffe trampling.
Along with this, animals with faster metabolisms have shorter lifespans because they create more metabolic waste and therefore they accumulate more damage to their bodies in the same amount of time. This also applies to carnivores: animals who eat meat have much higher rates of cancer than their herbivorous counterparts. (Source: https://www.nature.com/articles/s41586-021-04224-5)
The only reason why larger animals generally have longer lifespans is because small prey animals can get away with short life’s and still successfully reproduce, while larger species take more time to grow up and reach maturity. Humans cannot have the two-year lifespan of a mouse because we’d still be toddlers by the time.
Primates in general have a lot of the characteristics that both allow and require longer lifespans: they’re larger than the average mammal, have slower metabolisms, eat mainly plants, and have long growing-up periods (primates are smart and need to do lots of learning to do things other species do by instinct) before maturity. A 100-pound chimp can live to its late 40s and early 50s with modern medical care.
However, humans are outliers and break the trend in a few of these aspects. Yes, we are the second largest primate after gorillas so that helps, but we have a faster metabolism than other apes (a 120-pound adult human will burn around 20% more calories than a 120-pound chimp, and 40% more than a similarly-sized gorilla), and are highly omnivorous with a significant portion of our diet being animal products, yet our lifespan is often double that of the other great apes. Even people living in the wild like hunter-gatherers regularly live to their late 60s and 70s, which is unheard of in any other primate, or really any mammal our size.
There are a lot of theories as to why this happened, but many people who study this agree that humans evolved such long lifespans because unlike other animals, grandparents are helpful to keep in a tribe in a survival and welfare sense: elders have deep knowledge of their environment which helps in times of crisis like droughts or natural catastrophes, they can help take some of the workload of raising kids so that the younger parents are more efficient at doing the more physically-demanding tasks, etc.
It is true that there seems to be a correlation between lifespan and metabolism, and generally larger animals have lower metabolism. But you have to consider animals of separate groups separately because ectotherms like turtles have very slow metabolisms compared with endotherms of similar sizes. Also, birds have very high metabolisms, but live surprisingly long lives.
So it’s best to compare mammals separately, and you certainly do see a correlation. For our metabolism and size, humans are big outliers. We live much longer than you would expect. So it’s not so much why big cats have short lifespans as much as it is why humans have such long lifespans.
Because when big cats go to big cat med school, they cheetah on the tests. Too many lives lost to iatrogencat errors.
Because they don’t need to live that long.
The only “goal” evolution has (if it can be said to have a goal at all) is to pass on genetic material. Every selective pressure is focused on this one event. Once you have passed on sufficient genetic material to more or less guarantee a third generation, then you become nothing more than a useless consumer of the resources that your progeny needs.
There are a lot of very different strategies based on this principle. For example, there’s the “shotgun” approach; have has many babies as possible but provide no parental care. Most babies will die this way, but from sheer numbers, a few will survive to reproduce. Opposite that is the “sniper” approach; have one or a few children, then spend a lot of resources on their development to adulthood. These are known as the “r/K selection theorem”. This is a spectrum, so there are a lot of species in between these two extremes.
Humans are on this spectrum at the extreme K end. Not only do we invest vast resources over many years to get our very few children to adulthood, we’ll even stick around to ensure their own reproductive success (menopause has been hypothesized to be part of this; pilot whale males with menopausal mothers in the pod have more reproductive success than males without). Cats are at the K end as well, but not nearly as extreme as we are. They provide enough parental care to a few litters to ensure grandkids, then pop off.
Other selective pressures focus on the individual’s ability to survive after parental care. Humans rely on a large brain for this, while cats rely on muscles and claws. Thanks to our brains, humans take *way* longer to reach sexual maturity than cats do. This means that even if a human didn’t live much past seeing grandkids, that would still mean a lifespan 3-4x that of cats.
In short, lifespans correlate more with time to the third generation than to size of the adult organism. It’s still not a very strong correlation, as there are many exceptions to the trend.
There’s some correlation between body size and lifespan in animals, but it’s mostly driven by the fact that insects are small and short-lived and the vast, *vast* majority of animals are insects. That correlation doesn’t hold up nearly as well when you start looking at animals vertebrate animals. Some of the longest-lived vertebrate animals are various species of parrot and rockfish, each of whom are about the size of a house cat and can live up to about 100 years.
Big cats like leopards have shorter lifespans due to factors such as high metabolic rates, environmental stresses, and predation pressures.
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