Sunday, December 4, 2016

Octopuses and the Puzzle of Aging

Around 2008, while snorkeling and scuba diving in my free time, I began watching the unusual group of animals known as cephalopods, the group that includes octopuses, cuttlefish and squid. The first ones I encountered were giant cuttlefish, large animals whose skin changes color so quickly and completely that swimming after them can be like following an aquatic, multi-armed television. Then I began watching octopuses. Despite being mollusks, like clams and oysters, these animals have very large brains and exhibit a curious, enigmatic intelligence.

I followed them through the sea, and also began reading about them, and one of the first things I learned came as a shock: They have extremely short lives — just one or two years.

I was already puzzled by the evolution of large brains in cephalopods, and this discovery made the questions more acute. What is the point of building a complex brain like that if your life is over in a year or two? Why invest in a process of learning about the world if there is no time to put that information to use? An octopus’s or cuttlefish’s life is rich in experience, but it is incredibly compressed.

The particular puzzle of octopus life span opens up a more general one. Why do animals age? And why do they age so differently? A scruffy-looking fish that inhabits the same patch of sea as my cephalopods has relatives who live to 200 years of age. This seems extraordinarily unfair: A dull-looking fish lives for centuries while the cuttlefish, in their chromatic splendor, and the octopuses, in their inquisitive intelligence, are dead before they are 2? There are monkeys the size of a mouse that can live for 15 years, and hummingbirds that can live for over 10. Nautiluses (who are also cephalopods) can live for 20 years. A recent Nature paper reported that despite continuing medical advances, humans appear to have reached a rough plateau at around 115 years, though a few people will edge beyond it. The life spans of animals seem to lack all rhyme or reason.

We tend to think about aging as a matter of bodies wearing out, as automobiles do. But the analogy is not a good one. An automobile’s original parts will indeed wear out, but an adult human is not operating with his or her original parts. Like all animals, we are made of cells that are continually taking in nutrients and dividing, replacing old parts with new ones. If you keep replacing the parts of an automobile with new ones, there is no reason it should ever stop running.

At least in principle, the puzzle of aging has been largely resolved, through some elegant pieces of evolutionary reasoning. Imagine some kind of animal with no tendency to decline in old age. It just keeps going, and keeps reproducing, until some accident or predator gets hold of it. In such a species, like any other, genetic mutations continually arise. Sometimes (very rarely) a mutation occurs that makes organisms better able to survive and reproduce; more often mutations are harmful and are filtered out by natural selection. But in some cases, a mutation arises that acts so late in an organism’s life that its effects are usually irrelevant, since the organism has already died for another reason, such as being eaten. Natural selection will have little effect on that mutation, so it will become either more common in the population, or less common, purely by chance.

Eventually, some mutations of this kind will become common, and everyone will be carrying them around. Then when some lucky individual does succeed in living a long time without being eaten, it will run into the (usually harmful) effects of these late-acting mutations. It will appear to have been “programmed to decline,” because the effects of those lurking mutations will appear on a schedule. The population has now evolved a natural life span.

That idea was sketched in the 1940s by a British immunologist, Sir Peter Medawar. A decade later, the American evolutionist George Williams added a second step. Mutations often have multiple effects, and these can differ in their timing. Consider a mutation that has good effects early in life and bad effects late. If the bad effects come late, after the organism has most likely perished because of external threats, then these bad effects will have less importance than the early benefits. This is a “buy now, pay later” principle, with payments coming due only after you have probably left the scene anyway. So mutations with that combination of effects — helpful early, harmful late — will be beneficial over all and will accumulate in the population. Then if an individual survives all the external threats and reaches old age, it will be hit with the bill.

The Medawar effect and the Williams effect work together. Once each process gets started, it reinforces itself and also magnifies the other. As some mutations are established that lead to age-related decline, they make it even less likely that individuals will live past the age at which those mutations act. This means there is even less selection against mutations that have bad effects only at that advanced age. As a result, that age becomes harder and harder to exceed.

In the light of all this, I think it is becoming clearer how octopuses and other cephalopods came to have their peculiarly poignant combination of features. Like their mollusk relatives, early cephalopods had protective outer shells, which they carried along as they prowled the oceans. Then, in some animals, the shells were abandoned. This had several interlocking effects. First, it gave rise to their unique, outlandish bodies — in the octopus, a body that can take on any shape at will. This created an opportunity for the evolution of finer behavioral control and large nervous systems. But the loss of the shell had another effect: It made the animals vulnerable to predators, especially fish.

That put a premium on the evolution of octopus wiles and camouflage. But there are only so many times those tricks will save the animal. Octopuses can’t expect to survive long. This makes them ideal candidates for the Medawar and Williams effects to compress their natural life spans. As a result, octopuses have ended up with their unusual combination: a large brain and a short life.

This view is supported by the recent discovery of an exception to the usual octopus pattern, an exception that illuminates the rule. The octopuses I’ve been talking about tend to live in shallow water. But in 2014 researchers at the Monterey Bay Aquarium Research Institute released some remarkable images of a deep-sea octopus they had watched with remote-controlled submarines. This one octopus brooded its eggs for over four years. Even allowing for the fact that everything tends to happen slowly at these depths, that’s a very long time. The total life span of this octopus might have been as long as 16 years.

The Medawar-Williams theory predicts that predation risks should be much less severe for this species than they are for shallower-water octopuses with shorter life spans. And the images taken by the Monterey researchers contain a strong clue that this is so: They show an octopus sitting out in the open with its eggs for years on end. It did not find itself a den. This suggests that this species has less to fear from predators than other octopuses do. As a result, evolution has tuned the life span of this species differently.

by Peter Godfrey-Smith, NY Times |  Read more:
Image: Marion Fayolle