1 Department of Sociobiology/Anthropology, Johann-Friedrich-Blumenbach Institute for Zoology und Anthropology, University of Göttingen, Göttingen, Lower Saxony, Germany.
2 Behavioral Ecology and Sociobiology Unit, German Primate Center, Leibniz Institute for Primate Research, Göttingen, Lower Saxony, Germany.
3 Department of Biology, Concordia University, Montreal, Quebec, Canada.
4 Department of Biology, Institute of Ecology and Evolution, University of Bern, Hinterkappelen, Bern, Switzerland.
5 Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, Ås, Viken, Norway.
6 Department of Preclinical Sciences and Pathology, Norwegian University of Life Sciences, Oslo, Oslo, Norway.
7 Natural Resources Institute Finland Luke, Reindeer Research Station, Kaamanen, Lapland, Finland.

Allonursing is the nursing of the offspring of other mothers. Cooperation is an emergent property of evolved decision rules. Cooperation can be explained by at least three evolved decision rules: 1) direct reciprocity, i.e. help someone who previously helped you, 2) kin discrimination, i.e. preferentially direct help to kin than to non-kin, and 3) generalized reciprocity, i.e. help anyone if helped by someone. We assessed if semi-domesticated reindeer, Rangifer tarandus, mothers allonursed according to the decision rules of direct reciprocity, generalized reciprocity and kin discrimination over 2 years. To assess if reindeer mothers allonursed according to the direct reciprocity decision rule, we predicted that mothers should give more help to those who previously helped them more often. To assess if reindeer mothers allonursed according to the kin discrimination decision rule, we predicted that help given should increase as pairwise genetic relatedness increased. To assess if reindeer mothers allonursed according to the generalized reciprocity decision rule, we predicted that the overall number of help given by reindeer mothers should increase as the overall number of help received by reindeer mothers increased. The number of help given i) increased as the number of help received from the same partner increased in the 2012 group but not in both 2013 groups, ii) was not influenced by relatedness, and iii) was not influenced by an interaction between the number of help received from the same partner and relatedness. iv) The overall number of help given increased as the overall number of help received increased. The results did not support the prediction that reindeer mothers allonursed according to the kin discrimination decision rule. The results suggest that reindeer mothers may allonurse according to the direct reciprocity and generalized reciprocity decision rules.