Behavioral, Ecological, and Evolutionary Aspects of Meat-Eating by Sumatran Orangutans (Pongo abelii)

Opportunistic meat-eating by urban folivorous-frugivorous monkeys

The consumption of vertebrate tissues and eggs (hereinafter "meat") is relatively common among some primates that are highly frugivorous or eclectic omnivores, but rare or absent in those that are highly folivorous. The Neotropical howler monkeys (Alouatta spp.) belong in the latter group. Here we report the consumption of meat by free-ranging urban black and gold howler monkeys (Alouatta caraya) and discuss the potential role of the consumed meat as a source of energy, protein, or micronutrients. We studied three groups of howler monkeys (comprising four to seven individuals), living in city squares (0.6, 1.5, and 1.9 ha) in south Brazil, from July 2022 to May 2023 (65 days; 797 h of observations). All of the study groups were spontaneously supplemented daily by people with variable amounts and types of food provided. Meat was only offered in the two larger squares. The groups' diets included leaves (42-49% scan sampling feeding records), fruit (3-20%), and flowers (2-5%) from 13 to 20 plant species, and considerable amounts of supplemented food (27-50%). We recorded 33 individual events of ingestion of supplemented cooked meat, three individual events of dove egg predation, and three bird nest inspections without egg consumption. All members of the two groups in the larger squares, except an infant male, ingested meat at least once. Meat accounted for 1% of total scan feeding records of both groups with access to this supplement. We conclude that whereas the opportunistic consumption of meat probably contributed only minor amounts of energy and protein to the study subjects, it may have benefitted them with micronutrients that are scarce in plant foods.

Slowly Making Sense: A Review of the Two-Step Venom System within Slow (Nycticebus spp.) and Pygmy Lorises (Xanthonycticebus spp.)

Since the early 2000s, studies of the evolution of venom within animals have rapidly expanded, offering new revelations on the origins and development of venom within various species. The venomous mammals represent excellent opportunities to study venom evolution due to the varying functional usages, the unusual distribution of venom across unrelated mammals and the diverse variety of delivery systems. A group of mammals that excellently represents a combination of these traits are the slow (Nycticebus spp.) and pygmy lorises (Xanthonycticebus spp.) of south-east Asia, which possess the only confirmed two-step venom system. These taxa also present one of the most intriguing mixes of toxic symptoms (cytotoxicity and immunotoxicity) and functional usages (intraspecific competition and ectoparasitic defence) seen in extant animals. We still lack many pieces of the puzzle in understanding how this venom system works, why it evolved what is involved in the venom system and what triggers the toxic components to work. Here, we review available data building upon a decade of research on this topic, focusing especially on why and how this venom system may have evolved. We discuss that research now suggests that venom in slow lorises has a sophisticated set of multiple uses in both intraspecific competition and the potential to disrupt the immune system of targets; we suggest that an exudate diet reveals several toxic plants consumed by slow and pygmy lorises that could be sequestered into their venom and which may help heal venomous bite wounds; we provide the most up-to-date visual model of the brachial gland exudate secretion protein (BGEsp); and we discuss research on a complement component 1r (C1R) protein in saliva that may solve the mystery of what activates the toxicity of slow and pygmy loris venom. We conclude that the slow and pygmy lorises possess amongst the most complex venom system in extant animals, and while we have still a lot more to understand about their venom system, we are close to a breakthrough, particularly with current technological advances.

