Apparatus for measuring rat body volume: a methodological proposition

Abstract

We propose a communicating-vessels system to measure body volume in live rats through water level detection by hydrostatic weighing. The reproducibility, accuracy, linearity, and reliability of this apparatus were evaluated in two tests using previously weighed water or six aluminum cylinders of known volume after proper system calibration. The applicability of this apparatus to measurement of live animals (Wistar rats) was tested in a transversal experiment with five rats, anesthetized and nonanesthetized. We took 18 measurements of the volume under each condition (anesthetized and nonanesthetized), totaling 90 measurements. The addition of water volumes (50–700 ml) produced a regression equation with a slope of 1.0006 ± 0.0017, intercept of 0.75 ± 0.81 ( R2= 0.99999, standard error of estimate = 0.58 ml), and bias of ∼1 ml. The differences between cylinders of known volumes and volumes calculated by the system were <0.4 ml. Mean volume errors were 0.01–0.07%. Among the live models, the difference between the volumes obtained for anesthetized and nonanesthetized rats was 0.31 ± 2.34 (SD) ml ( n = 90). These data showed that animal movement does not interfere with the volume measured by the proposed apparatus, and neither anesthesia nor fur shaving is needed for this procedure. Nevertheless, some effort should be taken to eliminate air bubbles trapped in the apparatus or the fur. The proposed apparatus for measuring rat body volume is inexpensive and may be useful for a range of scientific purposes.