Newborn kittens and piglets show some notable similarities in suckling-associated behaviours. Both develop a pattern of teat use in which each member of the litter predominantly occupies one or two particular nipples or teats, both show agonistic behaviour towards littermates during suckling, and both are born with ‘weapons’–kittens with sharp claws and piglets with sharp needle teeth. Traditionally, the differential use of nipples or teats has been thought to be due to differential quality of the mammary glands. However, neither our findings in kittens nor an examination of the literature on piglets support this view. As mammary glands usually are of equal quality and as there are normally more glands than young, there seems little reason for littermates to fight. Furthermore, there is also little evidence that individual performance in suckling contests or use of ‘weapons’ is related to weight gain or to dominance relations. Given the present lack of satisfying adaptive explanations for the seemingly aggressive behaviour and use of weapons by kittens and piglets, we suggest these to be epiphenomena of developmental processes anticipating the use of these behaviours and structures in later adaptive contexts.