Drivers of Three Most Charismatic Mammalian Species Distribution across a Multiple-Use Tropical Forest Landscape of Sumatra, Indonesia

Tropical Rainforest Heritage sites of Sumatra are some of the most irreplaceable landscapes in the world for biodiversity conservation. These landscapes harbor many endangered Asiatic mammals all suffering multifaceted threats due to anthropogenic activities. Three charismatic mammals in Sumatra: Elephas maximus sumatranus, Pongo abelii, and Panthera tigris sumatrae are protected and listed as Critically Endangered (CR) within the IUCN Red List. Nevertheless, their current geographic distribution remains unclear, and the impact of environmental factors on these species are mostly unknown. This study predicts the potential range of those species on the island of Sumatra using anthropogenic, biophysical, topographic, and climatic parameters based on the ensemble machine learning algorithms. We also investigated the effects of habitat loss from current land use, ecosystem availability, and importance of Indonesian protected areas. Our predictive model had relatively excellent performance (Sørensen: 0.81-0.94) and can enhance knowledge on the current species distributions. The most critical environmental predictors for the distribution of the three species are conservation status and temperature seasonality. This study revealed that more than half of the species distributions occurred in non-protected areas, with proportional coverage being 83%, 72%, and 54% for E.m. sumatranus, P. abelii, and P.t. sumatrae, respectively. Our study further provides reliable information on places where conservation efforts must be prioritized, both inside and outside of the protected area networks, to safeguard the ongoing survival of these Indonesian large charismatic mammals.

The cost of chewing: The energetics and evolutionary significance of mastication in humans

Any change in the energetic cost of mammalian mastication will affect the net energy gain from foods. Although the energetic efficiency of masticatory effort is fundamental in understanding the evolution of the human masticatory system, nothing is known currently about the associated metabolic costs of chewing different items. Here, using respirometry and electromyography of the masseter muscle, we demonstrate that chewing by human subjects represents a measurable energy sink. Chewing a tasteless odorless gum elevates metabolic rate by 10 to 15% above basal levels. Energy expenditure increases with gum stiffness and is paid for by greater muscle recruitment. For modern humans, it is likely that mastication represents a small part of the daily energy budget. However, for our ancestors, before the onset of cooking and sophisticated food processing methods, the costs must have been relatively high, adding a previously unexplored energetic dimension to the interpretation of hominin dentofacial fossils.

Slow loris (Nycticebus borneanus) consumption by a wild Bornean orangutan (Pongo pygmaeus wurmbii)

Vertebrate predation and consumption by wild Bornean orangutans (Pongo pygmaeus spp.) is rare. In contrast to recorded observations of slow loris consumption by Sumatran orangutans (Pongo abelii), no cases of this have been previously published for Bornean orangutans in the wild. In 2017, we observed the capture and consumption of a slow loris (Nycticebus borneanus) by an adult unflanged male Bornean orangutan at Tuanan Orangutan Research Station, which is located in the Kapuas region of Central Kalimantan. The unflanged male was together with an adult female and her 3.5-year-old offspring throughout the event. However, despite the mother and her offspring watching the male closely and occasionally begging while he consumed the loris, he resisted all food-taking attempts. This study reports, to the best of our knowledge, the first documented case of slow loris predation and consumption by a Bornean orangutan, and thus provides an important data point for understanding primate predation on other primate species.

Trophic ecology of a Late Pleistocene early modern human from tropical Southeast Asia inferred from zinc isotopes

Tam Pà Ling, a cave site in northeastern Laos, has yielded the earliest skeletal evidence of Homo sapiens in mainland Southeast Asia. The reliance of Pleistocene humans in rainforest settings on plant or animal resources is still largely unstudied, mainly due to poor collagen preservation in fossils from tropical environments precluding stable nitrogen isotope analysis, the classical trophic level proxy. However, isotopic ratios of zinc (Zn) in bioapatite constitute a promising proxy to infer trophic and dietary information from fossil vertebrates, even under adverse tropical taphonomic conditions. Here, we analyzed the zinc isotope composition (⁶⁶Zn/⁶⁴Zn expressed as δ⁶⁶Zn value) in the enamel of two teeth of the Late Pleistocene (63-46 ka) H. sapiens individual (TPL1) from Tam Pà Ling, as well as 76 mammal teeth from the same site and the nearby Nam Lot cave. The human individual exhibits relatively low enamel δ⁶⁶Zn values (+0.24‰) consistent with an omnivorous diet, suggesting a dietary reliance on both plant and animal matter. These findings offer direct evidence of the broad utilization of resources from tropical rainforests by one of the earliest known anatomically modern humans in Southeast Asia.

