Division of labor

The central theme in our work on division of labor is understanding the degree of individual variation in cooperative groups that is adaptive, and dissecting the processes that make it so. Counterintuitively, several of our studies on bumble bees and Temnothorax ants have found that conventional explanations, such as higher individual efficiency of specialists, don’t hold, and that specialization is generally much lower than often portrayed for social insects.
Interestingly, other than the classic ‘division of labor to increase work output’ as proposed by Adam Smith (for humans), we have proposed other processes, both adaptive and non-adaptive, that may lead to division of labor: in particular, the interaction of comparative advantage effects and cost of worker production, and the possibility that ‘unemployed’ workers are actually crucial for effective task allocation. We also demonstrated that inactive workers can make up a high proportion of colony workforce, and we discuss additional explanations, such as fast-fluctuating task demands, traffic congestion, and low worker quality (perhaps because of immaturity).

size variation

Bumble bee workers show extreme size variation within colonies. We showed that this is not a lab artefact, and that it is specific to workers (not queens or males), suggesting it is not a necessary result of a disorganized nest, for example (see Spatial Pattern below). The evolutionary benefit of such size variation however is unclear. Smaller workers appear to store more fat, and this may drive longer survival in some conditions, but generally large workers perform better (and live longer). Incidentally we show that under unpredictable food availability, workers generally store more fat.

Several manuscripts on this topic are in review - these suggest that small workers may be cheap, low quality workers for tasks where high quality workers are not worth their cost.

Kelemen EP, Skyrm K, Dornhaus A 2022, ‘Selection on size variation: more variation in bumble bee workers and in the wild’, Insectes sociaux 69: 93-98 - pdf - field-raised bumble bee colonies produce the same variation in worker body size as lab-raised ones; and variation is much higher in workers than males or queens, suggesting different selection on body size consistency

Couvillon MJ, Jandt J, Duong N, Dornhaus A 2010 ‘Ontogeny of worker body size distribution in bumble bee (Bombus impatiens) colonies’, Ecological Entomology 35: 424-435 - pdf - with the exception of the very first worker generations (not studied here), bumble bee colonies produce the same degree of variation in worker body size throughout ontogeny, and produce varying body sizes even in simultaneously raised workers (contradicting the idea that standing size variation may reflect change in size across generations)

Couvillon MJ, Dornhaus A 2010 ‘Small worker bumble bees (Bombus impatiens) are hardier against starvation than their larger sisters', Insectes sociaux 57: 193-197 - pdf - when deprived of food in the lab, larger workers died earlier than smaller ones (support for the idea that perhaps there is a performance-robustness tradeoff across worker sizes)

Couvillon MJ, Jandt J, Bonds J, Helm BR, Dornhaus A 2011 ‘Percent lipid is associated with body size but not task in the bumble bee Bombus impatiens’, Journal of Comparative Physiology A 197: 1097-1104 - pdf - smaller workers store more fats; larger workers more often forage and smaller ones more often nurse, and ‘nursing’ in social insects is often associated with lipid storage (and foraging with leanness) - but here we found that size, not task, was the better explanation for differences in lipid storage

Jandt J, Dornhaus A 2014 'Bumblebee response thresholds and body size: does worker diversity increase colony performance?', Animal Behavior 87:97-106 - pdf - worker variation did not increase colony performance; instead, average worker preference for a task seems to predict task performance; one of two colony level manipulation studies from our lab showing this same lack of benefit of division of labor (the other is Kelemen et al. 2020, below)

Kelemen EP, Davidowitz G, Dornhaus A, 2020, ‘Size variation does not act as insurance in bumble bees; instead, workers add weight in an unpredictable environment’, Animal Behaviour 170, 99-109 - pdf - found no increased performance at whole-colony level of worker variation under different feeding regimes; unclear whether the result from Couvillon & Dornhaus 2010 was replicated here, but did find that lipid storage plastically responded to predictability of food.

