RODENTS
Although most rodents are considered omnivorous they exhibit varied feeding strategies (Table 14.4). The blackfooted tree-rat (Mesembriomys gouldii), has been categorised as frugivorous, but has a varied diet and a digestive anatomy that suggests a diet higher in fibre.
Like the greater stick-nest rat (Leporillus conditor), the blackfooted tree-rat has broad molars, a large forestomach, a relatively short small intestine, long large intestine and a large, highly folded caecum to maximise digestion of cellulose (Jackson 2003). Australian mice are expected to have lower BMRs than non-native species which may be due to their adaptations to more arid environments and more tolerance for thermolability (Tomlinson 2016). However, at least one species, the ash-grey mouse (Pseu- domys albocinereus) had higher than expected BMR and used torpor outside their TNZ to conserve energy (Barker et al. 2012).The small proportion of animal matter and fruit cited in natural rodent diets has led to the development of omnivorous zoo diets that consist of a variety of fresh fruit and vegetables, seeds, nuts, invertebrates and commercial rodent cubes or even dog kibble. The widespread use of high energy commercial rodent cubes, designed for breeding rats and mice, and excessive fruit, have been implicated as possible causes of obesity and diabetes. Previously, domestic mice and rats were considered the best animal models for Australian rodents but as there is no minimum fibre requirement listed for omnivorous rodents, guinea-pigs (Cavia porcellus) may be a more appropriate model (Table 14.1). When faced with energy dense diets, guinea-pigs compensate by increasing insulin levels to maintain a normoglycaemic state (Opazo et al. 2005). It is likely that prolonged elevation of insulin levels, may lead to β-islet cell exhaustion, leading to inadequate insulin production and Type 2 diabetes mellitus (T2DM), a process seen in native rats and described in the desert gerbil (Psammomys obesus) (Kaiser et al.
2005a). An alteration in insulin that incurs insulin insensitivity common to these glucose-intolerant, herbivorous rodents could account for the T2DM reported in managed care (Vardhan et al. 2012; Nehar et al. 2015).Anti-diabetogenic diets that limit high fat nuts and seeds and replace fruit, dried fruit and rodent cubes with lower sugar vegetables, kangaroo pellets and apple-flavoured psyllium (Plantago arenaria) pellets (Pachylax, Vetafarm, Wagga Wagga, NSW) had marked effects on all signs of T2DM in greater stick-nest rats and blackfooted tree-rats (Lendon et al. 2015). Ad lib browse and chaff as well as fibre supplements or feeds with anti- hyperglycaemic benefits may help achieve the 15% fibre recommended by the National Research Council for guinea-pigs and reduce the risk of T2DM (NRC 1995; Kaiser et al. 2005b; Ziai et al. 2005; Winarto and Anisah 2009; Lebas et al. 2010; Sasidharan et al. 2011; Pothuraju et al. 2014; Wu et al. 2014).
13.