Sepsis and the evolution of human increased sensitivity to lipopolysaccharide

Among mammals, humans are exquisitely sensitive to lipopolysaccharide (LPS), an environmentally pervasive bacterial cell membrane component. Very small doses of LPS trigger powerful immune responses in humans and can even initiate symptoms of sepsis. Close evolutionary relatives such as African and Asian monkeys require doses that are an order of magnitude higher to do the same. Why humans have evolved such an energetically expensive antimicrobial strategy is a question that biological anthropologists are positioned to help address. Here we compare LPS sensitivity in primate/mammalian models and propose that human high sensitivity to LPS is adaptive, linked to multiple immune tactics against pathogens, and part of multi-faceted anti-microbial strategy that strongly overlaps with that of other mammals. We support a notion that LPS sensitivity in humans has been driven by microorganisms that constitutively live on us, and has been informed by human behavioral changes over our species' evolution (e.g., meat eating, agricultural practices, and smoking).

Meat eating by nonhuman primates: A review and synthesis

Most nonhuman primates prey on vertebrates. Meat-eating, defined as ingestion of vertebrate tissue, occurs in 12 families, ≥39 genera, and ≥89 species. It is most common in capuchins (Cebus and Sapajus spp.), baboons (Papio spp.), bonobos (Pan paniscus), and chimpanzees (Pan troglodytes) and modestly common in blue monkeys (Cercopithecus mitis), callitrichids (Callithrix spp. and Saguinus spp.), and squirrel monkeys (Saimiri spp.). It is uncommon in other cercopithecines, rare in other haplorhines and in lemurs, and virtually absent in colobines. Birds are the prey class eaten by the most species (≥53), followed by reptiles (≥48), amphibians (≥38), mammals (≥35), and fish (≥7). Major hypotheses for the importance of meat eating are that it is (1) mainly an energy source, especially (1a) when plant-source foods (PSFs) with high energy return rates are scarce (energy shortfall hypothesis); (2) mainly a protein source; and (3) mainly a source of micronutrients scarce in PSFs. Meat eating bouts sometimes provide substantial energy and protein, and some chimpanzees gain substantial protein from meat monthly or annually. However, meat typically accounts for only small proportions of feeding time and of total energy and protein intake, and quantitative data are inconsistent with the energy shortfall hypothesis. PSFs and/or invertebrates are presumably the main protein sources, even for chimpanzees. Support is strongest for the micronutrient hypothesis. Most chimpanzees eat far less meat than recorded for hunter-gatherers, but the highest chimpanzee estimates approach the lowest for African hunter-gatherers. In fundamental contrast to the human predatory pattern, other primates only eat vertebrates much smaller than they are, tool-assisted predation is rare except in some capuchins and chimpanzees, and tool use in carcass processing is virtually absent. However, harvesting of small prey deserves more attention with reference to the archaeological and ethnographic record.

The zooarchaeology and paleoecology of early hominin scavenging

Questions about the timing, frequency, resource yield, and behavioral and biological implications of large animal carcass acquisition by early hominins have been a part of the "hunting-scavenging debate" for decades. This article presents a brief outline of this debate, reviews the zooarchaeological and modern ecological evidence for a possible scavenging niche among the earliest animal tissue-consuming hominins (pre-2.0 Ma), revisits some of the questions that this debate has generated, and outlines some ways to explore answers to those questions with evidence from the archaeological record.