Couvillon MJ, Fitzpatrick G, Dornhaus A 2010 ‘Ambient air temperature does not predict whether small or large workers forage in bumble bees (Bombus impatiens)', Psyche 2010, doi:10.1155/2010/536430 - pdf - tested the hypothesis that larger bees may be more prone to overheating (16deg C vs 36deg C) but did not find this to be the case.

Westling JN, Harrington K, Bengston S, Dornhaus A 2014 ‘Morphological differences between extranidal and intranidal workers in the ant Temnothorax rugatulus, but no effect of body size on foraging distance’, Insectes sociaux 61: 367-369 - pdf - although Temnothorax are often considered ‘monomorphic’, we show they do exhibit quite a bit of size variation among workers, and this may be associated with task. However, the latter did not replicate in Charbonneau et al. 2017, Integrative and Comparative Biol.

see also Jandt & Dornhaus 2009 for first comprehensive analysis of bumble bee division of labor

Inactive workers

It has long been known that many workers in social insect colonies appear ‘inactive’; we demonstrate thoroughly that some individuals are consistently inactive, in field and lab, and across contexts.

Why? Inactive workers may be too immature to work effectively, or young workers may serve as food storage or be more likely to be selfish.
Inactive workers, as a group, can also function as a ‘reserve force’, replacing active workers on demand, as previously hypothesized; however evidence for this is mixed.

Dornhaus A, Holley J-A, Pook VG, Worswick G, Franks NR 2008 'Why do not all workers work? Colony size and workload during emigrations in the ant Temnothorax albipennis', Behavioral Ecology and Sociobiology 63: 43-51 - pdf - some workers work a lot, many only a little (often more than half do not participate in emigrations at all); and this is more pronounced in smaller colonies (perhaps contrary to intuition; this result is correlative); quorum thresholds in emigration seem to be set relative to colony size

Charbonneau D, Dornhaus A 2015 ‘Workers ‘specialized’ on inactivity: Behavioral consistency of inactive workers and their role in task allocation’, Behavioral Ecology and Sociobiology 69: 1459-1472 - pdf - inactivity is the behavioral state that most differentiates workers; differs between workers and colonies and negatively correlates with all other tasks; circadian rhythm does not explain this

Charbonneau D, Hillis N, Dornhaus A 2015 ‘‘Lazy’ in nature: ant colony time budgets show high ‘inactivity’ in the field as well as in the lab’, Insectes sociaux 62: 31-35 - pdf - time budgets in field colonies do not differ from those in lab colonies [paper of the year in Ins soc]

Charbonneau D, Dornhaus A 2015 ‘When doing nothing is something. How task allocation strategies compromise between flexibility, efficiency, and inactive agents’, Journal of Bioeconomics 17: 217-242 - pdf - conceptual review of possible adaptive explanations for large numbers of inactive workers, particularly that of fast-fluctuating workloads

Charbonneau D, Poff C, Nguyen H, Shin MC, Kierstead K and Dornhaus A, 2017. 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, Integrative and Comparative Biology, 57(3), pp.649-667 - pdf - systematic test of the predictions of 6 hypotheses for why there are so many inactive workers in ant colonies is consistent with the idea that inactive workers may be young, possibly too immature or corpulent to work effectively

Charbonneau D, Sasaki T and Dornhaus A, 2017. Who needs ‘lazy’ workers? Inactive workers act as a ‘reserve’ labor force replacing active workers, but inactive workers are not replaced when they are removed, PloS one, 12(9): e0184074 - pdf - empirical demonstration of the often proposed idea that inactive workers are a reserve force that can recover colony function if previously active workers are removed; inactive workers are not replaced when removed, indicating that they did not perform some hidden function while ‘inactive’

Jandt JM, Robins NS, Moore RE, Dornhaus A 2012 ‘Individual bumblebees vary in response to disturbance: a test of the defensive reserve hypothesis’, Insectes sociaux 59: 313-321 - pdf - inactive bumble bee workers are not particularly likely to respond to disturbance directly or by increasing guarding behavior afterwards; thus they do not seem to act as ‘reserve’, at least for defense

see also Pinter-Wollman et al. 2012 on the reserve hypothesis