The Coevolution of Pine Squirrels (Tamiasciurus) and Conifers

Field observations of discriminatory feeding behavior of squirrels indicate that in the process of maximizing their own feeding efficiency squirrels act as selective agents on several characteristics of plant reproduction. Calorimetric measurements of the energy content of plant reproductive organs confirm the evolutionary effects of discriminatory feeding by squirrels. The reproductive characteristics of conifers which are affected include several details of cone anatomy, the number of seeds per cone, the time at which cones shed their seeds, the thickness of seed coats, and annual fluctuations in the size of cone crops. A prolonged evolutionary interaction between rodent—like, arboreal mammals and the reproductive organs of the modern genera in the family Pinaceae is consistent with the fossil record. A general analysis of selective pressure influencing terrestrial plant reproduction indicates that seed eaters could have a marked influence on the evolution of plant reproduction. Among the interrelated selective pressures that influence the coevolution of conifers and squirrels, the physical environment is the independent variable. The effect of one predator feeding on two species of prey is the divergent evolution of defense mechanisms in the two prey species. However, one species of predator can cause convergent evolution in the pattern of variation in the production of prey, which may also be considered a defense mechanism.