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In most species.. females invest more in their offspring than males. In mammals, this also holds true after the birth of young since only mothers can lactate and fathers are thus more likely to desert and seek additional mating opportunities. These opportunities are largely dependent on resource distribution and the resulting distribution of females. The emerging social and mating systems of different species give rise to different benefits of investment in paternal care. Paternal care should have evolved in relation to the frequency of losing paternities; to other males. Hence, in species living in one-male systems with little opportunity for female extra-pair copulations, paternal care should be more common than in those species living in multi-male groups. The guinea pies (Caviinae) comprise closely related species exhibiting different social organizations and mating systems. The amount of paternal behaviour and offspring-directed aggression was investigated in three species kept under laboratory conditions: the polygynous Cavia aperea, the promiscuous Galea musteloides and a newly described monogamous species, (Galea monasteriensis sp. nov). All animals were kept in groups of one adult male and one adult female with their nonweaned offspring. Cavia aperea as well as G. monasteriensis males showed a high incidence of paternal behaviour. which consisted primarily of playing with offspring and grooming them. In contrast. G. musteloides males never engaged in paternal behaviour but directed more aggressive behaviour towards their offspring. Mating with multiple partners and multiple paternities are common in G. musteloides whereas in the former two species there is a much higher certainty of paternity. We conclude that paternal behaviour in guinea pigs probably co-evolved in alignment with the certainty of paternity that males of a species encounter under natural conditions.