We found that lactating P. coquereli exploited a nutritionally diverse set of foods that varied widely for all measured nutrients and included many high fiber foods. The PCA indicated that available protein, fiber and metabolizable energy accounted for over 91% of the variation among these foods. Our analysis revealed two potential categories of foods in our dataset, visually represented in Figure 2. The relationship between ME and ADF (Figure 3) and AP and ADF (Figure 4) visually demonstrates the separation between the clusters for fiber. However, estimated ME and AP shows considerable overlap between the two clusters, suggesting that sifaka foods could be described by a nutritional gradient. This approach is supported since some foods were moderate to higher in protein and metabolizable energy while lower in fiber, and other foods were lower in protein and metabolizable energy while higher in fiber. The gradient approach may better reflect the continuous nature of nutrient values, particularly for foods on the cluster boundaries (Figures 1, 2, and 4). However, the six foods overlapping in protein and fiber factor space do differ in NDF (Table 4) and the two clusters vary in the proportion of fiber bound to ADF. Both these factors support the hypothesis that these foods cluster into at least two nutritionally distinct groups. We propose that these two food types will have different physiological and metabolic effects, with cluster 1 foods contributing more to the ingesting sifaka’s nutritional status directly while cluster 2 foods will affect nutritional status through effects on the sifaka gut microbiome.
Protein and fiber were the most consistently variable nutrients in the sifaka foods, which also varied considerably in the protein-to-fiber ratio. Primates are estimated to require a minimum of 14% protein per dry matter basis for reproduction, 7—11% for growth and development 36, and 6.4—8% crude protein in their diet to satisfy maintenance nutritional requirements 37. The cluster 1 foods consumed by lactating P. coquereli had a mean of 12.0% available protein, which exceeds minimum protein requirements for primate maintenance, and growth and development, while nearly meeting the estimated reproductive nutritional requirements. Cluster 1 foods had a high ratio of AP-to-CP, supporting the hypothesis that they are good protein sources.
Lactating P. coquereli appear to have a diet quite high in fiber (means of 38.2% NDF, 27.8% ADF) with a relatively low protein-to-fiber ratio without experiencing adverse effects and routinely consumed high fiber foods during the lean season (Table 3). Frequently consumed foods of gestating ring-tailed lemurs (Lemur catta) during the dry season contained less than 21% ADF 3. During lactation, eight of ten of the most frequently consumed foods contained less than 30% ADF, while none of the foods contained over 50% ADF 3. The black-and-white ruffed lemur (Varecia variegata), consumed fruits, leaves and flowers with ADF content of approximately 30% 38. The average ADF content of leaves eaten by the larger-bodied Indri (Indri indri) was 53%, and the fruit, leaves and flowers consumed by diademed sifakas (Propithecus diadema) averaged between 30 and 50% ADF 39. The fiber levels for these larger lemur species are comparable to our results for P. coquereli. Although, the sifakas did include many high protein/low fiber foods in their diet, suggesting that exploiting different foods has functionally distinctive physiological and metabolic consequences.
High fiber food consumption may be a residual effect of lactating P. coquereli unselectively exploiting the foods available in the forest during the lean season. We emphasize that this also has biological relevance, since it provides an assessment of seasonally available nutrients consumed during the critical period of infant development. During the lean season in a dry deciduous forest the availability of foods high in available protein and metabolizable energy may be insufficient, thereby constraining females to select difficult to digest resources to meet energy requirements. Perhaps the increased demand placed by lactation in conjunction with the food constraints of the lean season force sifakas living in dry deciduous forest to ingest the high fiber foods.
However, Propithecus spp. are hindgut fermenters 5 with highly specialized gut microbiomes that vary depending on seasonal fruit availability 40. Dietary plant fiber only become nutritious after its microbial conversion into vital nutrients like short-chain fatty acids 41, facilitated by specific cellulose-degrading microbes present in the sifaka gut and an increased functional capacity for fiber metabolism 42. The specialized morphology of hindgut fermenters (enlarged caecum and elongated colon) could enable the efficient digestion of fibrous materials, increasing nutrient extraction from difficult to digest resources. Our findings are consistent with previous studies that have shown sifakas to be seasonally flexible folivores, a novel dietary strategy that may mitigate potential energetic deficits 43–45. Recent evidence demonstrates sifakas possess molecular adaptions to folivory including rapidly evolving gene pathways that aid in xenobiotic metabolism and nutrient absorption, which may assist in the detoxification of plant compounds while maximizing nutritional gain from leaves 46. This capacity for augmented nutrient uptake 46 would be advantageous to foraging throughout periods of pronounced seasonality in Madagascar 18,47.
