Carnivore (diet) refers to any species that ingests animal tissues, either wholly or in part.
A carnivore is defined as an organism that consumes animal tissue. With specific reference to the carnivore diet, the term relates to foodstuffs ingested, digested, absorbed, and utilized by an organism and identifies that ingested foodstuff consists wholly, or in part, as tissues derived from other animal species.
The dietary niche inhabited by a given species is an important aspect of ecological classification. What nutrients an animal requires and how an animal acquires those nutrients constitute the dietary niche of a given species. Carnivory is viewed as one of the three major dietary classifications, the other two being omnivory and herbivory. The term carnivore is derived from the Latin carn, meaning “flesh,” and vor, “to devour,” and essentially means “flesh eating.” Consequently, carnivorous organisms obtain their dietary nutrient and energy requirements from animal tissues, either exclusively or in part. Those animal tissues can be insect (insectivore), invertebrate (invertivore), avian, lizard, amphibian, fish (piscivore), or mammalian in origin. Hematophagy (the consumption of blood) can also be viewed as a form of carnivory. Carnivory is viewed as forming the third trophic level, and carnivores are typically regarded as secondary consumers, although carnivores that prey and feed on other carnivorous species are typically classed as tertiary consumers and apex predators.
Evolution of the Carnivore Dietary Strategy
For one species to consume the tissues of another, specific adaptations are required to acquire, consume, and effectively digest the tissues of the ingested species. The consumption of animal tissues represents an effective form of both energy and nutrient acquisition. Indeed, carnivory has appeared across different taxonomic orders, in several mammalian clades (Van Valkenburgh 1999), and at multiple times during evolutionary history. The repeated appearance of carnivory as a dietary strategy demonstrates its success as a nutritional niche, with the consumption of animal tissues representing a nutrient-rich food source in terms of protein, fat, vitamins, minerals, and other key nutrients. Carnivory can also be viewed as an effective dietary strategy that did not necessitate the evolution of the highly specialized additional physical, physiological, and microbial adaptations typically needed to digest vegetation, as observed in the ruminants and hindgut fermenters (Stevens and Hume 1995).
However, the carnivore dietary strategy has been dependent upon the development of key evolutionary adaptations for successful predation, scavenging, ingestion, and processing of foodstuffs. Consequently, a range of physical, physiological, metabolic, and behavioral characteristics are observed across those species with a carnivorous diet. Indeed, the characteristic dentition of members of the mammalian order Carnivora, that includes enlarged canines and a carnassial apparatus, is also evident in the fossil record of the family Miacidae, the earliest members of this order that appeared over 40 Ma (Wang and Telford 1994). Indeed, evidence suggests that there is striking similarity in dental adaptations for carnivory in different groups of flesh-eating species, both extinct and extant (De Muizon and Lange-Badre 2007). There is now a wide variety of adaptations supportive of carnivory that can be identified in extant species, and the consumption of animal tissues remains widespread in nature.
Classification of Carnivores
Carnivorous organisms can be broadly classified based on the approximate percentage of animal-derived tissue in their diet. Contrary to popular belief, not all carnivores have a diet that exclusively consists of tissue derived from other animal species. Carnivorous species with greater than 70% of their diet consisting of animal tissue are defined as hypercarnivores; those with between 30% and 70%, mesocarnivores; and species with a dietary composition of less than 30% animal tissue, hypocarnivores (Van Valkenburgh 2007). It is also worthy to note that the term mesocarnivore is sometimes used to refer to carnivorous species that are of a medium morphological size, such as foxes and other canid species, for classification purposes.
