Scientists Revealed The Bizarre Reason Animals Age Faster Than Humans

Researchers from the school of medicine at the University of California have actually looked into this question properly. It’s a complicated matter, but we’ll do our best to wrap our heads around it. The scientists analyzed the way in which molecules known as “methyl groups” heaped together in specific parts of the human genome. They then looked at the same thing in the dog genome and compared the two. While many of us might struggle to fully understand that, we can still look at their results. They basically worked out, through this process, that a dog year isn’t the same as seven people years.

The researchers looked at the genetics of 100 or so Labrador retrievers of different ages for the study, from puppies to old dogs. Based on that, they found that methyl groups accumulate in the dog genome really rapidly in the first year of a pooch’s life. In essence, this means they age really quickly during this period. But this is where things get interesting, because they also found this process slows down as a dog gets older. In other words, aging for dogs is by no means a straightforward, steady process.

To try and conceptualize this in a way we might understand, we can say a one-year-old pup is not equivalent to a seven-year-old human. In fact, it’s actually more like 30. That’s right! A one-year-old dog is so mature, in terms of its own species, that it’s equivalent to a human being of 30. But after that first year of a dog’s life, the aging process slows down quite considerably. That means that by the time a dog is 14, it’s about the same as a person being in their mid-70s.

Of course, this study revolved around quite a limited sample size. A group of 100 dogs really isn’t that much to go on, plus they were all the same breed. To gain a more accurate and comprehensive understanding of how dogs of all types age, a range of breeds will need to be studied — not just Labradors! Still, this is definitely a start toward gaining a better understanding of how our best friend works. Their experience of aging, it seems, is just so different to anything we’d recognize in ourselves.

The researchers found that they could make pretty accurate comparisons between dogs and humans at both the early and late stages of their lives. For instance, they could say a seven-week-old puppy had developed to broadly the same extent as a nine-month-old human. A 12-year-old dog, meanwhile, was about the same as a 70-year-old person. Obviously this isn’t to say our two species’ development is comparable, but in terms of age this is how things work out. But when they looked at adolescent dogs and people, plus middle-aged subjects, too, things didn’t plot out so neatly. The middle years of a dog’s life don’t seem to have an easy equivalent in human years.

Academics in this field have praised the work done by the researchers at the University of California. Professor Lucy Asher from Newcastle University is one such expert: although she wasn’t involved in the study, she was intrigued by its findings all the same. As she remarked to British newspaper The Guardian, “If we think about aging in terms of how old our cells are, this new paper is really useful in matching up human and dog years.” She did point out, though, that the findings strictly related to aging, rather than other forms of development. In other words, don’t be shocked that a one-year-old dog, who is about 30 in human terms, still acts like a puppy who loves play.

On this point, Dr. Asher said, “Whilst a 30-year-old human might have cells of an analogous ‘age’ to a one-year-old dog, many dogs won’t be fully grown at this time and they will still have unsettled hormones and behavior associated with puberty... The development of dogs is not just a shortened version of the human development, which is why it’s difficult to find a clear match-up between a dog’s age and a human’s age.” It’s probably worth bearing all this in mind, as, while it can be useful and interesting to make comparisons between our species and dogs, it’s not always simple: we’re very different in lots of ways.

The fact that the researchers in this study have figured out a specific way of measuring the aging process could prove genuinely useful. Far from just being interesting, this could have real-world applications, as senior author Professor Trey Ideker has pointed out in a statement. He said, “There are a lot of anti-aging products out there these days, with wildly varying degrees of scientific support. But how do you know if a product will truly extend your life without waiting 40 years or so? What if you could instead measure your age-associated methylation patterns before, during and after the intervention to see if it’s doing anything?”

One way to think about what Dr. Ideker and his colleagues have done here is to say they’ve given us a glimpse at what we might call an “epigenetic clock.” In other words, they’ve landed upon a method of analysis that means a cell, tissue, or wider organism’s age can be discerned. This epigenetic clock provides clues to a genome’s age in a somewhat similar way to how wrinkles on a person’s face can give pointers to roughly how old they are. This is a tantalizing prospect for experts in the field of development and aging.

