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Body size as a latent variable in a structural equation model: thermal acclimation and energetics of the leaf-eared mouse

Roberto F. Nespolo^*, Matías Arim and Francisco Bozinovic

Centro de estudios avanzados en Ecología y Biodiversidad, Departamento de Ecología, Facultad Ciencias Biológicas, Pontificia Universidad Católica de Chile, PO Box 6513677, Santiago, Chile

View larger version (19K): [in a new window]   Fig. 1. Path diagram of the `indirect' model tested with measured data. BMR, basal metabolic rate; NST, non-shivering thermogenesis; MMR, maximum metabolic rate; C, thermal conductance; mbBN, body mass recorded at the moment of BMR and NST measurement; mbM, body mass recorded at the moment of MMR measurement; mbC, body mass recorded at the moment of C measurement; u, residual error. The broken line shows the additional paths from variables other than mb, which were considered as indicators of body size: Lf (foot length) and Lb (body length). These variables were measured only for cold-acclimated individuals (the `small' dataset). Latent variables are in circles and manifest variables in rectangles. The small diagram at the upper left identifies the path model in Table 3 (see Results).

View larger version (20K): [in a new window]   Fig. 2. Path diagram of the `direct–indirect' model tested with observed data. Symbols and broken lines as in Fig. 1.

View larger version (16K): [in a new window]   Fig. 3. Adjusted means (± 1 S.E.M., N=108) of basal metabolic rate (BMR), non-shivering thermogenesis (NST), maximum metabolic rate (MMR) and thermal conductance in cold- (15°C) and warm- (30°C) acclimated individuals. Significant differences between acclimation temperatures were found in all cases (ANCOVA, P<0.01; see Results).

View larger version (33K): [in a new window]   Fig. 4. Correlations between body mass and oxygen consumption parameters. (A) MMR, maximum metabolic rate; (B) NST, non-shivering thermogenesis; (C) BMR, basal metabolic rate; (D) C, thermal conductance. Squared correlation coefficients are shown for cold- (open symbols, top) and warm- (closed symbols, bottom) acclimated individuals in each case. ***, P<0.001.

View larger version (20K): [in a new window]   Fig. 5. Path coefficients adjusted for model 1, cold-acclimated animals (Table 3). Values are means ± 1 S.E.M. Asterisks denote significant coefficients (P<0.05). Latent variables are in circles; manifest variables are in rectangles. Adjustment statistics are from Table 3 (only non-significant models are presented). Abbreviations as in Fig. 1.

View larger version (28K): [in a new window]   Fig. 6. Path coefficients adjusted for model 2 (indirect effects of latent body size on mass), warm-acclimated individuals (see Table 3) Values are means ± 1 S.E.M. Asterisks denote significant coefficients (P<0.05). Latent variables are in circles; manifest variables are in rectangles. Adjustment statistics are from Table 3 (only non-significant models are presented). Abbreviations as in Fig. 1.

View larger version (28K): [in a new window]   Fig. 7. Path coefficients adjusted for model 2 (indirect effects of latent body size on mass), cold-acclimated individuals (see Table 3). Values are means ± 1 S.E.M. Asterisks denote significant coefficients (P<0.05). Latent variables are in circles; manifest variables are in rectangles. Adjustment statistics are from Table 3 (only non-significant models are presented). Abbreviations as in Fig. 1.

View larger version (25K): [in a new window]   Fig. 8. Path coefficients adjusted for model 1 (direct effects of latent body size on mass), cold-acclimated individuals including foot length (Lf) and body length (Lb) as proxies for body size (see Table 3). Values are means ± 1 S.E.M. Asterisks denote significant coefficients (P<0.05). Latent variables are in circles; manifest variables are in rectangles. Adjustment statistics are from Table 3 (only non-significant models are presented). Abbreviations as in Fig. 1.

View larger version (33K): [in a new window]   Fig. 9. Path coefficients adjusted for model 2 (indirect effects of latent body size on mass), cold-acclimated individuals including foot length (Lf) and body length (Lb) as proxies for body size (see Table 3). Values are means ± 1 S.E.M. Asterisks denote significant coefficients (P<0.05). Latent variables are in circles; manifest variables are in rectangles. Adjustment statistics are from Table 3 (only non-significant models are presented). Abbreviations as in Fig. 1.

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