Social organization of necrophoresis: insights into disease risk management in ant societies

Abstract

Insect societies, which are at a high risk of disease outbreaks, have evolved sanitary strategies that contribute to their social immunity. Here, we investigated in the red ant Myrmica rubra, how the discarding of nestmate cadavers is socially organized depending on the associated pathogenicity. We examined whether necrophoresis is carried out by a specific functional group of workers or by any nestmates that may become short-term specialists. By observing the behavioural profiles of tagged individuals, we assigned half of the colony members to functional groups (foragers, intermittent-foragers, domestics, nurses and inactives). Following the introduction of uninfected or sporulating corpses into the nest, intermittent-foragers were the functional group most involved in necrophoresis, as they touched, moved and discarded more cadavers. Interestingly, sporulating corpses induced a more generalized response in workers from all functional groups, thereby accelerating their rejection from the nest. The individuals contacting corpses were also prophylactically engaged in more grooming behaviour, suggesting the existence of hygienist workers within ant colonies. These findings raise questions about a trade-off existing between concentrating health risks on a few workers who are highly specialized in necrophoresis and exposing a larger population of nestmates who cooperate to speed up nest sanitization.

Keywords: Beauveria bassiana; Myrmica rubra; generalist entomopathogen; social immunity; waste management; work organization.

Figure 1.

Diagram of the nest from different perspectives. (a) Top view. (b) Screenshot of a video used during behavioural analyses. (c,d) Side view of the nest prior to the introduction of corpses (c) and during their introduction (d). In red (a,c), the mobile column inserted in the centre of the nest allows to free a space for the further introduction of corpses. In blue (d), the mobile plexiglass plug that closes the roof opening during the observation of necrophoresis.

Figure 2.

Experimental procedure. The study included three experimental sessions represented in blue, red and green. Days 1 and 2: tagging of workers. Days 3 and 4: rest period to stabilize the ant population inside the nest and to habituate workers to the tags. Days 5 to 7: recording of tagged workers behaviour inside the nest (three times per day for 3 min). Day 8: introduction of 10 uninfected cadavers in the nest and observation of necrophoresis by ants. Days 9 and 10: rest period before the second introduction of cadavers. Day 11: introduction of 10 sporulating cadavers in the nest and observation of necrophoresis. Days 5 to 21: each day, dead workers were counted, taken out of the colony and put in sporulation.

Figure 3.

Principal component analysis of all behavioural profiles. (a) The first component allows to differentiate between internal workers (left), ‘intermittent-foragers’, and ‘foragers’ (right). (b): Components 2 and 3 allow to differentiate between ‘nurses’ (top), ‘inactives’ (bottom left) and ‘domestics’ (bottom right). Note that the assignment of tagged workers to one of the five functional groups was done at the colonial level using a centroid clustering method (see electronic supplementary material).

Figure 4.

Proportion of tagged workers contacting or not the corpses per functional group. An individual is considered as having contacted the corpse when touching it at least once. Black bars and letters: uninfected corpses. Green bars and letters: sporulating corpses. Full bars: come in contact with cadavers at least once. Striped bars: never contacted cadavers. Significantly different proportions among groups share no common letters (Pearson χ²-test followed by a pairwise nominal independence test as post hoc). Above each functional group, significantly different proportion within group between the uninfected and sporulating conditions are displayed with a star (McNemar tests).

Figure 5.

Percentage of tagged workers in each functional group that transported at least one cadaver. Black bars and letters: uninfected cadavers. Green bars and letters: sporulating cadavers. Significantly different post hoc comparisons between groups share no common letters (χ²-test followed by pairwise nominal independence test as post hoc, α = 0.05, fdr correction). Above each functional group, lines with the corresponding p-value indicate the results of the McNemar test between the sporulating and uninfected condition within each functional group.

Figure 6.

Contribution of individuals to necrophoresis according to the level of specialization. Shades of black and white: uninfected corpses. Shades of green: sporulating corpses. Significantly different proportion between conditions are displayed with stars (χ²-tests). See electronic supplementary material, table S3 for details about individual specialization across functional groups.

Figure 7.

Survival curve per functional group. Mortality was calculated over the individuals that still bore their tag at the end of experiment. X-scale represents days after introduction of contaminated carcasses. The Kaplan–Meier test shows a significant difference in the mortality dynamics of foragers (in red) compared with other functional groups (in black).

Figure 8.

Proportion of prophylactic self-grooming per functional group depending on whether individuals later had contact with cadavers. (a) Uninfected condition. (b) Sporulating condition. The proportion of time spent performing self-grooming was measured for each tagged ant during behavioural analyses, prior to the introduction of corpses. Each functional group is divided into individuals who touched the cadavers at least once (Contact) and workers who never touched them (No). Significant differences within group are indicated by stars (Wilcoxon test, α = 0.05). Significantly different post hoc comparisons among functional groups share no common letters (black = uninfected condition, dark green = sporulating condition, lowercase letters = contact, capital letters = no contact. Kruskal–Wallis test followed by paired Nemenyi as post hoc, α = 0.05, Tukey correction).