Seasonal resource categorisation and behavioral adaptation among chimpanzees: Implications for early hominin carnivory

Seasonality plays a critical role in determining the yearly dietary variability of many nonhuman primates living in tropical and subtropical environments. Much previous research has emphasised the seasonal importance of both preferred resources-eaten whenever available-and fallback foods-eaten during periods of scarcity to compensate for an insufficient availability of preferred resources. However, previous discussions of this dichotomy have often overlooked why different populations of the same taxon may exhibit a different level of engagement with identical resources, especially those that require additional technological investment by virtue of being embedded. Similarly, not enough attention has been given to diachronic trends in the incorporation of novel resources to seasonal consumption patterns among non-human primates. In this paper, we present a systematic framework for understanding the spatio-temporal relationships between preferred and fallback resources, explicitly through the lens of landscape knowledge and seasonal fluctuations in quality and availability among chimpanzee communities. We argue it is the interplay between resource quality and the available knowledge pertaining to its exploitation that will determine the categorisation of a resource. In this regard, the accumulation of further information through encounter, experimentation, and behavioural (including technological( innovation enables resources with high nutritional potential to attain preferred status. We end with an exploration of the gradual consolidation of the hominin carnivory niche in the Early Pleistocene of East Africa, to demonstrate the utility of our framework-specifically the interplay between seasonality and the concept of landscape knowledge-for understanding behavioural change in the archaeological record.

Estimates of absolute crown strength and bite force in the lower postcanine dentition of Gigantopithecus blacki

Gigantopithecus blacki is hypothesized to have been capable of processing mechanically challenging foods, which likely required this species to have high dental resistance to fracture and/or large bite force. To test this hypothesis, we used two recently developed approaches to estimate absolute crown strength and bite force of the lower postcanine dentition. Sixteen Gigantopithecus mandibular permanent cheek teeth were scanned by micro-computed tomography. From virtual mesial cross-sections, we measured average enamel thickness and bi-cervical diameter to estimate absolute crown strength, and cuspal enamel thickness and dentine horn angle to estimate bite force. We compared G. blacki with a sample of extant great apes (Pan, Pongo, and Gorilla) and australopiths (Australopithecus anamensis, Australopithecus afarensis, Australopithecus africanus, Paranthropus robustus, and Paranthropus boisei). We also evaluated statistical differences in absolute crown strength and bite force between the premolars and molars for G. blacki. Results reveal that molar crown strength is absolutely greater, and molar bite force absolutely higher, in G. blacki than all other taxa except P. boisei, suggesting that G. blacki molars have exceptionally high resistance to fracture and the ability to generate exceptionally high bite force. In addition, G. blacki premolars have comparable absolute crown strength and larger bite force capabilities compared with its molars, implying possible functional specializations in premolars. The dental specialization of G. blacki could thus represent an adaptation to further facilitate the processing of mechanically challenging foods. While it is currently not possible to determine which types of foods were actually consumed by G. blacki through this study, direct evidence (e.g. dental chipping and microwear) left by the foods eaten by G. blacki could potentially lead to greater insights into its dietary ecology.

Diet and Activity Budget in Colobus angolensis ruwenzorii at Nabugabo, Uganda: Are They Energy Maximizers?

INTRODUCTION

Colobine monkeys are specialized folivores that use foregut fermentation to digest leaves. The slow process of fermentation forces them to spend a lot of time resting and to minimize their energy expenditure to subsist on a lower-quality diet.

METHODS

We recorded the diet and activity budget of Colobus angolensis ruwenzorii, which form a three-tiered multi-level society, at Lake Nabugabo, Uganda, over 12 months using scan sampling on adults and subadults, to determine whether they utilize the energy minimization strategy typical of colobines.

RESULTS

We found that the annual diet was primarily comprised of high-quality food resources (young leaves 65% and fruit 31%), and fruits were the only plant part the monkeysselected when available. Both the fruits and young leaves of some species were preferred food items in some months, and mature leaf consumption correlated negatively with preferred food availability. Mature leaves appear to be a fallback food for this population but are rarely relied upon (3%). The C. a. ruwenzoriiat Nabugabo spent less time resting (40%) and more time moving (25%) than is typical for other species of black-and-white colobus.

DISCUSSION/CONCLUSION

The high-quality diet of this population appears to allow them to utilize an energy maximization strategy. Their reliance on food items that tend to be clumped in space and time likely explains the frequent fission-fusion behaviour that we observe between core units. Our findings demonstrate that the foraging strategies of colobines may be more flexible than was previously thought and illustrate how food availability and distribution can impact primate social organization.

