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Group-living and cooperative breeding in the mandarin vole Lasiopodomys mandarinus were proposed to be an adaptation (or preadaptation) to the fossorial mode of life (Smorkatcheva, 1999). This hypothesis involved indirect benefits that gain offspring by helping their mother in tunnel construction among the factors promoting cooperative breeding. Answer to the question “which animals do provide the most help?” may allow us to understand whether helpers derive direct or indirect benefits from their action. In this study we compared the contributions to nest-residence, digging, bringing objects, eating and the digging/eating rates between different sex-age categories. We observed groups composed of pair of breeders, 1-9 weaned offspring and unweaned pups in artificial tunnel systems. Only daughters older 35 days participated extensively (along with their fathers) in transport and burrow construction. Overall, the digging/eating rate was greater in daughters than in mothers. This provides evidence that young females perform some excess workload to be used up, potentially, by reproductive female. Sons, independently of their age, and daughters under 35 days were engaged very little (lesser than other members of families) in burrow construction and transport. Sons under 60 days, daughter older 60 days and fathers were the major baby-sitters. Sex differences in degree of the most expensive activities are inconsistent with the kin-selection hypothesis, but can be explained in the framework of delayed reciprocity or group augmentation hypothesis. Another (non-alternative) explanation of the revealed sex-bias is that burrow construction would be parental, not only alloparental, investment for daughter should it attain a breeding position at natal territory. KEY WORDS: Lasiopodomys mandarinus, subterranean rodent, cooperative breeding, sex-biased helping.
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