Assessing the Pelvic Canal and Iliac Flare as Indicators of Gut Size

Paleoanthropologists hypothesize that the iliac flare of the pelvis reflects the size of the digestive tract ("gut"). However, gut size might be related to the pelvic canal because the bony canal anchors the pelvic floor muscles that support the abdominal weight. Here, we test if canal size or iliac flare can be used to infer gut size variation.

We used 3D landmarks to measure iliac flare, midplane, and outlet dimensions. We used CT volumetry to determine gut volume in living humans (N = 83) and multiple regression to assess the relationship of gut volume (GV) with the pelvis and body weight (BW).

In our female sample, we found (1) there is a small but significant relationship between cranial iliac flare, BW, and GV (r² = 0.20, p < 0.01) and (2) between the anteroposterior dimension of the canal outlet, BW, and GV (r² = 0.17, p < 0.01). In our male sample, we found (1) a relationship between all flare metrics, BW, and GV (r² = 0.45-0.48, p < 0.001) and (2) a relationship between the mediolateral dimensions of the outlet, BW, and GV (r² = 0.56, p < 0.001).

We conclude that neither iliac flare nor the pelvic canal size is reliable for quantitative estimates of gut volume, but cranial flare and outlet dimensions have weak associations with gut size if sex is known. We should be mindful of how sex influences the gut-pelvis relationship when considering the gut size of fossil hominins.