At the cellular level, we typically think of senescence as an orderly transition into quiescence or death. Aging, a functional decline in cellular and organismal activity, is often associated with senescence. Both processes are typically influenced by the passage of time, but aging and senescence do not always go hand-in-hand. Two recent papers illustrate the distinction between senescence and aging and touch upon the broader question of the adaptive purpose of these processes.
Rueppell et al. addressed a paradox in the well-studied biology of the honey bee Apis mellifera. These researchers found that, despite a higher mortality risk, chronologically older workers bees do not display significant age-related declines in physiological properties. One explanation might be that older bees drop dead according to a secret internal chronometer that doesn’t influence their function (aging without senescence). Alternatively, the authors suggest that increased mortality risk in older bees may result from age-related division of labor. Worker bees perform various tasks to maintain the colony, including hive-building, nursing of larvae and foraging for food. It is known that foraging bees tend to be older individuals, who may be cannier but also expose themselves to greater risk of predation or mishap. The higher extrinsic mortality of older bees despite an absence of measurable aging might thus result from the dangers of leaving the hive, a privilege accorded only to the oldest of workers.
This hypothesis has found experimental support from a rather cruel study of another eusocial organism, the ant Myrmica scabrinodis. Moron et al. (I kid you not) manipulated the physiological aging of worker ants by injuring them mechanically or with myrmo-toxic carbon dioxide (the bioethics committee must have rejected their application for the use of a magnifying glass). These treatments, while not fatal, shortened the life expectancy of their hapless victims. Injured workers were marked and placed into artificial colonies with equal numbers of unmarked, age-matched healthy workers. The experimenters then carefully observed how the injured ants spent their time and how long they lived. Not only did injured ants die faster than healthy ants, but they spent more of their time foraging on the outskirts of the colony. The authors propose that physiological damage triggers a behavioral change that maximizes the utility of the individual to the colony— individuals that are likely to die soon take on the most dangerous work.
Both studies support the idea that the behavior of eusocial insects undergoes age-related change— this can be thought of as social senescence. In ants, physiological damage that may (or may not) model the natural aging process triggers a behavioral change that makes the best of a bad situation. In bees, cryptic age-related physiological changes promote risk-taking in ostensibly healthy animals. The obvious question is whether these changes can be reversed. For instance, it may be possible to extend the lifespan of forager bees by engineering of colony demographics to decrease the need for foraging.