THE tree-shrews (family Tupaiidae), small squirrel-like mammals occurring throughout South-east Asia, are currently classified1 into two subfamilies: Ptilocercinae (nocturnal pen-tailed tree-shrews; genus Ptilocercus) and Tupaiinae (diurnal tree-shrews; genera Tupaia, Anathana, Dendrogale, Lyongale, Urogale). The overall systematic status of the family is still controversial. The tree-shrews are variously allocated to the Primates2–3, Insectivora or to an entirely separate order of mammals4–6. Recently the unusual maternal behaviour of one tree-shrew species, Tupaia belangeri, together with other reproductive characteristics, has been interpreted as further evidence excluding tree-shrews from the Primates1. Key features of maternal behaviour in captivity are that the mother deposits her young in a separate nest subsequently visited for suckling only once every 48 h. If such behaviour and its functional correlates are typical of all tree-shrew species, arguments based on reproductive peculiarities could be more confidently integrated with considerations of methodology, general morphological evidence7, detailed information on the central nervous system8 and the results of biochemical investigations9 to reach a balanced assessment of the phylogenetic relationships of tree-shrews.