Compact Bone Changes in Cold-exposed Rats ALPHONSE RIESENFELD D e p a r t m e n t of Anthropology, Hunter College of the C i t y University of N e w Y o r k , nnd i n s t i t u t e of Applied Biology, N e w Yor k KEY WORDS Compact bone reduction . R a t s . Experiment . Cold. ABSTRACT Rats exposed to cold stress show a reduction of relative cortical thickness i n their long bones. T h e reaction resembles reduction of cortical thickness observed i n Asiatic h u m a n s of t h e Mongoloid race, in whom many morphological features have been postulated to be cold-adaptive. However, until it c a n be established that the physiological processes underlying the experimental a n d phylogenetic condition a r e similar, n o extrapolation c a n be attempted from the one to the other. In a paper dealing with compact bone in Chinese and Japanese, Garn et al. ('64; Garn, '71) found that these Asiatics whether American-born or born abroad showed lesser amounts of compact bone than Americans of European descent. Since the differences appear even under conditions of adequate calcium intake, the authors attribute the difference to genetic factors rather than to limiting nutrients. Ericksen ('72), who studied long bones in Alaskan Eskimos and Pueblo Indians, found that cortical bone was lost with age in both populations with some differences according to sex and specific long bone involved. It is common knowledge that a number of morphological features in Mongoloids have been interpreted as cold-adaptive in nature, and the validity of these interpretations has been strengthened by the fact that many of these cold responses could be duplicated in experimental cold-exposure of rats (Steegmann and Platner, '68; Riesenfeld, '73). The present analysis was undertaken to determine possible changes of compacta thickness in experimentally cold-stressed rats. MATERIALS AND METHODS The right and left femora of normal control rats of the Wistar variety (30d and 300) and of cold-exposed Wistar rats ( 2 5 d and 300) were used. All animals were fed the same Purina rat diet and all were six months old at the time of sacrifice. Temperature condition of cold-exposed rats was 38 "F 2 average humidity was 65% , and all conditions of housing were the same a s * O , AM. J. PHYS. ANTHROP..44: 111-112 those described in detail in my previous study (Riesenfeld, '73). The femur was selected because of its large size. The bones were sectioned exactly at the midpoint of their shaft. The thickness of compacta was measured with specially constructed Helios dial calipers with horizontally opposing points and the antero-posterior diameter of the shaft at midpoint was measured with regular Helios dial calipers. All measurements were accurate to 0.01 mm. Both absolute measurements are reported a s well as the indices relating cortical thickness to midpoint diameter. RESULTS Table 1 indicates that cold females show a significantly lesser absolute thickness of the compacta and a significantly greater antero-posterior diameter of the shaft than normal controls. Males show the same trend but the absolute differences are not significant. When cortical thickness was related to total antero-posterior diameter of the shaft, that index for relative cortical thickness was significantly reduced in cold-exposed rats of both sexes, the reduction being more pronounced in females than in males. This observation is reminiscent of the finding of my previous study on the effect of cold exposure (Riesenfeld, '73) according to which a number of responses to experimental cold stress were considerably more pronounced in female than in male rats. The exact physiological etiology of cortical reduction under experimental cold stress and in the phylogenetic situation is not known 111 112 ALPHONSE RIESENFELD TABLE 1 Meitn, stctndard devicctions crnd t-tests of femoru of normctl cind cold-exposed ruts Normal Cortical thickness Mean S.D. 0.70 0.63 0.08 0.08 0.67 0.57 0.10 0.08 1 .89 4.78 2 3.02 2.79 0.33 0.19 3.09 2.89 0.25 0.18 1.05 2.75 1 8 23.48 3.11 2.97 21.55 19.61 2.16 2.38 3.16 1 6.152 8 0 A-P d i a m e t e r d P 100 X (Thicknessldiameter) 9 22.82 I 2 Cold S.D. Mean t p c 0.01, p s 0.001, at present and neither is its adaptive advantage in the phylogenetic condition. Until it can be established that the underlying physiological processes are similar in both, no extrapolation can be attempted from the experimental to the phylogenetic situation. LITERATURE CITED Erickseii. R1. t'. 1972 A c o m p a r a t i v e study o f c o r tical bone involution in t w o aboriginal Amer- ican populations ( a b s t r a c t ) . Am. J Phys. Anthrop.. 37: 436. C;;irn. S. M . 1971 H u m a n Kaces. Charles C T h o m a s . Publisher. Springfield. 111. C a m . S. M.. E. M . Pao a n d M. E . Rihl 1964 Compact bone in Chinese and J a p a n e s e . Science. 1 4 3 : 1439-1440. Kicsc~rifeld.A 1973 T h e effect of e x t r e m e tempera t u r e s and starvation on t h e body proportions of t h e r a t . Am. J . Phys. Anthrop.. 39: 427460. S t e e g m a n n . A . T.. a n d W . S. P l a t n e r 1968 Experi m e n t a l cold modification of cranio-faclal morphology. Am. J . Phys. Anthrop.. 2 8 . 17-30.