Reproductive state and rank influence patterns of meat consumption in wild female chimpanzees (Pan troglodytes schweinfurthii)

An increase in faunivory is a consistent component of human evolutionary models. Animal matter is energy- and nutrient-dense and can provide macronutrients, minerals, and vitamins that are limited or absent in plant foods. For female humans and other omnivorous primates, faunivory may be of particular importance during the costly periods of pregnancy and early lactation. Yet, because animal prey is often monopolizable, access to fauna among group-living primates may be mediated by social factors such as rank. Wild chimpanzees (Pan troglodytes) across Africa habitually consume insects and/or vertebrates. However, no published studies have examined patterns of female chimpanzee faunivory during pregnancy and early lactation relative to non-reproductive periods, or by females of different rank. In this study, we assessed the influence of reproductive state and dominance rank on the consumption of fauna (meat and insects) by female chimpanzees of Gombe National Park, Tanzania. Using observational data collected over 38 years, we tested (a) whether faunivory varied by reproductive state, and (b) if high-ranking females spent more time consuming fauna than lower-ranking females. In single-factor models, pregnant females consumed more meat than lactating and baseline (meaning not pregnant and not in early lactation) females, and high-ranking females consumed more meat than lower-ranking females. A two-factor analysis of a subset of well-sampled females identified an interaction between rank and reproductive state: lower-ranking females consumed more meat during pregnancy than lower-ranking lactating and baseline females did. High-ranking females did not significantly differ in meat consumption between reproductive states. We found no relationships between rank or reproductive state with insectivory. We conclude that, unlike insectivory, meat consumption by female chimpanzees is mediated by both reproductive state and social rank. We outline possible mechanisms for these patterns, relate our findings to meat-eating patterns in women from well-studied hunter-gatherer societies, and discuss potential avenues for future research.

Speech-like rhythm in a voiced and voiceless orangutan call

The evolutionary origins of speech remain obscure. Recently, it was proposed that speech derived from monkey facial signals which exhibit a speech-like rhythm of ∼5 open-close lip cycles per second. In monkeys, these signals may also be vocalized, offering a plausible evolutionary stepping stone towards speech. Three essential predictions remain, however, to be tested to assess this hypothesis' validity; (i) Great apes, our closest relatives, should likewise produce 5Hz-rhythm signals, (ii) speech-like rhythm should involve calls articulatorily similar to consonants and vowels given that speech rhythm is the direct product of stringing together these two basic elements, and (iii) speech-like rhythm should be experience-based. Via cinematic analyses we demonstrate that an ex-entertainment orangutan produces two calls at a speech-like rhythm, coined “clicks” and “faux-speech.” Like voiceless consonants, clicks required no vocal fold action, but did involve independent manoeuvring over lips and tongue. In parallel to vowels, faux-speech showed harmonic and formant modulations, implying vocal fold and supralaryngeal action. This rhythm was several times faster than orangutan chewing rates, as observed in monkeys and humans. Critically, this rhythm was seven-fold faster, and contextually distinct, than any other known rhythmic calls described to date in the largest database of the orangutan repertoire ever assembled. The first two predictions advanced by this study are validated and, based on parsimony and exclusion of potential alternative explanations, initial support is given to the third prediction. Irrespectively of the putative origins of these calls and underlying mechanisms, our findings demonstrate irrevocably that great apes are not respiratorily, articulatorilly, or neurologically constrained for the production of consonant- and vowel-like calls at speech rhythm. Orangutan clicks and faux-speech confirm the importance of rhythmic speech antecedents within the primate lineage, and highlight potential articulatory homologies between great ape calls and human consonants and vowels.

