In the midwestern United States, grassland habitat is fragmented by row-crop agriculture and urbanization. In spring and early summer, grassland animals facing a decision to either enter a fallow crop field or detour around it likely balance trade-offs between predation risk and travel costs, similar to forest animals encountering a clear-cut. We tested if Franklin's ground squirrel (Poliocitellus franklinii), a grassland mammal, based gap-crossing decisions on lengths of alternate movement routes or on energetic constraints by translocating radiocollared adult squirrels across crop fields and tracking their routes home. Giving-up densities of food resources from a field experiment indicated that Franklin's ground squirrels perceived a higher risk of predation in crop fields than in grass, but squirrels did not appear to compensate for risk by adjusting travel speed through crop fields. Body mass was the only predictor of gap crossing; lighter squirrels were more likely than heavier squirrels to cross crop fields. Squirrels did not appear to base gap-crossing decisions on detour efficiency (distance across gap divided by distance of detour). Additionally, likelihood of homing within 24 h of release decreased with increasing crossing distance. The importance of body mass in gap-crossing decisions by Franklin's ground squirrels suggests that movements through heterogeneous landscapes are partly explained by trade-offs between predation risk and energetic constraints.
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Vol. 93 • No. 5