Regulation of bone mineral density in the grey squirrel, Sciurus carolinensis
Many studies have attempted to determine what triggers the grey squirrel to damage trees (Kenward & Parish, 1986; Kenward et al., 1996), with a view to informing preventive approaches; however, the underlying causal mechanism is still unknown. It has been suggested that bark stripping represents an attempt to utilise inner bark as a source of carbohydrate when other food resources are lacking (Kenward, 1983; Kenward, Parish, Holm, & Harris, 1988; MacKinnon, 1976); however, bark stripping still occurs in semi‐wild enclosures when food is provided ad libitum, and bark is of low calorific value (Kenward, 1982). In some cases, bark stripping may be the result of agonistic behaviour between individuals, for instance young males are often implicated (Taylor, 1966, 1969). This does not however preclude other drivers for bark stripping such as the seeking of a trace nutrient that may be deficient (Allen, 1943).
The calcium hypothesis proposes that grey squirrels ingest phloem to ameliorate a seasonal calcium deficiency (Nichols, Drewe, Gill, Goode, & Gregory, 2016). Calcium is found in greater quantities than any other inorganic element in plants (McLaughlin & Wimmer, 1999). It is present in the phloem of tree bark in large quantities and precipitates as calcium oxalate (CaOx) crystals in the cell vacuoles of many species of angiosperm trees (Borchert, 1990), acting as a store as excess calcium is sequestered (Franceschi & Nakata, 2005; Hudgins, Krekling, & Franceschi, 2003). These crystals can occur in most plant families, including oak, Quercus robur and poplar, Populus tremula (Trockenbrodt, 1995), both of which are known to be susceptible to damage by the grey squirrel (Rowe & Gill, 1985). It is possible that juveniles and pregnant adult females of some grey squirrel populations have an increased demand for dietary calcium during the bark‐stripping season (Nichols, 2016) of April‐July (Gurnell, 1987; Shorten, 1957).
If, as the calcium hypothesis suggests, grey squirrels are damaging trees to ingest calcium, it would be expected that grey squirrels can utilise CaOx. However, not all mammals can utilise CaOx because it is poorly absorbed unless it is broken down into its constituent parts (Hossain, Ogawa, Morozumi, Hokama, & Sugaya, 2003). The resultant oxalate proves problematic as it is a dietary deterrent for grey squirrels (Schmidt, Brown, & Morgan, 1998), and oxalic acid can be poisonous to mammals (Blackwell, 1990), unless it is degraded. This breakdown can be achieved by some mammals using symbiotic microbes in the gut such as Oxalobacter species (Palgi, Taleisnik, & Pinshow, 2008). The pack rat, Neotoma albigula, and the fat sand rat, Psammomys obesus, for instance can achieve this feat (Shirley & Schmidt‐Nielsen, 1967), and it is possible that grey squirrels could also. The CaOx complex is practically inert (Hossain et al.