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Hey Mr. Lion, How Old Are You—A Guide to Aging Animals

It seems that every year, the world will discover a newest oldest animal. About a decade ago, people found a 405-year-old clam named as Ming. Then a giant tortoise called Jonathan was touted as the world’s oldest living creature—until people started to question about its identity. There are also accounts of 115-year-old reptiles and 150-year-old wales and while, the famous Galapagos tortoise, Lonesome George, seems to be relatively younger compared with the aforementioned two.

It is not overly difficult to determine these animals’ ages. Just like all clams, Ming grows tree-like rings each year since it was alive. The tortoise, Jonathan and George, are all documented to have appeared in photographs and diaries over the years. The bowhead whale, which is called the longest living mammal on Earth, is discovered to have a century-old harpoon pin lodged inside of its body.

However, determining other animals’ ages, especially those born in the wild, becomes a very difficult task. By using X-rays, zoologist could look for growth markers in the skeletal structure, or they can easily find out an animal’s age after death through examining certain biological markers on an autopsy. While, without tissue samples and X-rays, it becomes much more difficult to get an idea on animals’ ages. Zoologist have to rely on visual cues, with some guesswork thrown in. Below comes a guide to how zoologists determine age for various species.

Orangutans grow wrinkles too.

Meredith Bastian, curator of primates and small mammals at the Philadelphia Zoo, says that a lot of primate aging has to do with teeth. “If I take a look at their teeth, I’ll get a pretty accurate idea about how old an animal is,” she says. Specifically speaking, Bastian is referring to primates’ molars. If the molars are worn-down, it indicates that a primate is older, or it might also indicate that a primate used to eat foods that require a lot of heavy-duty chewing.

Bastian also looks for primates’ skin. She noted:”They have very similar characteristics with humans. You will be able to differentiate a baby, a juvenile and later stages of life by looking at their wrinkly skin.”

In wild male orangutans, zoologists mainly look for a thing called flange, also known as cheek pads, which are the only visible on sexually mature dominant males. When they become older, their flanges sad—like our jowls.


An image of a male orangutan with large flange. Image source: nationalgeographic.com

While, Bastian is not 100 percent accurate, since there also exist sexually mature males that don’t have flanges.

She noted:” People used to think that only flanged males can mate because the flange helps them emit long calls to attract females. This is the dominant male strategy. However, unflanged males have sneakier mating strategy—they mate away from the flanged males and try not to get themselves caught.”

Female orangutans do not have flanges, but much like humans, they do wrinkles and lose bone density. “If there are more stressed, they may have less hair.” Bastian adds.

What else can be markers to identify orangutans’ ages? The answer is their eyes. Baby orangutans have a white circle around their eyes and it disappears gradually over the time. Hence, if you see whites in an orangutan’s eyes, then it has not finished weaning yet.

Another use for cat hair.

Tammy Schmidt, curator of carnivores and ungulates at the Philadelphia Zoo, said:” To determine cats’ ages, you will want to start with the hairs. As a cat ages, its hair gets dry, brittle and gray. It is also true for every cat ranging from house cats to big cats such as eldely tiger and lion.”

Of course, no one would get too close to an elderly tiger or lion to observe hairs. But it is probable to see the difference in their fur coats from a distance. Schmidt noted:” The hair gets duller because a cat is less likely to take care of its hairs as it ages.”

There are some other clues to determine cats’ ages, but they might be harder to see.

Schmidt added:” A carnivore such as a tiger or lion is made to move secretly and sly about what is happening to them, so you need to put all the pieces together to solve the puzzle.” These pieces include muscle tone—animals become less toned when they are older, as well as how the tail fits between its hips. Schmidt said:” You will also want to look at their ribs and how they are moving. Older animals are going to have more pronounced stepping due to their diminished eyesight.”

You can’t know a living fish’s age.

Kara Hilwig, the supervisor for the Alaska State Fish and Game lab, reveals that the secrets to aging fish lies in their ears. Hilwig’s team recently successfully aged a 200-year-old rockfish captured in Alaska. In order to age the rockfish, Hilwig sliced its head and removed two tiny ear bones called otoliths, which have annual growth rings that are similar to those found in a clam or a tree. The otoliths are helpful in determining how old the fish is.

One caveat: The fish must be dead.

Hilwig says:” We break the bones into half and put them over a flame. And this is how you can discern this annual feature.”


The left image indicates the location of the otoliths in a fish head; the right image represents the technology of break and burn by the Alaska State Fish and Game lab—break the otoliths and put them over a flame, but this is not the only approach to discern the annual feature. An example of alternative methods is superior isotope measurements. Image source: mtalab.adfg.alaska.gov;marinebiodiversity.ca

You want to tell the age of a fish without slicing its head off and digging around the ear bone? Forget about that. “It will be very difficult to determine the age of a fish accurately without the otolith.” Hilwig says.

In the same time, it is also essential to make sure the fish live in a temperature-fluctuating environment because the otoliths only grow in summer. “For fish down in the tropics, we can’t find distinct signatures, so it would be much harder to determine their ages.” Hilwig added.

Editor’s note: Various fish live in different environment and hence, the corresponding annual rings on the otoliths are different.  Generally, tropic fish live in an environment that changes little throughout the year, thus the annual ring is not obvious. While, there do have some research suggesting that annual rings can grow on the otoliths due to shifts between dry season and rainy season in tropical waters.

ReferenceNationalGeographicHow Old Is That Lion? A Guide to Aging Animals