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Energetics reveals physiologically distinct castes in a eusocial mammal


Eusociality, which occurs among mammals only in two species of African mole-rat, is characterized by division of labour between morphologically distinct ‘castes’1. In Damaraland mole-rats (Cryptomys damarensis), colony labour is divided between ‘infrequent worker’ and ‘frequent worker’ castes2. Frequent workers are active year-round and together perform more than 95% of the total work of the colony, whereas infrequent workers typically perform less than 5% of the total work3. Anecdotal evidence suggests that infrequent workers may act as dispersers, with dispersal being limited to comparatively rare periods when the soil is softened by moisture4,5. Here we show that infrequent workers and queens increase their daily energy expenditure after rainfall whereas frequent workers do not. Infrequent workers are also fatter than frequent workers. We suggest that infrequent workers constitute a physiologically distinct dispersing caste, the members of which, instead of contributing to the work of the colony and helping the queen to reproduce, build up their own body reserves in preparation for dispersal and reproduction when environmental conditions are suitable.

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This research was funded by the National Research Foundation, Republic of South Africa (N.C.B.) and a University of Pretoria Post Doctoral Research Fellowship (M.S.). We thank S. van Cutsem and H. Hjarvard de Fine Licht for assistance in the field, and F. Dalerum and G. Gutjahr for comments on the manuscript. Experimental procedures and animal husbandry practices were approved by the Animal Ethics Committee, University of Pretoria. Author Contributions M.S., conception of research, fieldwork, organization of study site, data analysis and manuscript preparation; J.R.S., isotope analysis and contribution to manuscript preparation; M.K.O., fieldwork; T.J.R., conception of research and contribution to manuscript preparation; N.C.B., conception of research, fieldwork, organization of study site, acquisition of funding and contribution to manuscript preparation.

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Correspondence to M. Scantlebury.

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Figure 1: Resting metabolic rate and daily energy expenditure for C. damarensis.


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