Variation in dietary fiber is a critical component to understanding gut microbiomes in folivores and has been shown to affect microbial diversity in P. coquereli 42. Sifaka gut microbiomes have been found to be significantly richer and more diverse in comparison to generalist and frugivorous lemurs 42. Less inter-individual variation in sifaka gut microbiomes is exhibited relative to frugivorous V. variegata and generalist L. catta, suggesting that sifakas may be less flexible in terms of their diet 42 and more susceptible to habitat disturbance 48.
Captive P. coquereli provisioned with a more diverse diet that included local wild plant species had significantly richer, more diverse gut microbiomes in comparison to when their standard diet was supplemented with winged-sumac only 48. Significantly higher concentrations of short-chain fatty acids, including acetate and propionate, and moderately greater concentrations of butyrate were present in P. coquereli colonic metabolomes when provisioned a more diverse diet 48. Additionally, the same study found that individuals given the opportunity to forage more naturally in forested enclosures, even for limited durations, maintained greater gut microbiome diversity relative to conspecifics without forest access (Green et al., 2018). This supports that fiber consumption can have a profound influence on gut microbiome structure and function. It is possible that a high-fiber diet is a requirement for sifakas to maintain their coevolved microbiota. We posit that many if not all the cluster 2 foods in our study may have a greater effect on the sifaka gut microbiome than a direct nutritional effect on the host animal. In other words, cluster 2 foods may be important for maintaining gut health by feeding the microbiome, while cluster 1 foods more directly affect the nutritional plane of the sifakas.
Sifakas are exceptionally difficult to maintain in captivity due to their specialized digestive anatomy and highly folivorous diet 49,50. Our results suggest that incorporating high-fiber foods (ADF greater than 30% or even 40%) into captive diets would better replicate foods consumed in the wild. Table 3 highlights two leading commercial products for leaf-eating primates in various life cycle stages, health, and seasonality versus our field data collected on lactating sifakas during the lean season. The commercial supplements contain higher concentrations of protein (CP) and lower concentrations of fiber. Both Marion and Mazuri provide their products as supplements to foraging and non-foraging fruit and vegetable produce diets. Because of this, percent nutritional values of the various nutrients do not represent the overall lemurs’ diet, but only that of the commercial product itself. Similarly, food selection in the wild depends on environmental factors and does not necessarily reflect the ideal composition for the health of sifakas without food supply constraints as in captivity. While we acknowledge the limitations of juxtaposing a partial wild diet to a partial captive diet, it is presented here to highlight the importance of incorporating nutritional diversity in captive diet design based on wild plant foods acquired by lemurs. We suggest that incorporating foods like the cluster 2 foods in this study may be helpful for dietary management of captive sifakas, possibly by improving gut health through effects on the microbiome.
Consistent with previous studies 11,29,51−53, our results confirm that botanical category (e.g., fruit versus leaf) is a poor means by which to assess the nutritional contribution a food will make to animals that consume it. Fruit is often equated with high water and high non-structural carbohydrate (sugar) content; however, wild fruits can be substantially different in nutrient profile from domesticated fruits, and often are similar to leaves, buds, and flowers, as seen in our study. The fruits in this study were not different from leaves in fiber content. The NDF content of fruit in our study ranged from 8.6–78.5% and the mean NDF for fruit (37.7%) was numerically higher than the mean NDF for leaves (35.8%). Sifakas ingest high fiber foods, whether those foods are classified as leaves, fruit, flowers, or buds. Our results also confirm that wild plant foods can vary seasonally in nutrient content, cautioning that the nutritional consequences of consuming some foods can differ by time of year.
In summary, infant-bearing P. coquereli’s employ a mixed-diet strategy consuming foods with wide ranges in percent nutrient content to compensate for nutrient deficiencies in multiple plant parts and food availability. Food sources clustered into two categories: high in protein and low-to-moderate in fiber; or high in fiber and low in metabolizable energy.