Parasite life-history traits should reflect past environmental
and host-related selective pressures acting to produce
strategies that maximize transmission success. The evolution of adult body
size and egg production in 804 species of
trematode parasites was investigated using independent contrasts derived
from
a phylogeny of trematodes. Contrasts in
trematode body size were positively correlated with contrasts in egg size,
and almost significantly correlated with contrasts
in numbers of uterine eggs. After controlling for body size, no relationship
existed between egg size and egg numbers,
suggesting that there is no trade-off between the two components of egg
production. Average host body mass and latitude
of the sampling site did not correlate with either trematode body size
or
egg size. Contrasts between trematode taxa
exploiting ectotherm hosts and their sister taxa exploiting endotherms
showed no consistent difference in either body size
or egg size. The effect of other variables on trematode life-histories,
such as the nature of the habitat in which eggs are
released, the site of attachment within the host's body, or the number
of
hosts involved in the life-cycle, could not be
evaluated statistically. The similarity in life-history traits among
members of given clades suggests that phylogenetic
constraints may have acted to limit or mask any adaptive changes expected
from changes in host-related or environmental conditions.