Blood, bulbs, and bunodonts: on evolutionary ecology and the diets of Ardipithecus, Australopithecus, and early Homo

Beginning with Darwin, some have argued that predation on other vertebrates dates to the earliest stages of hominid evolution, and can explain many uniquely human anatomical and behavioral characters. Other recent workers have focused instead on scavenging, or particular plant foods. Foraging theory suggests that inclusion of any food is influenced by its profitability and distribution within the consumer's habitat. The morphology and likely cognitive abilities of Ardipithecus, Australopithecus, and early Homo suggest that while hunting and scavenging occurred, their profitability generally would have been considerably lower than in extant primates and/or modern human hunter-gatherers. On the other hand, early hominid diet modelers should not focus solely on plant foods, as this overlooks standard functional interpretations of the early hominid dentition, their remarkable demographic success, and the wide range of available food types within their likely day ranges. Any dietary model focusing too narrowly on any one food type or foraging strategy must be viewed with caution. We argue that early hominid diet can best be elucidated by consideration of their entire habitat-specific resource base, and by quantifying the potential profitability and abundance of likely available foods.

Detection of termites and other insects consumed by African great apes using molecular fecal analysis

The consumption of insects by apes has previously been reported based on direct observations and/or trail signs in feces. However, DNA-based diet analyses may have the potential to reveal trophic links for these wild species. Herein, we analyzed the insect-diet diversity of 9 feces obtained from three species of African great apes, gorilla (Gorilla gorilla gorilla), chimpanzee (Pan troglodytes) and bonobo (Pan paniscus), using two mitochondrial amplifications for arthropods. A total of 1056 clones were sequenced for Cyt-b and COI gene libraries, which contained 50 and 56 operational taxonomic units (OTUs), respectively. BLAST research revealed that the OTUs belonged to 32 families from 5 orders (Diptera, Isoptera, Lepidoptera, Coleoptera, and Orthoptera). While ants were not detected by this method, the consumption of flies, beetles, moths, mosquitoes and termites was evident in these samples. Our findings indicate that molecular techniques can be used to analyze insect food items in wild animals.

Broad-scale phylogenomics provides insights into retrovirus-host evolution

Genomic data provide an excellent resource to improve understanding of retrovirus evolution and the complex relationships among viruses and their hosts. In conjunction with broad-scale in silico screening of vertebrate genomes, this resource offers an opportunity to complement data on the evolution and frequency of past retroviral spread and so evaluate future risks and limitations for horizontal transmission between different host species. Here, we develop a methodology for extracting phylogenetic signal from large endogenous retrovirus (ERV) datasets by collapsing information to facilitate broad-scale phylogenomics across a wide sample of hosts. Starting with nearly 90,000 ERVs from 60 vertebrate host genomes, we construct phylogenetic hypotheses and draw inferences regarding the designation, host distribution, origin, and transmission of the Gammaretrovirus genus and associated class I ERVs. Our results uncover remarkable depths in retroviral sequence diversity, supported within a phylogenetic context. This finding suggests that current infectious exogenous retrovirus diversity may be underestimated, adding credence to the possibility that many additional exogenous retroviruses may remain to be discovered in vertebrate taxa. We demonstrate a history of frequent horizontal interorder transmissions from a rodent reservoir and suggest that rats may have acted as important overlooked facilitators of gammaretrovirus spread across diverse mammalian hosts. Together, these results demonstrate the promise of the methodology used here to analyze large ERV datasets and improve understanding of retroviral evolution and diversity for utilization in wider applications.

The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality

Recent data suggest that the human body is not such a neatly self-sufficient island after all. It is more like a super-complex ecosystem containing trillions of bacteria and other microorganisms that inhabit all our surfaces; skin, mouth, sexual organs, and specially intestines. It has recently become evident that such microbiota, specifically within the gut, can greatly influence many physiological parameters, including cognitive functions, such as learning, memory and decision making processes. Human microbiota is a diverse and dynamic ecosystem, which has evolved in a mutualistic relationship with its host. Ontogenetically, it is vertically inoculated from the mother during birth, established during the first year of life and during lifespan, horizontally transferred among relatives, mates or close community members. This micro-ecosystem serves the host by protecting it against pathogens, metabolizing complex lipids and polysaccharides that otherwise would be inaccessible nutrients, neutralizing drugs and carcinogens, modulating intestinal motility, and making visceral perception possible. It is now evident that the bidirectional signaling between the gastrointestinal tract and the brain, mainly through the vagus nerve, the so called “microbiota–gut–vagus–brain axis,” is vital for maintaining homeostasis and it may be also involved in the etiology of several metabolic and mental dysfunctions/disorders. Here we review evidence on the ability of the gut microbiota to communicate with the brain and thus modulate behavior, and also elaborate on the ethological and cultural strategies of human and non-human primates to select, transfer and eliminate microorganisms for selecting the commensal profile.

