While allonursing, the provision of milk to non-offspring by females, involves a potential cost to their own offspring, allosuckling, the suckling from females other than their own mother may allow offspring to compensate for previous deficiencies in maternal milk. We tested this hypothesis in farmed guanacos. Under the compensation hypothesis we predicted that mothers of calves exhibiting allosuckling should be in poorer physical condition and should exhibit relatively low acceptance rates to filial sucking attempts compared to mothers whose calf did not allosuckle. We also predicted that calves exhibiting frequent allosuckling should show similar or greater rates of gain in body weight. but similar total (or final) weight in the long term than calves that nursed from their mothers exclusively. We examined the potential effects of sex and order of birth dates of calves on allosuckling, and the effect of female Success during agonistic encounters with other females on allonursing. Two stable groups of 15 and 14 mother-offspring pairs of farmed guanacos were studied from birth to approximately 3 months of age. Allosuckling events comprised 5.7% of all suckling events. Allonursing was performed by 52% of dams and 62% of calves exhibited allosuckling. We found similar gain rates in body weight and total weight at 60 days of age between allosuckling calves and filial sucking calves, irrespective of whether their mothers allonursed or not (P > 0.1). Body weight of mothers whose calf allosuckled was significantly lower than that of mothers whose calves nursed from them exclusively (P = 0.02). In addition, the percentage of acceptance of filial suckling bouts was significantly lower for allosuckling calves (P = 0.004). There was no correlation between the frequency of allonursing and the success of dams during agonistic encounters (P > 0.22). Our findings are consistent with the hypothesis that guanaco calves used allosucking to compensate for previous deficiencies in maternal milk. (C) 2009 Elsevier B.V. All rights reserved.