Age: Is it really just a number?

Allen et al. 2019. Muscular apoptosis but not oxidative stress increases with old age in a long-lived diver, the Weddell seal” Journal of Experimental Biology.

You’ve likely heard of “dog years” before, but what about seal years? Do wild animals age the same way that pets and humans do? Turns out – we don’t always know!

All animals – humans included – experience a certain number of stressors and threats in their lifetime. These threats might look very different depending on the animal. A house mouse, for example, might have to worry about avoiding your housecat, or finding enough crumbs in your kitchen to make a meal. A snake might worry about avoiding a large bird of prey, and finding enough mice to eat. As humans we don’t generally worry so much about avoiding predators or catching enough food, but we have stressors in our lives as well (I live in the San Francisco area, and I think driving here is giving me grey hairs!).

Webcomic from: Abstruse Goose. Link here to original content.

So, what do these stressors mean for age? For us humans, aging usually means feeling slower, maybe more back aches, and often we develop diseases of degeneration. This means more visits to the doctor, medications, “slowing down” the pace of our lives, and sometimes assistive devices such as wheelchairs, canes, or hearing aids. Wild animals don’t have these luxuries, though! An old mouse can’t use a cane to get around – it will simply be slower, which should make it an easier target for that housecat. Following this logic, we expect that we generally won’t see “old” animals in the wild because their declining health will make them more susceptible to predation or less capable of finding or catching enough food to eat.

There are a number of wild species, however, that live for quite a long time. Elephants and tortoises are well known examples, but did you know that Weddell seals can live well into their twenties? Weddell seals are the mammal with the southernmost breeding range, and the population breeding in McMurdo Sound, Antarctica (77oS) has been studied for many years. A project run by Montana State University has been tagging pups every season, so almost all of the animals that haul out on the sea ice annually are of known age.

Weddell seals napping on McMurdo Sound, Antarctica. Weddell seals can swim further under the seasonal shorefast sea ice than any other mammal, which means they can give birth to their pups on top of the ice far away from predators such as killer whales and leopard seals. NMFS permit #19439, ACA permit #2016-005.

A few years back, researchers started to ask whether any of these “old” Weddell seals showed signs of aging. But what does aging look like in a seal? Weddell seals are already pretty grey, so looking for grey hairs isn’t a great place to start. Plus, they wanted to know how aging would affect the animals. Adult Weddell seals don’t worry much about predators – they are approximately 3 meters long and weigh around 400 kg – too big to be lunch for most other animals!

Adult Weddell seals are HUGE animals! They are often over 3 meters long and can weigh upwards of 400 kilograms. They’re pretty mellow and relaxed on land, but they’re super athletes when it comes to holding their breath underwater. NMFS permit #19439, ACA permit #2016-005.

Weddell seals do need to eat, though, and they have to be fairly quick to catch their meals of fish and squid. Swimming speed can be affected by muscle health, so researchers looked for differences in the swimming and steering muscles of “old” (17-26 years old) seals compared to “younger” adults (9-16 years old). They found that old seals had “stiffer” muscles than younger seals – more of the muscle was made up of a structural protein called collagen, with an overall decrease in the number of muscle cells that are responsible for muscle contraction. This might mean that the seals can generate less force in their swimming muscles, making it more energetically costly for them to swim (spoiler: it didn’t change how long they dove for!).

One of the lingering questions from this earlier study was what drives the changes in the muscle that were observed between old and young seals? Are the cells dying in old seals? Are they accumulating more injury, somehow? A popular theory in aging is that as our cells get older they produce more reactive oxygen species – compounds that cause oxidative damage to cells and potentially kill them. This is why “antioxidants” (think of foods like blueberries) are popular! Were the changes in old seal muscle caused by reactive oxygen species? Or were the seals mostly protected from this type of damage, since reactive oxygen species are something they deal with while diving anyway?

Paper in a nutshell–what did we find?

To investigate this question, a group of researchers from University of California Berkeley, Texas A&M University, University of Alaska Fairbanks, University of Nevada Las Vegas, and the Alaska SeaLife Center, looked at the same muscle samples that were previously studied, and we found that markers of a certain type of cell death – apoptosis – were twice as high in the swimming muscle from old seals compared to young seals! But when we looked for markers of damage from reactive oxygen species – oxidative damage – there were no differences in the age groups. So the cells in swimming muscles of old animals are dying more than the same cells in young animals, but they’re not dying from oxidative stress. (So what are they dying from? We don’t know yet!). This might be because old seals don’t seem to lose any of their antioxidant capacity with age. All of the antioxidants and other protective proteins that we looked at were just as high in the old seals as in the young seals!

NMFS permit #19439, ACA permit #2016-005.

This is pretty cool, because in older humans there is often a decline in antioxidant capacity that allows oxidative damage to accumulate. So Weddell seals do get old – there are visible, measurable changes in their muscles with age – but they don’t follow the oxidative stress path to old age that we had expected! Sometimes the best part of science is being wrong, because now we get to ask… if not oxidative stress, then what?

Written by: Kaitlin Allen, PhD Student in the Vázquez-Medina Lab,University of California, Berkeley

Cover photo taken under NMFS permit #19439, ACA permit #2016-005.

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