RESUMEN
Allosuckling, the suckling of offspring from females other than their own mother, has been reported in a number of mammalian species, including reindeer. The causes and function of this behaviour are still being investigated. We monitored 25 doe-calf pairs of semi-domestic reindeer Rangifer tarandus over 10 weeks to test three allosuckling/allonursing hypotheses: (1) milk theft, calves opportunistically allosuckle; (2) mismothering, misdirected maternal care; and (3) kin-selected allonursing. A calf soliciting an allosuckling bout was categorized as non-filial (NF), and a calf soliciting a suckling bout from its mother was categorized as filial (F). We recorded 9757 solicitations, of which 5176 were successful F bouts and 1389 were successful NF bouts. The rejection rates were greater for NF than F calves. The proportions of antiparallel positions adopted were greater for F than NF calves. The odds of an allobout were lower for calves arriving 1st, 3rd, 4th, 5th or 6th than for those arriving 2nd, but the odds did not vary with position adopted and relatedness. Our results provided support to the milk-theft hypothesis, whereas limited support for the mismothering hypothesis was found. Our results did not support the hypothesized kin selection function of allosuckling in reindeer. This article is part of a Special Issue entitled: insert SI title.