Plant Secondary Compounds as Diuretics: An Overlooked Consequence1

Australian perennial shrub species add value to the feed base of grazing livestock in low- to medium-rainfall zones

Australian native perennial shrubs that are adapted to drought and infertile soils contribute nutrients to grazing systems that would otherwise support limited ruminant productivity. In this study, we report the nutritive value of 39 Australian shrub species of the genera Atriplex, Rhagodia, Maireana, Chenopodium, Enchylaena, Acacia, Eremophila, and Kennedia. Edible foliage was sampled in winter and summer, and there was little difference in nutritive value between seasons. The in vitro organic matter digestibility of most shrub species was 40–70%. Most species contained medium to high levels of crude protein (12–22% of dry matter, DM) and high concentrations of sulfur (2–8 g/kg DM). In an 8-week grazing experiment in which Merino wethers grazed a ‘shrub system’ containing four shrub species and a sown inter-row of annual pasture, the sheep gained weight during autumn without supplementary feeding. By comparison, sheep fed senesced volunteer pasture and supplementary cereal grain only maintained weight. The forage shrubs provided up to 50% of the total DM intake of sheep grazing the ‘shrub system’ and made a modest contribution to the digestible energy intake of the animals and a large contribution to their crude protein and mineral intake. Considering the timely and predictable provision of limiting nutrients and benefits such as gut health and the provision of shade and shelter, we suggest that Australian shrub species can make a valuable addition to the feed base of low- to medium-rainfall zones in southern Australia.

Tolerance to dietary phenolics and diet breadth in three seed-eating birds: implications for Graminivory

The ability to cope with plant secondary compounds (PSCs) has profound implications for an animal's behavior. In the present study, we assessed the tolerance to dietary phenolics in three seed-eating birds: Zonotrichia capensis, Saltatricula multicolor, and Diuca diuca, which differ in their diet breadth. Seeds in their habitat have distinct chemical composition: grass seeds have less PSCs, specifically, less total phenolics than forb seeds. Based on the detoxification limitation hypothesis and using published data of the natural history of these birds in the central Monte desert, we postulate that predominantly and exclusively graminivorous birds such as D. diuca and S. multicolor, respectively, are less tolerant due, in part, to a lower detoxification capacity than those with greater diet breadth, Z. capensis. To achieve this goal, we measured the food intake of diets varying in their concentration of tannic acid (TA). Indicators of tolerance were body mass change, food, TA and water intake, and glucuronic acid output throughout the experiment. Zonotrichia capensis performed better along the tolerance experiment: it maintained body mass from 0 to 4% TA diet, food and TA intake were higher than the other two species at the end of the experiment, and glucuronic acid output by Z. capensis was greater than D. diuca and S. multicolor from 2% TA diet until the end of the experiment. Our results suggest that Z. capensis is the most tolerant species and this physiological trait may explain their greater diet breadth.

The limit to the distribution of a rainforest marsupial folivore is consistent with the thermal intolerance hypothesis

Models of impacts of climate change on species are generally based on correlations between current distributions and climatic variables, rather than a detailed understanding of the mechanisms that actually limit distribution. Many of the vertebrates endemic to rainforests of northeastern Australia are restricted to upland forests and considered to be threatened by climate change. However, for most of these species, the factors controlling their distributions are unknown. We examined the role of thermal intolerance as a possible mechanism limiting the distribution of Pseudochirops archeri (green ringtail possum), a specialist arboreal folivore restricted to rainforests above an altitude of 300 m in Australia's Wet Tropics. We measured short-term metabolic responses to a range of ambient temperatures, and found that P. archeri stores heat when ambient temperatures exceed 30°C, reducing water requirements for evaporative cooling. Due to the rate at which body temperature increases with ambient temperatures >30°C, this strategy is not effective over periods longer than 5 h. We hypothesise that the distribution of P. archeri is limited by interactions between (i) the duration and severity of extreme ambient temperatures (over 30°C), (ii) the scarcity of free water in the rainforest canopy in the dry season, and (iii) constraints on water intake from foliage imposed by plant secondary metabolites and fibre. We predict that dehydration becomes limiting for P. archeri where extreme ambient temperatures (>30°C) persist for more than 5 h per day over 4-6 days or more. Consistent with our hypothesis, the abundance of P. archeri in the field is correlated with the occurrence of extreme temperatures, declining markedly at sites where the average maximum temperature of the warmest week of the year is above 30°C. Assuming the mechanism of limitation is based on extreme temperatures, we expect impacts of climate change on P. archeri to occur in discrete, rapid events rather than as a slow contraction in range.

The influence of dietary tannin supplementation on digestive performance in captive black rhinoceros (Diceros bicornis)

Free-ranging browsers such as the black rhinoceros (Diceros bicornis) consume a diet that contains tannins, whereas the diets offered to them in captivity consist mostly of items known to contain hardly any such secondary plant compounds. Tannins could have potentially beneficial effects, including the chelation of dietary iron (iron storage disease is a common problem in black rhinos). Here, we tested the acceptance, and the consequences on digestion variables, of a low-dose tannin supplementation in captive animals. Eight black rhinoceroses from three zoological institutions were used. Faecal output was quantified by total faecal collection. Diets fed were regular zoo diets supplemented with either tannic acid (T, hydrolysable tannin) or quebracho (Q, condensed tannins); overall tannin source intake increased at 5-15 g/kg dry matter (DM) in relation to regular zoo diets. Adaptation periods to the new diets were >2 months. Additional data were taken from one hitherto unpublished study. Data were compared to measurements in the same animals on their regular zoo diets. All animals accepted the new diets without hesitation. There was no influence of tannin supplementation on digestion coefficients of DM and its constituents, or faecal concentrations of short-chain fatty acids or lactate. Water intake did not increase during tannin supplementation. Should the inclusion of dietary tannin sources be an objective in the development of diets for captive rhinoceroses, moderate doses such as used in this study are unlikely to cause relevant depressions of digestive efficiency and will not interfere with bacterial fermentation in a relevant way.

Ingestion of juniper foliage reduces metabolic rates in woodrat (Neotoma) herbivores

Ingestion of plant secondary compounds by herbivores is predicted to increase resting or basal metabolic rates. We tested this hypothesis with two species of woodrat herbivores, Neotoma stephensi and Neotoma albigula, consuming diets of juniper (Juniperus monosperma), which is rich in plant secondary compounds. In nature, N. stephensi specializes on juniper, whereas N. albigula consumes a variety of plant species including juniper. We measured resting metabolic rates (RMR) of woodrats on control, 25% juniper and a treatment containing the maximum tolerable dose of juniper (50% juniper for N. albigula and 70% juniper for N. stephensi). Ingestion of a juniper diet resulted in decreased RMR in both species of woodrats. We propose several potential mechanisms for metabolic depression of Neotoma on juniper diets. Our novel results underscore the need for more studies utilizing plant-based diets to determine the general effect of plant secondary compounds on metabolic rates of herbivores.