It wasn’t actually Dr. Ideker’s idea to focus on analyzing dogs: that brainwave came from a colleague named Tina Wang, who, at that point in time, was a graduate student. As Dr. Ideker explained in an article on the University of California’s website, “We always look at humans, but humans are kind of boring. So, she convinced me we should study dog aging in a comparative way.” Once he was sold on the idea, the pair then set about bringing other experts on board the project. And people like Professor Danika Bannasch and Elaine Ostrander were happy to help and offer their specific and very relevant expertise.

Dr. Ideker thinks dogs are especially interesting animals to consider, as their lives are so closely entwined with our own. They’re arguably closer to us than any other species — hence their status as “man’s best friend” — which means they’re often exposed to the same sorts of conditions as us. They live in the same places as us, and on top of everything else they even receive a comparable level of healthcare. In other words, for anyone who wants to compare human physiology with that of another species, dogs are ideal.

But beyond the comparisons with humans, this research could have a material benefit for dogs, too. That’s because the old notion that dogs age seven years for every one year in humans has actually played a part in how vets treat the animals. But think about it: if that idea is fundamentally wrong, then those treatments won’t necessarily work as well as they should. By gaining a more accurate way of conceptualizing a dog’s life cycle, vets might really be better placed to help them.

The caveat with all this, of course, is that this study alone isn’t enough to totally rewrite our understanding of how dogs age. Dr. Ideker himself has acknowledged this point, noting the fact that studies involving breeds besides the Labrador will be necessary. He’s planning to do just that, and he’s confident his current thinking on the matter will be proven accurate. He’s also admitted to a new understanding of his own pet dog as a result of his work. He noted, “I have a six-year-old dog. She still runs with me, but I’m now realizing that she’s not as ‘young’ as I thought she was.”

Obviously Dr. Ideker and his colleagues are far from the only researchers to take an interest in how aging works in different animals. Why is it that human beings tend to live so much longer than so many other animals? Our pets’ life expectancy, as a general rule, is just a fraction of our own, but the same is true for many wild animals, too. Well, the question of why that is has been at the center of many experts’ minds. And one possible explanation has to do with brain size and how it relates to the rest of the body’s dimensions.

The bigger an animal’s brain in relation to their body, it seems, the more likely they are to enjoy a longer lifespan. Not only that, but they tend to reach sexual maturity at a steadier rate and they generally produce a lower number of offspring. Humans have really big brains compared to the rest of their body, and on average we tend to reach a little under 80 years of age. Capuchin monkeys also have a big brain-to-body ratio, and they live to about 50.

To turn our attention back to dogs again, you might look at a big pooch and presume its large brain will mean it will live longer than a smaller dog. But anyone familiar with the animals will know that’s not the case at all. It’s actually smaller dogs who tend to outlive their bigger counterparts, and the brain-to-body ratio theory holds true here, too. In other words, smaller dogs actually have larger brains relative to their bodies than the big pups do.

A small breed of dog might be expected to live for up to 15 years, whereas a medium dog might make it to 14 and a big one might get to 13. Obviously these are general figures, and some individuals vary. It must also be noted, though, that we can’t just take size alone here. The dog’s specific breed matters: for instance, a Great Dane might be lucky to make it past ten years of age. Compared to other big dogs their life expectancy is quite a bit shorter.

Another thing worth noting about bigger dogs is that their cells might be more likely to grow in an abnormal way compared to smaller breeds. In effect, this means they are more prone to developing cancer. It’s just a sad fact that bigger breeds tend to lose their lives to cancer more than smaller ones. But, of course, providing a big dog with a good diet and regular exercise can really help extend its lifespan. Everyone needs to live healthily, and that goes for our pets, too.

We’ve spent a lot of time thinking about dogs, but what about man’s other best friend? Well, cats have a fairly similar brain-to-body ratio as dogs, and their lifespans are broadly similar. Again, though, different breeds can expect different outlooks. A Birman cat, for instance, reaches an average of 16 years, whereas a Maine coon might generally only get to 11 or 12. On top of that, another major thing to consider is the living situation of a given cat. Those that live outdoors are at greater risk of coming into contact with toxins and viruses, and they tend to live about five years fewer than those who enjoy a life indoors.