The term carnivore is often used to refer to the mammalian order Carnivora, which includes the terrestrial flesh-eating mammals (suborder Fissipedia), such as canids and felids, and the aquatic mammals that feed on fish (suborder Pinnipedia), such as seals and walruses. However, a carnivore diet is not exclusive to this order, and equally, not all members of Carnivora are obligate hypercarnivores (i.e., they absolutely require animal tissues in their diet), and some demonstrate substantial adaptions for other dietary niches. The giant panda (Ailuropoda melanoleuca) is one such species, a specialist bamboo feeder that retains the innate characteristics of a carnivore digestion system but has evolved other characteristics associated with the digestion of cellulose, including a symbiotic microbiome population, capable of cellulose metabolism (Zhu et al. 2011).
There are also many species with a carnivorous dietary strategy that are not members of the order Carnivora, including birds, insects, lizards, amphibians, fungi, fish, and even some plants. Carnivorous plant species trap and digest insects and small animals as a source of nutrients and energy. These plant species demonstrate high levels of physiological and morphological similarity as adaptations in support of carnivory (Ellison and Gotelli 2009). Genomic analysis also reveals molecular similarities in carnivorous plant species, suggesting convergent evolution and limited evolutionary pathways toward plant carnivory (Fukushima et al. 2017).
Similarly, the class Insecta contains approximately 14,000 species known to feed on blood (Adams 1999). These organisms are highly adapted for blood feeding and are responsible for significant levels of morbidity and mortality in human and animal populations because of disease transmission, annoyance, and reduced productivity (Lehane 2005). Consequently, while carnivory is often associated with mammalian members of the order Carnivora, it is a dietary strategy not exclusive to that order, and the consumption of animal tissues can be observed in a much wider taxonomic range of species.
Indeed, evidence suggests that “opportunistic carnivory” is sometimes observed in species otherwise considered to be entirely herbivorous in their dietary strategy, without deleterious consequences. Domestic rabbits have been observed to consume animal carcasses suggesting both opportunity and behavioral adaptations (Clauss et al. 2015). Similarly, snowshoe hare (Lepus americanus) have been observed to scavenge from carcasses of lynx and other hare, possibly as a strategy to deal with dietary inadequacy, especially in winter months (Peers et al. 2018). Deer have also been noted to consume tissues and indulge in osteophagia (consumption/chewing of bone) from carcasses in an opportunistic manner (Meckel et al. 2017). Evidence of species not traditionally viewed as carnivorous, consuming animal tissues in this way, demonstrates the diversity of species in which a carnivore diet (either whole or in part) can be observed.
Obligate Carnivores and Facultative Carnivores
Clearly, the proportion of animal tissue in the diet of carnivores varies. Obligate carnivores are those species that depend entirely upon animal tissue for the supply of their nutrient requirements, whereas facultative carnivores will also consume and obtain nourishment from non-animal-derived material. The definition of facultative carnivore or omnivore remains obscure, with the amount of animal-derived dietary material versus plant-derived material not being fully defined. However, facultative carnivores typically demonstrate physical adaptations in support of carnivory, such as a relatively short, simple gastrointestinal tract, elongated canine teeth, and a carnassial apparatus that might not be observed in a more omnivorous species such as swine (Sus scrofa). Obligate carnivores (e.g., felids and mustelids) typically demonstrate specific metabolic and biochemical specializations that further limit their ability to survive without animal-derived material in their diet (Legrand-Defretin 1994). This has ramifications in considering ecological niche survival and in the formulation of diets for captive and companion animals, where obligate carnivores have very specific nutritional requirements that need to be fulfilled in prepared dietary provision.
Conversely, while facultative carnivore species typically retain physical and physiological characteristics of carnivory (dentition, simple digestive system, scavenging/hunting behavior), some species also demonstrate genetic signatures that are indicative of an evolutionary adaption to other sources of nutrition. This supports a nutritional ecological niche that includes the consumption of a range of foodstuffs and an ability to derive appropriate nourishment from them. The domestic dog (Canis lupus familiaris), for example, has a molecular signature supporting adaption to the presence of carbohydrate, specifically starch, in their diet (Axelsson et al. 2013). These findings support the development of specific dietary adaptations in otherwise carnivorous species, based on their ecological niche and parallel evolution with other species, including humans (Perry et al. 2007).
