Feeding ecology and diet

Most of the information available on sigmodontine diet comes from analysis of stomach contents (in which, usually a small number of individuals were studied) and not from field observation or experimental studies. As expected from such a diverse group, sigmodontines show a large range of diets. Sigmodontines are omnivorous (e.g., Zygodontomys brevicauda), grassivorous (e.g., Reithrodon typicus), primarily frugivorous (e.g., Thomasomys aureus), primarily granivorous (e.g., Elig-modontia typus), and primarily animalivorous (e.g., Oxymycterus nasutus, Rheomys mexicanus); still others consume large amounts of fungus (e.g., Chelemys macronyx).

Mice and rats of the ichthyomyine tribe are among the most distinctive sigmodontines with regard to their diet because they feed on aquatic organisms (Voss, 1988). In much of the scientific literature ichthyomyines are referred to as the fish-eating sigmodontines. However, though ichthyomyines feed on fishes, they represent a relatively uncommon item of the ichthyomyine diet. Ichthyomyines primarily feed on arthropods, of which the vast majority is part of the stream benthic fauna, including crabs, amphipods, and several insect orders (larvae, ninphae, and adults). Other less common ichthyomyine food items are tadpoles and salamanders. Ichthy-omys exhibits feeding plasticity: in the lowlands, crabs are an important food item, while in the highlands, where crabs are absent, it may be largely insectivorous.

After entering the water Ichthyomys pittieri uses its whiskers to explore the substrate, searching for, and identifying live prey. The rat attacks its prey when they move after being touched. Small prey (up to 0.8 in long; 20 cm) are seized with the forepaws and are usually eaten immediately, while sitting on its hindquarters in the shallow water. Larger prey are pinned to the bottom with forepaws, bitten repeatedly until moribund, and then carried out of the water where they are consumed.

The feeding behavior of the omnivorous water rat Necto-mys squamipes was studied by observations of captive animals (Ernest and Mares, 1986). The rat picks up immobile items (e.g., leaves) with the incisors, then sits back on its haunches and holds the food in the forepaws, to bite and chew the food. Mobile items (e.g., cockroaches, Blattidae) are caught on the ground by jumping on it with the forepaws. Flying insects are caught close to the ground and eaten head first, and entirely. The water rat takes mobile prey in the water (e.g., tadpoles) with the forepaws. After the prey is secured, it is eaten like the immobile items.

Combining analyses of stomach contents and cafeteria tests, Castellarini and collaborators (1998) established that the vesper mouse Calomys venustus is omnivorous, and that it shows a tendency to folivory in spring and autumn and to granivory in summer. C. venustus does not consume the leaves that are abundant in the habitat, or consume them in low proportions, rather, it shows a high preference for seed consumption under conditions of equal food availability.

0 0

Post a comment