Body Size‐Prey Relationships in Insectivorous Marsupials: Tests of Three Hypotheses

Abstract

There is a strong positive correlation between the body sizes of dasyurid marsupials and the mean sizes of the invertebrate prey. This study tests three hypotheses proposed to explain this relationship, using 21 species of dasyurids (body mass range: 5–200 g) throughout continental Australia: (1) the maximum prey size that can be physically handled increases with dasyurid size due to the restricted gapes of forces of biting of smaller dasyurids; (2) the prey size that maximizes rate of energy intake varies with dasyurid size, with animals preferentially consuming the most profitable prey; and (3) prey sizes encountered during foraging vary with dasyurid size. Field observations of dasyurids were made at 20 study areas between January 1980 and February 1991. Most dasyurids were live—trapped, weighed, and sexed before being released at the trap site. We then tracked each animal and sampled potential prey in their foraging microhabitats. Dasyurids diets were examined using scats analysis. We measured rate of energy intake for captive dasyurids by offering different—sized cockroaches, measuring processing times, and calculating the energy content of cockroaches used. The first hypothesis was rejected. Both large and small dasyurids could grasp and pierce the largest prey offered in the laboratory, which were similar in size to the largest prey potentially available in the field. The second and third hypotheses were supported. Individuals of the smaller species maximized rates of energy gain in captivity by feeding preferentially on small prey, and their foraging trails in the field traversed microhabitats where mean potential prey size encountered was small (2–4 mm long). In contrast, larger dasyurids preferred large prey, obtained a greater rate of energy gain from them, and foraged in microhabitats where prey lengths averaged @>6 mm. There was a tendency for larger dasyurids to include progressively larger prey in their diets than were available, on average, along their foraging trails. This may reflect increasing selectivity for larger prey. However, it may also reflect a reduced susceptibility to predation on increased competitive ability for large dasyurids in productive microhabitats, and hence reflect a size—based advantage in encountering large prey. These results indicate that net energy yield and the use of microhabitats with different prey sizes are the most important determinants of the body size—prey size relationship in dasyurid marsupials.