Are you strong enough to carry that seed? Seed size/body size ratios influence seed choices by rodents

Granivorous rodents prefer large seeds because they offer a higher nutrient content, but such preferences may be counterbalanced by the higher costs of handling and transporting larger seeds. We hypothesized that this compromise underlying foraging decisions depends simultaneously on the size of both seeds and rodents. A given seed offers a fixed amount of resources, but the costs of handling and transporting that seed could increase as the rodent size decreases. We designed a laboratory experiment to assess explicitly whether seed choices depend on the size of both seeds and rodents. As a model, we used two different-sized species, the Algerian mouse Mus spretus (8–15 g) and the wood mouse Apodemus sylvaticus (18–40 g), that face the decision of choosing Holm oak (Quercus ilex) acorns from 0.5 to 15 g under natural conditions. The probability of seed removal decreased as the ratio of seed size to rodent size increased in both rodent species. Seeds heavier than 60% of the rodent's mass led to substantial decreases in acorn removal and seeds heavier than 70% of the rodent's mass were in no case removed. This critical value limited the choice of the largest seeds by the smaller species, but not by the larger one. Therefore, the larger species selected larger acorns and with a wider variance in size than the smaller species. Our results show the importance of considering jointly the sizes of seeds and rodents in the study of foraging decisions and seed dispersal behaviour by granivorous rodents.