Effects of selection for high voluntary wheel running on sexual dimorphism in the sacrum of Mus musculus

Abstract

Comparative studies have shown that pelvic girdle (os coxae and sacrum) morphology may be influenced by sex and/or locomotion and that a possible evolutionary trade‐off exists between reproductive constraints and locomotor activity. Previous unpublished work explored how sexual dimorphism in the os coxae responds to locomotor activity in Mus musculus through analyses of mice from an artificial selection experiment that bred mice for high voluntary wheel running. Three of the eight lines from this experiment were sampled, representing control (C), selected (HRNORMAL), and selected mini‐muscle (HRMINI) groups, with the mini‐muscle individuals possessing a Mendelian recessive mutation that causes a 50% decrease in skeletal muscle mass. Based on prior comparative work on the pelvis and the specific results from the os coxae of these mice showing differing patterns of dimorphism in shape, density and bone volume, we hypothesized that a similar pattern would be present in the sacrum. Using 122 specimens from the aforementioned experiment, we obtained shape and size (mass and centroid size) metrics of the sacra and compiled these with previously recorded body masses and lengths. ANOVAs of size metrics showed significant differences in body length, mass and sacral centroid size between the lines and sexes. On all metrics, both HR lines were smaller than the C line and the HRMINI line was smaller than the HRNORMAL line. In addition, females were smaller than males in all metrics. Finally, there was no significant sex‐by‐line interaction for either body size metrics or centroid size, indicating no differential sexual dimorphism patterns in these variables between lines.Results of MANOVAs showed that sex and line, but not body mass, had a significant effect on sacral shape, and there was no significant sex‐by‐line interaction. Further, we calculated a proxy measurement of sacral density using a ratio of sacral mass to sacral centroid size. ANOVAs showed a significant effect of sex, line, and the sex‐by‐line interaction, with females showing a greater mean bone density than males (a result opposite that found in a similar proxy metric in the os coxae). Overall, these results support the hypothesis that sacral morphology is influenced by sex, selection for high locomotor activity, and the presence of the mini‐muscle mutation in a complex manner, which affects specific skeletal elements in different ways.Support or Funding InformationMurdock Charitable Trust