EVOLUTIONARY RESPONSES OF PLANTS TO SEED-EATERS: PINE SQUIRREL PREDATION ON LODGEPOLE PINE.

Recent papers by Smith (1970) and Janzen (1969, 1971) have emphasized different strategies by plants in their evolutionary responses to seed predators. Janzen, while working with insect seed predators, concluded that seed predators may select primarily for smaller seed size by killing a higher per cent o,f the seed crop of trees with fewer but larger seeds. Smith, on the other hand, studying squirrels, proposed that predators discriminate between tre,es on the basis of which tree will give them the highest feeding rate. The basis for this discriminatory behavior could be factors other than s,eed size, such as the production of chemical deterrents or protective tissue with the seed size remaining constant. The close correlation between se,ed size and the physical environment (Salisbury, 1942; Baker, 1972) and the evidence that seed size is one of the least variable of plant characters (Harper, 1961) indicates that selection for seed size is not likely to be one of the "lines o,f least resistance" (Stebbins, 1967) for seed predator selection to follow. Assuming, as Baker (1972) has shown, that certain plant communities exhibit an optimum seed size, it appears that if both seed size and effectiveness of seed protection are variable, selective pressure from seed predators will select for greater energy expenditures in se,ed protection with the seed size remaining constant (Smith, 1970). This selective trend would result in an individual plant producing fewer seeds due to the increased production of protective devices. It is quite possible that the differing theo,ries of Smith and Janzen are, to a large degree, reflective of the different predatorprey systems with which each worked. Likewise, it is possible that each prediction is correct in describing its respective system, and that neither can be expanded to a generalized theory. However, before such conclusions can b,e made, more information is needed in regard to identifying which plant characters most closely resemble predator independent factors, primarily controlled by the physical environment, and which characters are more likely to be predator dependent. The evolutionary responses of plants, to seed predators can be determined only by identifying the criteria by which predators discriminate, if they do, between varying prey items. This research is an effort at such a determination. The pine squirrel (Tamiasciurus hudsonicus) shows discrimination in its feeding behavior among the cones of lodgepole pines (Pinus contorta). This discrimination is based primarily on the number of seeds per cone (Hatt, 1929; Smith, 1968, 1970), which has been shown to vary by a factor of eight (Smith, 1968). Variation between trees has also been shown to, exist in the hardness of cones, the size of cones, and the symmetry or shape of cones (Critchfield, 1957), all of which have been observed to influence the feeding efficiency of Tamiasciurus (Smith, 1970). However, the degree of variation in seed size and its relationship to the effects of seed predators have not been previously studied. Thus, a critical aspect of this study centers around the idea that, if the degree of variation in the cone and seed characters is great enough to allow a squirrel to discriminate among cones, then an effective selective agent exists for determining cone and seed characteristics. Personal observations of squirrel feeding behavior during the summer of 1971 and