Oral Processing Behavior of Diana Monkeys (Cercopithecus diana) in Taï National Park, Côte d'Ivoire

Primates are hypothesized to "fall back" on challenging-to-process foods when preferred foods are less available. Such dietary shifts may be accompanied by changes in oral processing behavior argued to be selectively important. Here, we examine the oral processing behavior of Diana monkeys (Cercopithecus diana) in the Taï Forest across their dietary breadth, testing relationships among food intake, fruit availability, preference, and oral processing behaviors including those involved in food ingestion and breakdown. We conducted 1,066 focal follows from April 2016 to September 2017 documenting frequencies of incisor, canine, and cheek tooth mastications (i.e., chews) per ingestive action (n = 11,906 feeding events). We used phenological survey and scan sample data collected between 2004 and 2009 to examine dietary preference and food availability. Our analyses show that Diana monkeys processed foods in significantly different ways (H2 = 360.8, p < 0.001), with invertebrates requiring the least oral processing, fruit requiring intermediate amounts, and leaves requiring the most oral processing. There was no relationship between fruit availability and consumption of preferred or nonpreferred fruits (p > 0.05); however, preferred fruits were processed with significantly fewer mastications (i.e., less chewing) than nonpreferred fruits (U = 6,557, p < 0.001). We thus demonstrate that, when preferred foods are scarce, Diana monkeys do not fall back on difficult-to-process foods. Changes in processing profiles occurred throughout the year and not solely when preferred foods were in short supply. Though preferred fruits required less processing than nonpreferred fruits, we found no relationship between fruit preference and fruit availability. Diana monkeys' lack of readily identifiable fallback foods may be attributable to the relatively high tree diversity and productivity of the Taï Forest. We conclude that Diana monkeys engage in resource switching, consuming a relatively easy-to-process diet year-round.

Responses to Spatial and Temporal Variation in Food Availability on the Feeding Ecology of Proboscis Monkeys (Nasalis larvatus) in West Kalimantan, Indonesia

Folivorous primates have long been assumed to experience food competition less acutely than frugivores due to their ability to eat leaves, an abundant resource in most forest systems. Consequently, the behavioural responses of leaf-eating primates to variation in food availability are less well characterised than those of frugivores. Recent empirical studies have demonstrated that many colobine species are more affected by food availability and distribution than previously thought; they employ multiple strategies to survive during periods of food scarcity. We studied a population of proboscis monkeys (Nasalis larvatus) over 16 months in three forest types in West Kalimantan, Indonesia, to examine their responses to temporal fluctuation and spatial variation in food availability. We examined how feeding behaviour was influenced by the availability of plants in botanical plots to identify important and preferred foods of proboscis monkeys across months and in different forest types. Proboscis monkeys consumed foods from 68 genera, comprising 35% young leaves, 27% unripe fruit, 12% flowers and 6% mature leaves. Consumption of plant parts and genera by proboscis monkeys varied in response to monthly changes in food availability but did not vary among forest types despite substantial differences in phenology and floristics among them. The monkeys preferred unripe fruits and flowers and used young and mature leaves as fallback foods in mangrove forests. Documentation of proboscis monkey responses to variation in food availability contributes to our understanding of how monkeys respond to changes in their environments due to climate change and habitat degradation.

Ontogenetic and functional modularity in the rodent mandible

The material properties of diets consumed by juvenile individuals are known to affect adult morphological outcomes. However, much of the current experimental knowledge regarding dietary effects on masticatory form is derived from studies in which individuals are fed a non-variable diet for the duration of their postweaning growth period. Thus, it remains unclear how intra-individual variation in diet, due to ontogenetic variation in feeding behaviors or seasonal resource fluctuations, affects the resulting adult morphology. Furthermore, the mandible is composed of multiple developmental and functional subunits, and the extent to which growth and plasticity among these modules is correlated may be misestimated by studies that examine non-variable masticatory function in adults only. To address these gaps in our current knowledge, this study raised Sprague Dawley rats (n=42) in four dietary cohorts from weaning to skeletal maturity. Two cohorts were fed a stable ("annual") diet of either solid or powdered pellets. The other two cohorts were fed a variable ("seasonal") diet consisting of solid/powdered pellets for the first half of the study, followed by a shift to the opposite diet. Results of longitudinal morphometric analyses indicate that variation in the mandibular corpus is more prominent at immature ontogenetic stages, likely due to processes of dental eruption and the growth of tooth roots. Furthermore, adult morphology is influenced by both masticatory function and the ontogenetic timing of this function, e.g., the consumption of a mechanically resistant diet. The morphology of the coronoid process was found to separate cohorts on the basis of their early weanling diet, suggesting that the coronoid process/temporalis muscle module may have an early plasticity window related to high growth rates during this life stage. Conversely, the morphology of the angular process was found to be influenced by the consumption of a mechanically resistant diet at any point during the growth period, but with a tendency to reflect the most recent diet. The prolonged plasticity window of the angular process/pterygomasseteric muscle module may be related to delayed ossification and muscular maturation within this module. The research presented here highlights the importance of more naturalistic models of mammalian feeding, and underscores the need for a better understanding of the processes of both morphological and behavioral maturation that follow weaning.