The evolution of negligible senescence, part II: Organisms that are remotely like us

There are organisms on this planet that are, for all practical purposes, biologically immortal. Last week we learned about a new body of evolutionary theory that purports to explain how negligible senescence could have evolved in at least some of these organisms: When a high density of adults prevents juveniles from finding optimal conditions for growing to maturity, it makes sense for the juveniles to adopt a strategy that one might call “wait for the next funeral”; in some cases this can generate an evolutionary “arms race” within a species as adults live longer and the juveniles who compete with them for resources must wait longer. According to this model, runaway natural selection can thus produce organisms that effectively never age.

In organisms that inspired the work — hydras, pine trees, and Arctic clams — it’s easy to see how there could be a population density-dependent effect on the ability of juveniles to mature: Young pine trees can’t get enough sunlight to get really big until a clearing forms in the forest canopy. Clam larvae need a clear bit of seabed to set down in, and they can’t do so if the entire seabed is covered with adult clams.

Both the body plans and life histories of these sorts of organisms are radically different from ours (and by “we” here I include any metazoan that can, you know, move around purposefully). It’s almost certain that the molecular and cellular particulars of their species-specific solutions to the senescence problem will not be helpful to organisms that are anything like humans. But does the evolutionary theory that explains the emergence of negligible senescence in trees and clams have anything to teach us about how long-lived species arise from short-lived stock? If so, are those lessons in any way portable to mammals?

Possibly. In the Discussion of the Seymour and Doncaster paper we talked about last week, the authors describe a similar sort of evolutionary logic acting in the selection of long lifespans in an ant (recall again that here “recruitment” means “development to reproductive maturity”):

Recruitment limitation is important in determining the population density of the exceptionally long-lived Harvester Ant Pogonomyrmex occidentalis, which has an average colony life expectancy of 17 y. Because nests are static structures and new queens almost never colonize their own nest, there is a clear advantage to a resident queen outliving her neighbors to implant offspring in their place.

Here’s an example, then, of a mobile animal with density-dependent recruitment issues similar to those of a redwood tree or a clam, and therefore an incentive to play the same sort of evolutionary waiting game. The story is subtly different — here an adult is waiting for her adult competitors to die so that her offspring can take over their spots — but the end result is the same: runaway selection for lifespans dramatically longer than those of other ant species. Note that the reproductive peculiarities of eusocial insects play a role here: The queen is waiting for a spot for one of her daughters to start a new colony, and is therefore the queen herself who must live long — the workers, who lack reproductive privilege, die relatively quickly. (By the way, this is not the first example of clever aging-related evolutionary tactics we’ve seen among the ants.)

What can this teach us about the evolution of delayed senescence in mammals? Possibly quite a lot: One famous example of a species with far greater longevity than similarly sized species of comparable body plan, the naked mole rat, is also territorial and eusocial. It is tempting to speculate that mole rat queens, like their peers among the harvester ants, have evolved long lifespans in order to wait out their competitors in other burrows. In the “exception that tests the rule” department, mole rat queens and workers have roughly the same lifespan — but unlike the case in ants, mole rat workers can acquire reproductive capacity when an old queen dies; hence it makes sense for all members of the colony to enjoy the same enhanced longevity.

Mole rats are no less similar to humans than lab mice are. Therefore, biogerontologists are very interested in learning the detailed mechanisms by which mole rats have delayed senescence, since it’s likely (more likely than for clams and trees, anyway) that these details might be of some practical use to us. I’ll close with a typically enthusiastic abstract about the value of studying the mole rat. From Rochelle Buffenstein:

… Negligible senescence is characterized by attenuated age-related change in reproductive and physiological functions, as well as no observable age-related gradual increase in mortality rate. It was questioned whether the longest living rodent, the naked mole-rat, met these three strict criteria. Naked mole-rats live in captivity for more than 28.3 years, ∼9 times longer than similar-sized mice. They maintain body composition from 2 to 24 years, and show only slight age-related changes in all physiological and morphological characteristics studied to date. Surprisingly breeding females show no decline in fertility even when well into their third decade of life. Moreover, these animals have never been observed to develop any spontaneous neoplasm. As such they do not show the typical age-associated acceleration in mortality risk that characterizes every other known mammalian species and may therefore be the first reported mammal showing negligible senescence over the majority of their long lifespan. Clearly physiological and biochemical processes in this species have evolved to dramatically extend healthy lifespan. The challenge that lies ahead is to understand what these mechanisms are.

Mole rats aren’t the only ones who can teach us about negligible senescence in mammals, but the other prominent candidate lacks eusocial behavior, reproductive privilege and any easily identifiable mechanism by which population density would affect maturation rates. In other words, we’re going to need a whole new model to explain whales.