Mad, bad and dangerous to know: the biochemistry, ecology and evolution of slow loris venom

Only seven types of mammals are known to be venomous, including slow lorises (Nycticebus spp.). Despite the evolutionary significance of this unique adaptation amongst Nycticebus, the structure and function of slow loris venom is only just beginning to be understood. Here we review what is known about the chemical structure of slow loris venom. Research on a handful of captive samples from three of eight slow loris species reveals that the protein within slow loris venom resembles the disulphide-bridged heterodimeric structure of Fel-d1, more commonly known as cat allergen. In a comparison of N. pygmaeus and N. coucang, 212 and 68 compounds were found, respectively. Venom is activated by combining the oil from the brachial arm gland with saliva, and can cause death in small mammals and anaphylactic shock and death in humans. We examine four hypotheses for the function of slow loris venom. The least evidence is found for the hypothesis that loris venom evolved to kill prey. Although the venom's primary function in nature seems to be as a defense against parasites and conspecifics, it may also serve to thwart olfactory-orientated predators. Combined with numerous other serpentine features of slow lorises, including extra vertebra in the spine leading to snake-like movement, serpentine aggressive vocalisations, a long dark dorsal stripe and the venom itself, we propose that venom may have evolved to mimic cobras (Naja sp.). During the Miocene when both slow lorises and cobras migrated throughout Southeast Asia, the evolution of venom may have been an adaptive strategy against predators used by slow lorises as a form of Müllerian mimicry with spectacled cobras.

The costs and benefits of flexibility as an expression of behavioural plasticity: a primate perspective

Traditional neo-Darwinism ascribes geographical variation in morphology or in behaviour to varying selection on local genotypes. However, mobile and long-lived organisms cannot achieve local adaptation this way, leading to a renewed interest in plasticity. I examined geographical variation in orang-utan subsistence and social behaviour, and found this to be largely owing to behavioural plasticity, here called flexibility, both in the form of flexible individual decisions and of socially transmitted (cultural) innovations. Although comparison with other species is difficult, the extent of such flexibility is almost certainly limited by brain size. It is shown that brains can only increase relative to body size where the cognitive benefits they produce are reliably translated into improved survival rate. This means that organisms that are very small, face many predators, live in highly seasonal environments, or lack opportunities for social learning cannot evolve greater flexibility, and must achieve local adaptation through selection on specific genotypes. On the other hand, as body and brain size increase, local adaptation is increasingly achieved through selection on plasticity. The species involved are also generally those that most need it, being more mobile and longer-lived. Although high plasticity buffers against environmental change, the most flexible organisms face a clear limit because they respond slowly to selection. Thus, paradoxically, the largest-brained animals may actually be vulnerable to the more drastic forms of environmental change, such as those induced by human actions.

The real females of human evolution

When "woman-the-gatherer" was first proposed as a counter to "man-the-hunter," we were only beginning to understand the many faces of primate females. In ensuing decades we have learned about the skills and talents of female chimpanzees in tool using, hunting, and transmitting behaviors from one generation to the next as teachers, and across space as newcomers to neighboring communities. The perspective of evolutionary time highlights the continuity of female lives from the origin of mammals to the origin of Homo sapiens. The combination of behavioral, fossil, and archeological information aid in reconstructing key ingredients acquired by females along the way that contributed to the success of our species.