There are, of course, pets that enjoy lifespans that are far more in line with what humans can reasonably expect to enjoy. Parrots can hit 50 years of age, while pink cockatoos can make it past 80. Different species of tortoise can expect to make it past the half-century mark, with some even likely to outlive their humans by a considerable margin. The Eastern box turtle, for example, can reach 138 years! That means someone will have to take look after them after you’ve checked out!

Moving beyond our land-based pets and into the ocean, we can encounter animals with truly mind-bending life expectancies. The ocean quahog can live past the century mark, and in some cases has been known to survive for 500 years! A red sea urchin can get to roughly 200, as can a rough-eyed rockfish. A Greenland shark’s life expectancy might stretch to 500 years, and it isn’t even considered mature until it’s 156!

Of all the long-living sea creatures, though, there’s one that really stands out beyond all others. Turritopsis dohrnii is a type of jellyfish, but it has an ability that really beggars belief. It was first recorded in the Mediterranean Sea towards the end of the 19th century, and today it tends to be known by a rather telling nickname. Because of the things it can do, we call this creature “the immortal jellyfish.”

The life cycle of Turritopsis dohrnii starts out pretty much the same as any other jellyfish. It begins as a larva, which we call a planula. This can swim around, until eventually it sinks to the bottom of the ocean where it develops into a group of polyps. These then give rise to a bunch of “medusae” that can swim around and share the same genetic make-up. The medusae are what we think of when we hear the word “jellyfish.” These then mature into adulthood within a few weeks. All normal so far, but here’s where things get interesting.

If the jellyfish is harmed in some way, it can literally turn the clock back on its development cycle. In other words, it can go back to being a polyp all over again. From there, the cycle begins again and it eventually becomes an adult medusa. But vitally, the genetic makeup of this new medusa is exactly the same as the old, damaged one. It’s technically the same animal, which means, in a sense, it has cheated death. Obviously scientists are very interested in the process, as understanding it may potentially lead to all kinds of amazing medical feats down the line.

The term we use for the process the immortal jellyfish undergoes is “transdifferentiation,” when a given cell with a specific purpose changes into another. If scientists can understand that better, it might help them in stem cell research. And this, in turn, might ultimately help us to replace damaged cells in people’s bodies with functional ones. It’s an intriguing idea with lots of potential, and at the heart of it is this incredible jellyfish.

But anyway, let’s pull the focus back to humans. As we all know, our species takes quite a long time to mature compared to others: our childhood lasts for an extremely long time. Researchers from Vanderbilt University in Nashville, Tennessee, have pondered the reasons for this and have come up with some ideas. They argue that it all has to do with the number of neurons we have in our cerebral cortexes. To put it plainly, we have a lot. As the author of the study related to this question, Suzana Herculano-Houzel, pointed out in a statement, “Whether you’re looking at birds or primates or humans, the number of neurons that you find in the cortex of a species predicts around 75 percent of all of the variation in longevity across species.”

That’s pretty notable, especially when we compare it to the use of body size or an animal’s metabolic rate to predict lifespan. These other markers might help you predict longevity with an accuracy of maybe 20 or 30 percent. And when you think of birds, you can see the issue plainly. These things can live for maybe ten times as long as mammals roughly the same size as them. The number of neurons, then, seems a much better way to judge how long animals might live for. And it’s also a good way to understand the period it takes for animals to mature.

In her study Dr. Herculano-Houzel analyzed 700-plus warm-blooded species, taking note of their life expectancies and the number of neurons in their brains. Using a color-coding system for the data, she soon identified that birds tended to live longer than mammals of comparable size. She also noted in a statement, “Likewise, for similar specific basal metabolic rates, parrots and songbirds live longer and take longer to reach sexual maturity than many mammalian species, especially non-primates.”

This called to mind research Dr. Herculano-Houzel had previously undertaken. She’d noted in the past that parrots and songbirds generally possessed a greater number of cortical neurons than primates of roughly the same size, who themselves had more than other types of animal with similar dimensions. This latest bit of research just added to her thinking from that first one. As she plainly put it herself, “The more cortical neurons a species has, the longer it lives: ...[it] doesn’t matter if it is a bird, a primate or some other mammal; how large it is; and how fast it burns energy.”

Some pretty left-field ideas have circulated in relation to human development over the years, and researchers have taken them very seriously. One such idea revolves around the role of the grandma within human society. Basically, this line of thinking says that the reason people don’t reach maturity for such a long time, relative to other animals, is because their grandmothers tend to play an important caregiving role with their grandchildren. This, the theory goes, may affect the onset of maturity as it helps kids to enjoy an extra-long childhood.

