Carnivory

Abstract

Carnivory is a form of animal behavior—specifically a feeding behavior—the habit of obtaining nutrients by consuming the tissue of other animals. This article treats carnivory not as a fixed label on a species but as the outcome of a foraging decision: an animal (or, remarkably, a plant) adopts a meat diet when the energy and nutrients returned by capturing and processing animal tissue exceed the costs of searching for, subduing, and handling it. It covers the economics of a flesh diet through optimal-foraging theory, the functional response that governs how intake scales with prey abundance, the energetic body-size constraints that separate small-prey from large-prey hunters, the convergent evolution of carnivory in plants on nutrient-poor soils, and the ecological reach of large carnivores through trophic cascades. A worked example and three interactive demonstrations accompany the text.

Keywords: carnivory, optimal foraging, functional response, prey choice, carnivorous plants

A lion ignores a hare it could easily catch and waits for a zebra. A shrew, by contrast, must eat almost its own body weight in insects every day or starve. A Venus flytrap, rooted in a sunlit bog, snaps shut on a fly and digests it for the nitrogen the soil cannot supply. Each is carnivorous, yet the decision each faces is different, and the difference is not arbitrary: it follows from the energy a meal returns set against what it costs to obtain.

This article treats carnivory as a feeding strategy governed by cost and benefit rather than as a taxonomic category. It begins with what the category covers and why eating other animals should be analyzed as an economic choice, then develops the optimal-foraging account of prey choice, the functional response linking intake to prey density, and the energetic constraints that body size imposes. It then turns to the independent, convergent evolution of carnivory in plants, and closes with the outsized ecological role of large carnivores. Throughout, being a carnivore is read as a solution to a nutritional problem, not a label fixed in advance.

What Carnivory Is

Carnivory is the practice of feeding on the tissue of other animals. In the Medical Subject Headings vocabulary it is classified as a feeding behavior, itself a form of animal behavior—a placement that treats it as one dietary option among several rather than a defining essence of a species. Many animals are facultatively carnivorous, eating meat when it is available and plant matter when it is not, and the interesting question is rarely whether a species can eat meat but when doing so pays.

Analyzing carnivory as a decision is the contribution of optimal-foraging theory, which models a forager as though it were maximizing the net rate of energy intake. Robert MacArthur and Eric Pianka's founding paper framed the problem as the optimal use of a patchy environment, asking which prey types a predator should pursue and which it should pass over to keep searching (MacArthur & Pianka, 1966). Eric Charnov formalized the patch-use side of the same logic in the marginal-value theorem, which specifies when a forager should abandon a depleting patch and move on (Charnov, 1976). Graham Pyke's later review assessed how well this body of theory survived contact with data, and found its core prey-choice predictions broadly supported (Pyke, 1984).

Read this way, the question is never simply whether an animal is a carnivore but which prey it should take, how its intake changes as prey become common or rare, and what its own body size will and will not allow it to eat. The sections that follow take those questions in turn.

The Economics of Prey Choice

The central result of optimal-foraging theory is counterintuitive and precise. Suppose a predator encounters two kinds of prey: a profitable type that returns much energy per unit of handling time, and a marginal type that returns less. When the predator meets a marginal item, it faces a choice—eat it now, or ignore it and keep searching for a profitable one. The right choice depends on a comparison between the energy rate of eating the marginal item and the energy rate expected from continuing to search for the profitable type (MacArthur & Pianka, 1966; Pyke, 1984).

Working the comparison through yields the zero–one rule: a prey type should either always be taken on encounter or never be, with no partial preference, and the decision for the marginal type hinges on the abundance of the profitable type alone. When profitable prey are common, the expected rate from specializing is high, so the predator should ignore marginal prey; when profitable prey are scarce, that rate falls below what the marginal type offers, and the predator should broaden its diet to include it. Strikingly, the abundance of the marginal type does not enter the decision at all—a prediction that distinguishes optimal-foraging theory sharply from the intuition that animals eat whatever they most often encounter (Charnov, 1976; Pyke, 1984). The worked example below makes the threshold explicit.

