The effect of diet on water and energy turnover rates of fourGerbillurusspecies in captivity

The influence of dehydration on the thermal preferences of the Western tiger snake, Notechis scutatus

Temperature selection in tiger snakes (Notechis scutatus) is strongly influenced by hydration state and this response varies between two distinct neighbouring populations on semi-arid Carnac Island and mainland wetland (Herdsman Lake). Fed and hydrated (control) Carnac Island snakes selected a preferred body temperature of 26.2+/-1.2 degrees C and an average maximum temperature of 32.5+/-0.5 degrees C in a photo-thermal gradient. Dehydrated Carnac Island snakes selected a significantly lower preferred body temperature (19.7+/-1.6 degrees C) and average maximum temperature (27.7+/-1.0 degrees C). Control Herdsman Lake snakes selected a preferred body temperature of 27.5+/-0.6 degrees C and an average maximum temperature of 33.3+/-0.4 degrees C. Dehydrated Herdsman Lake snakes selected a significantly lower preferred body temperature (23.3+/-1.1 degrees C) and a lower average maximum temperature (31.8+/-0.6 degrees C). Thermal depression (decreased preferred body and average maximum temperatures) in response to dehydration was greater for Carnac Island than Herdsman Lake snakes. As decreases in temperature and activity can reduce water loss, our laboratory data suggest that the survival of the relict population of tiger snakes on Carnac Island is associated with thermoregulatory modifications, which may have the effect of enhancing water conservation in this waterless habitat.

Digestive efficiency of a generalist avian feeder, the Cape White-eye (Zosterops pallidus)

Digestive processes determine whether the particular diet of a bird is utilized efficiently and whether energetic demands are met. Assimilation efficiency is often used as an index of whether a diet is digested optimally. Studies on the digestive processing of generalist feeders are scarce. Cape White-eyes (Zosterops pallidus) have a diverse diet of fruit, nectar and insects. The nutrient contents of these three diets vary considerably and require quite different digestive processing. This study compared the digestive efficiencies of Cape White-eyes on these three diets by measuring transit times and assimilation efficiency. Cape White-eyes lost body mass significantly when fed fruit, while they maintained and gained body mass on nectar and mealworm diets, respectively. Assimilation efficiency varied significantly between the three diet types (nectar>mealworms>apples). When given a choice of diets, Cape White-eyes selected the diet, which was most efficiently digested and yielded the greatest energetic reward. Diet preference trials further showed that Cape White-eyes regulated daily energy intake. Assimilation efficiency depends on the accessibility of nutritional contents of a diet. Cape White-eyes did not maximize assimilation efficiency. Instead, they adjusted transit time to maximize the rate of energy gain per gram of food in order to maintain energy balance.

Wax digestion by the lesser honeyguide Indicator minor

Records of the lesser honeyguide feeding on beeswax in the wild are common. However, little is known about the ability of these birds to digest wax. We studied the wax preference, digestive efficiency, retention time and gut structure of the lesser honeyguide in the laboratory. Wax was found to be eaten daily by the lesser honeyguide. Birds ate significantly more new beeswax and foundation wax than old, black beeswax, but required an additional source of protein to maintain body mass. In the absence of other feed, the digestive efficiency for foundation wax was above 90%, with a long transit time of 256 min. Examination of the digestive tract showed no specialisation and no crop. The presence of digestive enzymes, including lipase, in the pancreas and small intestine and very few microbes in the digestive tract of the lesser honeyguide suggests that wax digestion occurs through a biochemical pathway with endogenous avian enzymes.

Water flux and energy use in wild house mice (Mus domesticus) and the impact of seasonal aridity on breeding and population levels

Water turnover rate (WTR), urine concentration and field metabolic rate (FMR) were examined in house mice, Mus domesticus, permanently inhabiting roadside verge areas and seasonally invading crops in semi-arid wheatlands in South Australia. FMR was approximately proportional to body mass^0.5 and mean values varied from 4.8 ml CO₂ g^-1h^-1 (2.9 kJ g^-1d^-1) in autumn and winter, to 7.0 ml CO₂ g^-1h^-1 (4.2 kJ g^-1d^-1) in maturing crops during spring. WTR was independent of body mass, indicating that larger mice were selecting a diet containing moister foods. WTR was low in summer and high in winter, and in mice from crops varied from 165 ml l^-1 body water d^-1 (122 ml kg^-1d^-1) to 1000 ml l^-1d^-1 (725 ml kg^-1d^-1). Seasonal changes in WTR were less extreme on the roadside, where a greater diversity of food was available. In the crops, breeding occurred throughout summer during two of three years, but the population increased only in the one summer when mice had marginally higher WTR. On the roadside breeding and population growth were continuous during summer, except in a drought year. Avcrage urine concentration was inversely related to WTR, and varied from 2.0 to 4.8 Osm l^-1. The data indicate that the water conserving abilities of mice equal those of many desert rodents. The water conserving abilities of mice living in crops during summer were fully extended, and in some years aridity limited breeding success and population levels. The degree of moisture stress to which mice are exposed during summer appears to depend not only on rainfall but also on other factors such as availability of food and shelter, and the level of weed infestation in crops.