Regulation of bone mineral density in the grey squirrel, Sciurus carolinensis: Bioavailability of calcium oxalate, and implications for bark stripping

Abstract

The damage caused when grey squirrels strip the outer bark off trees and ingest the underlying phloem can result in reduced timber quality or tree death. This is extremely costly to the UK forestry industry and can alter woodland composition, hampering conservation efforts. The calcium hypothesis proposes that grey squirrels ingest phloem to ameliorate a seasonal calcium deficiency. Calcium in the phloem predominantly takes the form of calcium oxalate (CaOx), however not all mammals can utilise CaOx as a source of calcium. Here, we present the results of a small-scale study to determine the extent to which grey squirrels can utilise CaOx. One of three custom-made diets containing calcium in varying forms and quantities (CaOx diet, Low-calcium carbonate (CaCO₃ ) diet and Control diet) were fed to three treatment groups of six squirrels for 8 weeks. Bone densitometric properties were measured at the end of this time using peripheral quantitative computed tomography and micro-computed tomography. Pyridinoline-a serum marker of bone resorption-was measured regularly throughout the study. Bone mineral density and cortical mineralisation were lower in squirrels fed the CaOx diet compared to the Control group but similar to that of those on the Low-calcium diet, suggesting that calcium from calcium oxalate was not effectively utilised to maintain bone mineralisation. Whilst no differences were observed in serum pyridinoline levels between individuals on different diets, females had on average higher levels than males throughout the study. Future work should seek to determine if this apparent lack of ability to utilise CaOx is common to a large sample of grey squirrels and if so, whether it is consistent across all areas and seasons.

Keywords: CaOx; low-calcium diet; micro-CT; pQCT; pyridinoline; tree damage.

Figure 1

Significant differences in distal femoral bone densitometric properties between cohorts of grey squirrels fed one of three treatment diets: CaOx diet (n = 6), Control diet (n = 6) or Low‐Ca diet (n = 6). (a) Total bone mineral density measured by pQCT (p = .0182, df = 15); (b) Mean total cortical cross‐sectional tissue perimeter measured by μCT. NS – p => .05, *p =< .05, **p = <.005

Figure 2

Average serum pyridinoline (PYD) levels in grey squirrels were significantly higher in females than males over the 10 weeks of the study. (Female n = 8, male n = 10). Bars indicate standard error of the mean. NS – p => .05, *p  =  <.05