Moribund Ants Leave Their Nests to Die in Social Isolation

Abstract

Animal societies provide perfect conditions for the spread of infections and are therefore expected to employ mechanisms that reduce the probability of transmitting pathogens to group members [1-4]. Death in nature rarely results from old age but commonly results from diseases. Leaving one's group to die in seclusion might be an efficient way of minimizing the risk of infecting kin. Anecdotal observations of moribund individuals deserting from their groups exist for several species, including humans (e.g., [5]), but have rarely been substantiated by quantitative analysis. Furthermore, to confirm that dying in solitude has evolved because of its altruistic benefits requires refuting the alternative explanation of pathogen manipulation of host behavior. Here we show that workers of the ant Temnothorax unifasciatus dying from fungal infection, uninfected workers whose life expectancy was reduced by exposure to 95% CO(2)[6, 7], and workers dying spontaneously in observation colonies exhibited the same suite of behavior of isolating themselves from their nestmates days or hours before death. Actively leaving the nest and breaking off all social interactions thus occurred regardless of whether individuals were infected or not. Social withdrawal might be a commonly overlooked altruistic trait serving the inclusive fitness interests of dying individuals in social animals.

Altruism can evolve when relatedness is low: evidence from bacteria committing suicide upon phage infection

“…While theoretical work convincingly revealed the conditions required for host suicide to evolve [39,40], empirical work has often been criticized [41,42]. In particular, the adaptive significance of host suicide has been challenged because studies involved eusocial Hymenoptera [43][44][45] or clonal aphids [46], where complex life histories impeded rigorous fitness tests and the exclusion of agarose concentration in growth medium bacterial cultures went extinct ** *** *** *** *** * *** *** *** *** ** *** *** *** *** Figure 3. (a) On solid agarose (0.4%, high relatedness) Escherichia coli l significantly outcompeted E. coli HK97 at all phage concentrations, even when initially rare (t-tests, from left to right: t 7 ¼ 3. rspb.royalsocietypublishing.org Proc R Soc B 280: 20123035 alternative explanations.…”

“…Possible examples include the observation that infected social insect workers leave the colony to die in isolation (i.e. altruistic self-removal [44,45]), a behaviour that could come at negligible costs because infected individuals might be strongly compromised in carrying out worker tasks anyway.…”

Dead ant walking: a myrmecophilous beetle predator uses parasitoid host location cues to selectively prey on parasitized ants

“…Adaptive suicide in social insects is relatively common as a pre-emptive strategy (e.g. honeybees using their stings to defend the colony), although it is notably difficult to evaluate in the context of host-parasitoid interactions [29,[35][36][37][38][39]. The host suicide hypothesis postulates that mature parasitoids emerging from hosts are more likely to infect host's kin than non-kin.…”

“…Previous studies have shown that dying ants will leave their nests or are restricted entry to their nests [29]. To determine whether M. xipe beetles are able to selectively locate individual parasitized ants within the coffee plantation, we constructed beetle traps from small plastic cups with plaster of Paris on the bottom to retain moisture and lids with holes large enough for the beetles to enter the traps (electronic supplementary material, S3).…”

Myrmica ants host highly diverse parasitic communities: from social parasites to microbes

“…auto-and allogrooming, alloor self-exclusion of infected individuals, corpse disposal and/or cemetery formation (Oi and Pereira, 1993;SchmidHempel, 1998;Heinze and Walter, 2010). However, increasing frequency of allogrooming, while reducing the amount of spores on individual level, may assist the spread of pathogens among nest-mates (Oi and Pereira, 1993;Reber et al, 2011;Konrad et al, 2012).…”