Heat and plant nutritional quality interact to constrain koala feeding

Context: Nutrition underpins all physiological and most ecological processes. In hot conditions, captive endotherms reduce intake and are less tolerant of plant secondary metabolites, reducing health and productivity. In the context of rapid global heating, there is an urgent need to understand how ambient temperature (T_a) affects feeding and diet selection in wild animals, particularly those of conservation concern. Aims: We aimed to determine whether T_a and the nutritional quality of available foods influence food intake and diet selection in wild koalas (Phascolarctos cinereus). Methods: We followed six free-ranging koalas continuously for 2 weeks per koala and recorded food intake per meal and hourly T_a. We also measured the foliar nutritional and chemical composition, namely concentrations of available nitrogen (availN) and formylated phloroglucinol compounds (FPCs), of all eucalypt trees in which koalas fed. We then investigated relationships between T_a, food intake, availN intake, food choice, and tolerance of FPCs. Key results: At warmer T_a, koalas reduced their daily food intake by reducing meal size without altering meal frequency. This reduction in meal size was greatest when koalas fed in trees with high foliar concentrations of FPCs, and they fed in high-FPC trees only when temperatures were below 19°C. Conclusions: T_a influences nutrient intake and food selection in wild koalas, with trees containing high FPC concentrations less likely to be eaten in hot weather. Implications: Increased T_a and more frequent and severe heat waves could alter foraging strategies and affect the capacity of wild koalas, and likely other endotherms, to obtain key nutrients, particularly when food plants contain high concentrations of plant secondary metabolites. This may have flow-on effects for the health of individuals and populations.