Head et al. reply

Replying to: J. M. K. Sniderman , 10.1038/nature08222 (2009) ; A. M. Makarieva, V. G. Gorshkov & B.-L. Li , 10.1038/nature08223 (2009) ; M. W. Denny, B. L. Lockwood & G. N. Somero , 10.1038/nature08224 (2009) Denny 1 and Sniderman 2 question our use of body size in Titanoboa cerrejonensis as an equatorial temperature proxy during the Palaeocene3, and Makarieva 4 suggest an adjustment to our temperature estimates. Denny et al.1 misinterpret the physiological model of our study5,6 and the implications of their body temperature (Tb) estimates relative to the thermal ecology of extant snakes. They assert that our model is inappropriate for large-bodied animals because the relationship between Tb and air temperature (mean annual temperature; MAT3) is not constant across different body sizes in poikilotherms. In fact, the model does not assume constancy of Tb relative to MAT. Changes in Tb (and thus body size) relative to MAT resulting from increasing thermal inertia with size are inherent in the model, as evidenced by the scaling of body length and MAT (see Fig. 3 in ref. 3). The model is accurate for the longest extant snake, Python reticulatus6, with a maximum body length ∼70% that of Titanoboa3, indicating that it is appropriate to use with animals approximating the sizes of the largest known snakes.