Why don’t all termite species have a sterile worker caste?

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Abstract
No general theory explains why a sterile worker caste is not found in all species of both Hymenoptera and Isoptera (Insecta). Recent empirical findings show that, in the termites (Isoptera), feeding outside the nest correlates well with the evolution of the sterile (true) worker caste from the non-sterile (false) worker caste. Here we explain the connection between food--nest separation and true worker evolution in termites, providing a general theory on the restricted distribution of the sterile worker caste in the Isoptera. A cost--benefit model suggests that there is a critical level of nest stability above which natural selection favours true workers over false workers, irrespective of genetical relatedness. Because food--nest separation tends to increase nest stability, this theoretical result implies that the less a termite species consumes its nest as food, the more likely is its nest stability to fall above the critical level and a true worker caste will evolve.

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