We tested some predictions of parental investment theory by studying t
he aggressive behaviour of colonial nesting chinstrap penguins (Pygosc
elis antarctica) against human intruders into their nesting territorie
s. We tested for differences in the aggressive behaviour of penguins a
ccording to offspring age (eggs vs. chicks), offspring number, nest lo
cation in the colonies (central vs. peripheral) and sex. Offspring age
was the main factor influencing nest defence, although nest location
and sex were also important. Chicks were defended more strongly than e
ggs, in accordance with changes in the reproductive value of offspring
, and this increase in aggressiveness was not related to revisitation
of the same individuals. The level of aggression of penguins breeding
in central sites was higher than that of peripheral birds, a differenc
e that could be due to the lower residual reproductive value of centra
l-nesting, probably older, birds. The stronger aggressiveness of males
could be due to a combination of factors related to sexual selection
and life-history traits. Offspring number did not affect the level of
nest defence.