Nutritional Requirements of the Carnivore
The obligate carnivore absolutely requires animal tissue as a source of key nutrients in their diet. Correspondingly, these species (felids are particularly well documented and studied) demonstrate a biochemistry and metabolism with limited intrinsic biosynthesis of specific compounds. These species are metabolically adapted for higher levels of protein metabolism and a limited ability to metabolize dietary carbohydrates (Zoran 2002). Their requirement for maintenance of glucose levels in the body is predominantly met via gluconeogenesis from protein precursors, rather than via the provision of dietary carbohydrates. This contrasts with many facultative carnivores that appear to have evolved metabolic strategies and pathways that permit the biosynthesis of many key compounds from alternative precursors, meaning that a dietary source of animal-derived nutrients is not always essential for survival.
For example, felids are well described as lacking the ability to synthesize adequate levels of both arachidonic acid and vitamin A (MacDonald et al. 1984). Consequently, an adequate dietary supply of these substances must be provided. A diet based on animal-derived tissues typically fulfils these nutritional requirements. Similarly, the amino acid taurine is also essential in feline diets and fox diets. Taurine is critical for several normal bodily functions, and deficiency symptoms typically manifest as feline central retinal degeneration (FRCD), reproductive issues, impaired fetal development, and heart issues, notably dilated cardiomyopathy (DCM). Taurine biosynthesis is extremely limited in obligate carnivores, in contrast to most herbivores and omnivores (Chesney 1985), supporting their dietary need for taurine. This is a consequence of the feline liver having a limited enzymatic ability to convert the precursor amino acids methionine and cysteine to taurine, combined with an obligate loss of taurine via the feces and intestinal microbe degradation (Morris and Rogers 1992). Mammalian tissue is rich in taurine, especially the brain, heart, and skeletal muscle, meaning that obligate carnivores will typically obtain enough dietary levels, providing there is adequate consumption of animal-derived tissue.
However, taurine appears to be dispensable in the diets of facultative carnivores such as the domestic dog that ingest adequate amounts of sulfur-containing amino acids to support taurine biosynthesis (Kramer et al. 1995; Delaney et al. 2001). Conversely, diets low in protein (Sanderson et al. 2001) or low in bioavailable precursor amino acids (Backus et al. 2003) can result in taurine deficiency and the appearance of clinical signs such as DCM.
Similarly, the maned wolf (Chrysocyon brachyurus) is a largely frugivorous canid of South America and in captivity is typically fed a higher proportion of animal tissue than would naturally be encountered. However, problems with cystinuria in captive individuals and the feeding of formulated diets to reduce this risk resulted in severe reductions in plasma taurine concentrations (Childs-Sanford and Angel 2006). Taurine supplementation to such diets for the captive maned wolf is now indicated. More recent reports of cases of DCM in domestic dogs with a correlative link to taurine metabolism, apparently linked to the formulation of specific commercial diets (Freeman et al. 2018), have raised further discussion about the dietary need for taurine in facultative carnivores, although it is likely that a range of biological variables including physiological, metabolic, genetic, and dietary factors are involved, and further investigation is required.
Characteristics of the Carnivore for Diet Acquisition and Processing
Organisms with a carnivore dietary strategy demonstrate specific adaptations to enhance their ability to obtain dietary animal tissue and nourishment from it. These adaptations are characteristic of many carnivorous species and are useful classification tools. However, the range of species in which carnivory is observed does make generalizations difficult.
Animal tissue included in the diet of carnivores can be obtained either via hunting and predation or by scavenging. Specific physical, physiological, behavioral, and metabolic adaptations are observed in carnivorous species as a result and are functionally important for the acquisition of dietary material (e.g., by hunting) as well as the processing of animal tissue in the digestive tract. The dentition of vertebrate carnivores is a prime example of such adaption, with highly specialized teeth specialized for killing, ripping, tearing, bone crushing, and varied degrees of grinding ability. Notably, animals with a more omnivore-like dietary strategy, such as the domestic dog (Canis lupus familiaris), have more teeth (42 compared to 30) than obligate carnivores such as felids. This is attributed to an increased number of molars and premolars, permitting increased mechanical processing of ingested material. Genomic signatures of carnivores also indicate a clear adaptation for the consumption and digestion of animal tissue, alongside adaptations for successful predation and other associated attributes (Kim et al. 2016).
