Allonursing, the nursing of nonfilial offspring, has been reported in a number of mammalian species; however, very few studies have investigated more than three hypotheses. The aim of our study was to investigate seven hypotheses explaining allonursing in captive giraffes. During 2007-2011, we observed 24 females and 37 calves in four zoological gardens in the Czech Republic, recording 2514 suckling events. We found that 83% of the females allonursed a nonfilial calf and 86.5% of calves allosuckled from a nonmaternal female; thus giraffes represent one of the highest occurrences of allonursing among nondomesticated mammals. The nonfilial calves more often allosuckled together with the filial ones than alone and tried to adopt positions where they may be harder to recognize, providing evidence for the milk theft hypothesis. In addition, the probability that a calf successfully allosuckled at least once from any female was higher when its mother allonursed successfully at least once than when she rejected calves. However, we found no evidence for the same rate of allonursing between reciprocal dyads of females and calves. Thus, we suggest that allonursing in giraffes is caused by offspring trying to steal milk and that females may tolerate this behaviour if reciprocal, in line with recent findings about giraffe sociality.