Well, that was the theory, but Dr. Herculano-Houzel thinks it’s invalid, based on her own research. She’s convinced the amount of neurons in an animal’s cortex correspondents with the length of time it takes to mature. Given the fact we have so, so many, it stands to reason it would take us a long time to grow up. The role of the grandmother may have some other significant impacts on human life, but in this particular case, Dr. Herculano-Houzel believes, the neurons are the important factor. She remarked, “Now we can say that humans spend just as long in childhood and live exactly as long after reaching maturity as you would expect for the number of neurons in our cerebral cortex.”

Dr. Herculano-Houzel went on, “It makes sense that the more neurons you have in the cortex, the longer it should take a species to reach that point where it’s not only physiologically mature, but also mentally capable of being independent. The delay also gives those species with more cortical neurons more time to learn from experience, as they interact with the environment.” And if, as Dr. Herculano-Houzel has suggested, more neurons in the cortex equates to a longer life, then older generations are more likely to be still alive when new generations are born. “Which means,” the academic said, “that grandma is still fundamental in the lives of those with plenty of cortical neurons; she’s just probably not the reason why our species is long-lived.”

So, the overall length of time an animal lives is tied to the number of cortical neurons they possess. That’s all well and good to say, but questions still remain. If we assume that theory is accurate, we still need to establish why it’s the case. Well, that’s exactly what Dr. Herculano-Houzel hopes researchers can now figure out. She said, “The data suggest that warm-blooded species accumulate damages at the same rate as they age. But what curtails life are damages to the cerebral cortex, not the rest of the body; the more cortical neurons you have, the longer you will still have enough to keep your body functional.”

Dr. Herculano-Houzel argues that, while the cortex plays a massive role in cognition, it does far more, too. She said, “The cortex is the part of your brain that is capable of making our behavior complex and flexible, yes, but that extends well beyond cognition and doing mental math and logic reasoning. The cerebral cortex also gives your body adaptability, as it adjusts and learns how to react to stresses and predict them. That includes keeping your physiological functions running smoothly and making sure your heart rate, your respiratory rate, and your metabolism are on track with what you’re doing, with how you feel, and with what you expect to happen next. And that, apparently, is a key factor that impacts longevity.”

Another theory Dr. Herculano-Houzel has tried to disprove through her research relates to brain size. For a long time, one prominent idea attempting to explain the incredible intelligence of humans was that the prefrontal cortex of our brains had grown to a larger extent than it had in other primates. Well, Dr. Herculano-Houzel doesn’t think that’s right. Yet again she places the emphasis on the number of cortical neurons we have. In general, though, our brains are fairly typical of any primate.

Dr. Herculano-Houzel explained, “People need to drop the idea that the human brain is exceptional. Our brain is basically a primate brain. Because it is the largest primate brain, it does have one distinctive feature: it has the highest number of cortical neurons of any primate. Humans have 16 billion compared with 9 billion in gorillas and orangutans and 6-to-7 billion in chimpanzees. It is remarkable, but it is not exceptional.”

Dr. Herculano-Houzel has her ideas about how humans came to possess these bigger brains — and therefore more cortical neurons — than our primate cousins. She thinks it all has to do with cooking! It’s our ability to cook our meals, she has argued, that means our ancestors were able to pass over an “energetic barrier” that kept other primate brains from growing bigger.

Dr. Herculano-Houzel has explained why cooking is so vital for us. She said, “By cooking, I mean cutting, dicing, smashing — all types of food preparation... Take a single carrot. If you eat it raw, it will take ten to 15 minutes of vigorous chewing and your digestive system will only capture about one third of the calories. But, if you cut the carrot up and cook it for a few minutes, it takes only a few minutes to consume and your body gets 100 percent of the calories.” In other words, cooking enables us to extract the maximum nutritional benefit from our diet and in a more timely fashion, too.

Anyway, all of this is to say, development and aging are complicated topics with which science is still grappling. Other animals’ experience of getting older is very different to ours, and the reasons for that are extremely varied and complex. But with ongoing study, maybe these remarkable processes will become slightly clearer and more comprehensible down the line.