Effects of ambient temperature on feeding by herbivorous marsupials

Abstract

In this thesis I show that the nutritional decisions of herbivores are tied to the thermal environment they experience. This pertains not just to the total energy eaten, but also to the macronutrients from which they choose to get that energy, and to the plant secondary metabolites (PSMs) that complicate the decision. I have reviewed the literature to show that the relationship between thermal physiology and nutritional ecology runs deeper than simply adjusting food intake to meet changes in metabolic rate. Ties between thermoregulation and nutrition in endothermic herbivores includes the thermogenic nature of PSM metabolism, PSM interference with thermosensing, potential mitochondrial uncoupling by PSMs, energy wasting by heat generation, and simple obligatory diet induced thermogenesis. I then present results on whether temperature-dependent toxicity applies to marsupial folivores ingesting diets with PSMs using captive feeding studies in common brushtail possums (Trichosurus vulpecula) and common ringtail possums (Pseudocheirus peregrinus). I show that when exposed to different ambient temperatures (10C, 18C, 26C) overnight there is no difference in intake of high-PSM diets when fed at different temperatures. However, if given one week of exposure time both brushtail and ringtail possums decrease intake at warmer ambient temperatures. I consider this pattern consistent with temperature dependent toxicity, however it differs slightly from previous literature. A mechanism put forward by others for temperature-dependent toxicity is the down regulation of hepatic enzymes in warmer conditions. I test whether this may be the underlying mechanism in possums by conducting a sleep-time assay using an hepatically metabolised anaesthetic agent as a proxy for PSM metabolism. I show that the rate of hepatic metabolism by possums is slower following one week of exposure to warm ambient temperatures (26C vs 10C), while there is no difference following overnight exposure. This supports previous work showing that TDT may be a result of down regulation of enzymes responsible for PSM metabolism. Following this I demonstrate using nutritional geometry that ambient temperature may influence the mixing of macronutrients to minimise heat generation. Brushtail possums chose to eat a diet lower in the more thermogenic macronutrient, protein at 26C, compared to possums fed at cooler temperatures (10C, 18C). The addition of a PSM (cineole) however removed this effect, so it may be that ingesting enough protein to deal with the protein cost of PSM ingestion overrides the desire to minimise the obligatory thermogenesis of a higher protein diet. I then consider data collected using radio collars fitted with microphones to determine how intake patterns are influenced by temperature and PSMs influence daily and per meal food intake in semi wild koalas (Phascolarctos cinereus). Koalas ingest 6g less overall per 1C increase in mean ambient temperature. This was due to a change in meal size rather than a change in meal number. In particular meal size was reduced by warm temperatures when leaves contain higher concentrations of a particular groups of PSMs (formylated phloroglucinol compounds). Since the PSM concentration and the concentration of protein were highly correlated, this resulted in animals consuming less protein in warm conditions. Following this I investigate whether there is evidence of mitochondrial uncoupling in cell-culture by a range of PSMs found in Eucalyptus leaves, and while I find some interesting results, I do not see any evidence for strong uncoupling, although this may be due to experimental design. Taken together the results in this thesis show that ambient temperature can have considerable impact on nutrition, and that warming ambient temperatures due to climate change are likely to result in new challenges for marsupial folivores, as well as other herbivore species ingesting PSM rich diets. Show more