Undying leafcutter-ant colonies: The oldest Atta texana leafcutter-ant colonies are estimated to grow older than 100 years

Abstract

Tropical Atta leafcutter-ant colonies reach a maximum lifespan of 15-20 years because each colony has only a single queen, queens live for maximally 15-20 years, and colonies do not requeen after queen death. At the northern range limit of leafcutter ants in Louisiana, however, we found that the mortality rate of Atta texana colonies is consistent with a life expectancy of 65-75 years and a maximum colony lifespan exceeding 100 years. Annual mortality of established A. texana colonies in Louisiana is only 1-2% compared to 10-25% in tropical Atta species. Exceptional longevity of A. texana colonies is due to (i) absence of Nomamyrmex army-ant predators specialized to raid Atta colonies; (ii) polygyny of A. texana colonies, creating a unique social organization in A. texana colonies that differs from the monogynous colonies typical for all other Atta species; and (iii) adoption of young queens by old colonies, possibly facilitated by low genetic diversity in range-limit populations of A. texana. Centenarian longevity of A. texana colonies was so far unknown because, from the 1930s to the turn of the century, research prioritized eradication of A. texana as a forestry pest, and landscape-wide pesticide application by aircraft eliminated old A. texana colonies. After the most harmful pesticides were banned 30-50 years ago, A. texana populations recovered, yet A. texana required decades to regain its earlier niche in the forest ecology. In undisturbed habitat, well-established old A. texana colonies with large territories appear to recruit young queens especially well. Old age of a colony therefore does not necessarily increase mortality as in typical lifespan-limiting processes where age drives senescence and mortality (old age [->] mortality); instead, well-rooted residency that is correlated to age of an A. texana colony increases the chance of rejuvenation via queen adoption, reduces or eliminates reproductive senescence of a colony, and consequently prolongs colony lifespan (well-established residency [->] rejuvenation [->] long life). Large size attained in old age imparts potentially unending life.