Who Are the “Lazy” Ants? The Function of Inactivity in Social Insects and a Possible Role of Constraint: Inactive Ants Are Corpulent and May Be Young and/or Selfish

Abstract

Social insect colonies are commonly thought of as highly organized and efficient complex systems, yet high levels of worker inactivity are common. Although consistently inactive workers have been documented across many species, very little is known about the potential function or costs associated with this behavior. Here we ask what distinguishes these "lazy" individuals from their nestmates. We obtained a large set of behavioral and morphological data about individuals, and tested for consistency with the following evolutionary hypotheses: that inactivity results from constraint caused by worker (a) immaturity or (b) senescence; that (c) inactive workers are reproducing; that inactive workers perform a cryptic task such as (d) acting as communication hubs or (e) food stores; and that (f) inactive workers represent the "slow-paced" end of inter-worker variation in "pace-of-life." We show that inactive workers walk more slowly, have small spatial fidelity zones near the nest center, are more corpulent, are isolated in colony interaction networks, have the smallest behavioral repertoires, and are more likely to have oocytes than other workers. These results are consistent with the hypotheses that inactive workers are immature and/or storing food for the colony; they suggest that workers are not inactive as a consequence of senescence, and that they are not acting as communication hubs. The hypotheses listed above are not mutually exclusive, and likely form a "syndrome" of behaviors common to inactive social insect workers. Their simultaneous contribution to inactivity may explain the difficulty in finding a simple answer to this deceptively simple question.

Division of labour promotes the spread of information in colony emigrations by the antTemnothorax rugatulus

“…workers that participate in the decision-making process underlying the colony emigration only passively when carried by a primary or carrier ant towards a candidate nest. Passive ants likely correspond to the lazy ants described in the genus Temnothorax in studies of task allocation (Charbonneau and Dornhaus 2015;Charbonneau et al 2017). We also found a small proportion of workers, 3-12% of the colony, that do not fit any of the previous behavioral castes.…”

“…Primary, carrier, passive, and fussy ants provide different contributions to the collective decisionmaking process undergoing a colony emigration that can be regarded as dividing the labor of gathering, spreading, and processing the information necessary to achieve a collective decision. While the behavior of workers (and therefore their membership in a particular behavioral caste) is stable over the course of the 5 colony emigrations we performed, we know that individual workers change the tasks they perform over time as a function of their developmental age (Tschinkel 1987;Seid and Traniello 2006;Tschinkel 2011;Kwapich andTschinkel 2013, 2016;Charbonneau et al 2017). Although the proportion of workers engaging in tandem running (i.e., primary ants) seems relatively stable (Richardson et al 2018;Pratt et al 2005Pratt et al , 2002, it remains to be shown that this is still true for each behavioral caste we identified.…”

Observation of plugging behaviour reveals entrance‐guarding schedule of morphologically specialized caste in Colobopsis nipponicus

“…Second, inactive majors may function as a "reserve" force. An empirical study on monomorphic ants demonstrated that inactive workers form a "reserve" force that becomes active when needed (Charbonneau et al, 2017). As entrance plugging is a vital task that significantly increases nest survival in C. nipponicus (Hasegawa, 1992), inactive majors may serve as "reserve" pluggers (backup) in case of the loss of the current "active" pluggers (Powell, 2016).…”

Ant nurse workers exhibit behavioral and transcriptomic signatures of specialization on larval stage

“…We wanted to track nurses for at least ten days because this time scale includes the entire amount of time that M. pharaonis workers tend to perform nursing behavior [14]. To track nurses over time, we anesthetized individuals with carbon dioxide and marked heads and abdomens of workers with a dot of paint using Sharpie extra-fine point, oil-based paint pens [6,22,48]. In each of five colonies, we uniquely painted 63 focal individuals with paint dots on their heads and abdomens using combinations of eight colors.…”

“…Worker variation in behavioral specialization can also occur independently of age and morphology [15,16]. This interindividual variability can be the result of genetic diversity among workers [17], environmental differences during early development [18,19], variation in adult nutritional state [20][21][22], prior experience [23], and the social environment [24].…”