BIOACCUMULATION AND THE EVOLUTION OF "AGE-REVERSED" DOMINANCE SYSTEMS IN ANIMALS, WITH A FOCUS ON NON-HUMAN PRIMATES: A NEW HYPOTHESIS Clara B. Jones August 2023
I have what I consider a robust & testable hypothesis for the evolution of "age-reversed" ["age-inverse"] dominance systems [dominance systems whereby younger adults are dominant to middle-aged & older adults; MA adults dominant to old adults; old adults subordinate to all adults younger than themselves, a dominance system so far documented only in howler monkeys & langurs--both leaf-eating taxa ... I asked ChatGPT to explain its origin ... the LLM suggested a few obvious & generic answers (e.g., female choice, differential access to food) ... nonetheless, the entity "said" something that really got me thinking--that the youngest, dominant animals must be competitively superior ... after much thought, & after rejecting several ideas, I settled on the explanation that toxicity from a folivorous diet builds up in individuals over time ["Bioaccumulation"]--from weaning to old age, leaving progressively older individuals with less Energy--fewer metabolic resources--to allocate to reproduction & survival, as well as, competition with same-sex adults for access to limiting resources [e.g., food, mates] ... this idea is consistent with the hypothesis that howlers are under greater, though significant, metabolic stress than Hanuman langurs since howlers' behavioral repertoire [including little aggression & a diverse display of stereotyped & ritualized signals] is highly truncated compared to langurs ... also, howlers don't have a modified gut; unlike langurs that have a pseudo-rumen ... the suggestion, then, is that, as individuals age, the "age-reversed" ["age-inverse"] dominance system emerges as a result of the age-dependent bioaccumulation of toxic compounds with metabolic consequences constraining/inhibiting allocation of Time & Energy to reproduction & survival as ageing advances ... fwiw, cows, grazers, with a rumen, also, exhibit this dominance system [SB Hrdy, personal communication], as well as, the social insects, Apis mellifera [honey bees] ... furthermore, in cercopithecines characterized by matrilines [e.g., baboons], the dominance hierarchy of sisters is "age-reversed" ["age-inverse"], with youngest sisters dominant to older sisters; mothers are dominant to offspring ... furthermore, Bioaccumulation may explain "wasteful" feeding whereby primates, other mammals, &, probably, other folivorous or herbivorous vertebrates, bite into one or a few items before throwing the item away; mantled howlers, for example, consistently throw away mango fruit after biting into the toxic fruit [many of us have experienced the unpleasant effects of consuming too many mangos] ... interestingly, these monkeys visit the same trees year after year without, seemingly, learning the general rule--"Mangos are toxic." ... finally, selection for behaviors/strategies opposing Bioaccumulation may help to explain the short duration of feeding bouts in food "patches" by some folivorous or herbivorous animals [e.g., folivorous gorillas] ... whether or not Bioaccumulation can explain the aforementioned cases deserves systematic investigation, research that, probably, can be easily effected in the field ...
Clara B. Jones
August 2023