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Table 4 Summary of interpretation of adaptation strategies given shape, soil, and body size

From: Getting a head in hard soils: Convergent skull evolution and divergent allometric patterns explain shape variation in a highly diverse genus of pocket gophers (Thomomys)

Taxa Soil type occupied Digging strategy used Evolutionary comments
T (T.) mazama Soft sandy soil - low clay, bulk density, and linear extensibility; high sand make this relatively easy to dig in Ancestral claw-digging Likely illustrates the primitive ancestral condition except in lateral humeral shape.
T (T.) monticola Soft sandy soil - lowest clay, bulk density, and linear extensibility; highest sand in the genus Ancestral claw-digging Arguably the easiest soil in the region, likely preserves the primitive ancestral condition for the genus.
T (T.) talpoides fisherii Friable clay soil - medium clay but high sand and low bulk density suggests it is not very compacted Derived tooth-digging despite body size While still appears to tooth-dig, like sister subspecies below, may be shifting back towards claw-digging in sandier soils
T (T.) talpoides quadratus Hard clayey soil - high clay and very low sand, low bulk density suggests it is not very compacted Derived tooth-digging despite body size A combination of allometric and non-allometric cranial rearrangement appears to produce derived tooth-digging shape
T. (M.) b. canus (Townsendii clade) Heavy sandy soil - low clay and high sand suggests the latter drives high bulk density Claw digging despite body size Intermediate soil appears to have selected for more for claw-digging adaptations
T. (M.) townsendii Heavy sandy soil - more clay than sister species above but still high sand Tooth-digging via size-increase alone In contrast to sister species above, intermediate soil appears to have selected more for tooth-digging adaptations
T. (M.) bottae laticeps Friable clay soil - medium clay but high sand and low bulk density make it easier to manipulate Claw-digging despite body size Diverging from the rest of its clade, intermediate soil appears to have selected for claw-digging adaptations
T. (M.) bottae navus Friable clay soil - medium clay and medium bulk density but high sand make it easier to manipulate Derived tooth-digging In contrast to sister subspecies above, intermediate soil appears to have selected for tooth-digging adaptations
T. (M.) bottae leucodon High clay soil - highest clay and linear extensibility with low sand and low bulk density Derived tooth-digging Arguably the hardest soils in the region appear to have selected for both tooth- and claw-digging adaptations
T. (M.) bottae saxatilis High clay soil - medium clay and linear extensibility with low sand and high bulk density Derived tooth-digging In slight contrast to sister subspecies, hard soils appear to have selected for a slightly more procumbent tooth-digging shape & less emphasis on claw-digging
  1. Values for soil conditions that impact digging. Percent clay is the part of soil texture that confers plasticity, and in high amounts, makes soil difficult to manipulate. Percent sand is the heaviest part of soil texture, and in high amounts makes soil heavy but easy to break apart. Bulk density is an indicator of soil compaction calculated by the dry weight of soil divided by its volumeā€”it can have high values due to compacted clay, or to a high percent of sand, the heaviest component of soil texture. Linear extensibility, a property of certain kinds of clay, quantifies the shrink-swell capacity of soil. This property causes soils to harden when dry, warm climatic conditions reduce the effective moisture in the soil