The gastrointestinal (GI) microbiome contributes significantly to host nutrition and health. However, relationships involving GI microbes, their hosts and host macrohabitats remain to be established. Here, we define clear patterns of variation in the GI microbiomes of six groups of Mexican black howler monkeys (Alouatta pigra) occupying a gradation of habitats including a continuous evergreen rainforest, an evergreen rainforest fragment, a continuous semi-deciduous forest and captivity. High throughput microbial 16S ribosomal RNA gene sequencing indicated that diversity, richness and composition of howler GI microbiomes varied with host habitat in relation to diet. Howlers occupying suboptimal habitats consumed less diverse diets and correspondingly had less diverse gut microbiomes. Quantitative real-time PCR also revealed a reduction in the number of genes related to butyrate production and hydrogen metabolism in the microbiomes of howlers occupying suboptimal habitats, which may impact host health.

Homeostatic responses of animals to environmentally induced changes in nutrient requirements provide a powerful basis for predictive ecological models, and yet, such responses are virtually unstudied in the wild. We tested for macronutrient‐specific compensatory feeding responses by free‐ranging golden snub‐nosed monkeys (Rhinopithecus roxellana) inhabiting high altitude temperate forests, where they experience a substantial difference in ambient temperature in cold winters vs. warmer springs. The monkeys had free access to natural foods throughout the year, and to ensure that any seasonal differences in nutrient intake were due to homeostatic compensation and not constraints on food availability, we studied the monkeys during periods in which they were provisioned with the same amount of supplementary foods in winter and spring. Thermoregulatory energy costs in winter and spring were calculated using partitional calorimetric estimations of convective and radiative heat loss obtained from thermal imaging of free‐ranging monkeys in situ. Daily nutrient intakes were measured using continuous focal follows (average 6.9 hr/day) of free‐ranging individuals (27 days in spring and 28 days in winter). We used a nutritional geometry framework to integrate these data and test three predictions: (a) In order to remain thermoneutral (balance heat loss with heat expenditure), golden snub‐nosed monkeys increase daily energy consumption during the winter compared to spring; (b) this increase is achieved specifically by increasing intake of the primary energetic nutrients, carbohydrate and lipid, relative to protein; and (c) the seasonal increase in ingested fat and carbohydrate calories will quantitatively match the additional thermoregulatory costs in winter compared with spring. Our results showed that daily metabolisable energy intake in winter (721.5 kJ/mbm) was 1.8 times that in spring (399 kJ/mbm). As predicted, this difference was specific to fat and carbohydrate, whereas seasonal protein intake did not differ significantly. Winter consumption of fat and carbohydrate was 326 kJ/mbm per day greater than in spring, a value that closely matched the seasonal difference in the daily energetic costs of thermoregulation (329 kJ/mbm). This is the first study to test for a match between nutrient‐specific homeostatic compensation and environmentally induced perturbations in nutrient requirements in free‐ranging animals and underpins the potential for the homeostasis framework to provide predictive power to ecological models.