Carnivorous mammalian species demonstrate flexibility in the diversity of prey consumed and thus in the adaptations required. Species that are hypercarnivorous (such as the polar bear, Ursus maritimus) tend to be large, show specific predatory adaptations and higher hunting energetic costs (Carbone et al. 2007), and demonstrate dentition capable of dealing with the processing of bones and other difficult tissues, as seen with early hyaenid carnivores (Werdelin 1989). Species that tend toward a hypocarnivorous strategy are typically small to medium-sized and often undertake frequent, solitary hunting behavior. Small carnivores appear to eat to gut capacity regularly, in contrast to larger species who typically demonstrate reduced predation frequency because of both gut capacity and increased prey size (De Cuyper et al. 2019), revealing that both ecological (including temporal and spatial) and physiological factors impact on dietary choices.
For carnivores that are dependent in hunting for prey acquisition, physiological characteristics permitting strength, sensory awareness, and speed are critical, in addition to behavioral adaptations likely to enhance hunting success. Behavioral adaptations may take the form of cooperative hunting strategies or solitary strategies. Scavenging behaviors can also be observed in species that do not hunt or in an opportunistic way by those that would otherwise hunt for prey.
The typical mammalian carnivore digestive system is short and simple in contrast to the highly sacculated and specialized digestive systems observed in many herbivorous species. This corresponds to the high digestibility of the carnivore’s diet compared to herbivores and the lack of requirement for prolonged retention of ingesta or significant levels of microbial fermentation. The stomach is considered simple but is often capable of expansion to permit the ingestion of larger prey items (Hume 2002). The small intestine is typically short, although it is notably longer in marine carnivores compared to terrestrial carnivores (Stevens and Hume 1995). If present, the caecum tends to be small, and the colon is also characteristically short.
In other species, such as carnivorous birds, amphibians, reptiles, and fish, the digestive systems are also simple and short, reflective of the high digestibility of ingesta. Carnivorous birds (especially raptors) have specialized beaks and talons to aid the separation of indigestible portions of prey or scavenged material prior to ingestion. Where indigestible material is consumed by carnivorous birds such as owls, it is separated in the gizzard and then egested as a formed “pellet” (Klasing 1998).
The carnivore diet is one that consists of tissues from other animals, either wholly or in part. Those tissues can be obtained via predation or scavenging. The carnivore dietary strategy is observed in a wide range of taxonomic orders and appears to have arisen multiple times during evolutionary history. As a dietary strategy, carnivory represents a dietary niche that exploits and utilizes highly digestible foodstuffs and requires few specific system adaptions to support prolonged retention, microbial fermentation, and other mechanisms to permit the digestion of otherwise indigestible material, as seen in many herbivorous species.
Predation is associated with the development of specific behavioral and physiological adaptations to enhance hunting success, although for many predators, prey availability is affected by temporal, spatial, and other ecological factors. Many predators are obligate carnivores and require animal tissues to supply key nutrients that cannot be synthesized within the body. Consequently, many of these species would be classed as hypercarnivores, with animal-derived tissues forming most of their dietary intake. Scavenging behavior is also observed in many species that actively ingest other animal tissues and can be either on an opportunistic or intentional basis, occasionally in species that would otherwise be unexpected to actively consume animal tissues.
Carnivory is a successful dietary strategy. It is observed across a wide diversity of species, which demonstrate a wide range of specific adaptions to support this life and diet strategy. Similarities are evident however, even across orders, at the physical, molecular, and nutrient requirement levels.
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