Introduction
According to W. D. Hamilton’s inclusive fitness theory (kin selection), a trait or behaviour is altruistic when the fitness cost of the actor is lower than the fitness benefit of the recipient which is directly proportional to the genetic relatedness between actor and recipient (rb>c ; r=relatedness, b=benefit for recipient, c=cost for actor; (West et al. 2007)). In insects, usually eusocial species such as ants, bees and termites are considered to show altruistic behaviour. This extends to the point that “an animal acting on this principle would sacrifice its life if it could thereby save more than two brothers, but not for less” (Hamilton 1963). Here, we report on our observations during exposure of the non-eusocial fruit flies (Drosophila melanogaster ) to insecticides arguing that first visitors of a contaminated site are able to detoxify the site to the benefit of the second visitors while they die.
Xenobiotics including plant secondary metabolites and insecticides challenge insects in their daily life as they may perturb cell, tissue and organ physiology at worst causing death. For survival, hence, they have developed elaborate structural and molecular defence mechanisms to escape or disarm xenobiotic toxicity (Gao et al. 2022a). First, the cuticle that covers the body and the endings of the digestive system serves as a barrier to some extent preventing xenobiotics penetration. If xenobiotics overcome the cuticle barrier, potent genetic and molecular programs are elicited for detoxification. The molecular players of the detoxification response have been studied extensively in various insect species. They act in concert in different internal tissues such as the fat body and the midgut. A key entry site of xenobiotics are the ends of the legs, the tarsa. These body parts are designed to sense the substratum with gustatory sensilla and need to have a cuticle with adapted higher permeability (Ling et al.2014; Dinges et al. 2021) and flexibility. A subtype of these sensilla, in addition, may have pores permitting uptake of small molecules. Thickening of the tarsal cuticle in response to continuous exposure to insecticides has been reported in mosquitos (Balabanidouet al. 2019). Thus, the tarsa are dynamic cuticular structures communicating with the proximal environment. Our finding suggest that an extra-corporeal detoxification mechanism may exist in insects that protects insects against their proximal environment. As protection extends to insects visiting the site of the toxic micro-environment after the first visit of their relatives, we consider this behaviour as altruistic.