The Functional Response

Prey choice decides what to eat; the functional response describes how much gets eaten as prey become more abundant. C. S. Holling's 1959 analysis established that a predator's intake rate does not rise without limit as prey density climbs, because every captured item must be handled—subdued, consumed, digested—and handling takes time during which no searching occurs (Holling, 1959). The result is Holling's disc equation, in which the number of prey eaten per unit time rises steeply at low prey density but bends over to a plateau as handling time comes to dominate the predator's budget.

This saturating, or Type II, response has far-reaching consequences. Because intake levels off, a predator cannot track an exploding prey population indefinitely, which limits its capacity to regulate prey numbers at high density and shapes the stability of the whole predator–prey interaction. The same handling-time logic connects directly to prey choice: the profitability of a prey type—its energy divided by its handling time—is exactly the quantity the diet model compares, so the functional response and the prey-choice rule are two faces of one economic account of the carnivore's time budget (Holling, 1959; Charnov, 1976).

Body Size and the Energetics of a Meat Diet

A carnivore's body size dictates both what it costs to live and what it is able to catch, and the two scale differently. Chris Carbone and colleagues showed that terrestrial carnivores fall into two energetic classes separated by a threshold at roughly 21.5 kilograms of body mass. Below it, a predator can meet its needs on small, invertebrate and small-vertebrate prey; above it, the energy returned by small prey can no longer cover the cost of a large body, and the animal is forced to hunt vertebrate prey near or above its own size (Carbone et al., 1999). This is why there are no cat-sized animals living on insects alone and no lion-sized animals living on mice: the arithmetic of intake against metabolic cost forbids it.

The same energetic logic scales up from the individual to the population. Carbone and John Gittleman found a common rule linking carnivore population density to the biomass of available prey: across species spanning several orders of magnitude in size, about the same quantity of prey biomass supports a given mass of predator, so large carnivores are necessarily rare (Carbone & Gittleman, 2002). Over evolutionary time, the demands of a flesh diet have repeatedly sculpted the same feeding apparatus—slicing carnassial teeth, powerful jaws—in lineage after lineage. Blaire Van Valkenburgh's paleobiological work documents this convergence, showing that hypercarnivorous forms evolved again and again in the Carnivora, often followed by extinction, in a recurring evolutionary cycle (Van Valkenburgh, 2007). Modern quantitative methods can now read diet directly from morphology: Silvia Pineda-Munoz and colleagues infer diet, carnivory included, from the three-dimensional shape of mammalian teeth (Pineda-Munoz et al., 2017).

Carnivory in Plants

Carnivory is not confined to animals. More than a dozen plant lineages have independently evolved the ability to attract, trap, kill, and digest animals—chiefly insects—and to absorb the nutrients released. Thomas Givnish and colleagues explained why with a cost–benefit model: building and running traps is expensive, and the investment pays only where the nutrients gained (principally nitrogen and phosphorus) are severely limiting yet light and water are abundant. That is precisely the condition of sunny, moist, nutrient-poor bogs, which is why carnivorous plants are concentrated there and nowhere else (Givnish et al., 1984). Aaron Ellison and Nicholas Gotelli later synthesized the evolutionary ecology of these plants, treating botanical carnivory as a textbook case of convergent evolution under a shared cost–benefit regime (Ellison & Gotelli, 2001).

That plants and animals arrive at carnivory by the same economic route—trapping animal tissue is worthwhile exactly when its nutritional return beats the alternatives—is the strongest evidence that carnivory is a strategy selected by cost and benefit rather than a trait belonging to any one kind of organism. The recent molecular work on how plant traps actually function is taken up in Current Directions below.

Large Carnivores and Trophic Cascades

The ecological consequences of carnivory extend far beyond the predator and its immediate prey. By killing and intimidating herbivores, large carnivores shape the abundance and behavior of the species below them, and those effects ripple down through the food web in what ecologists call a trophic cascade. William Ripple and colleagues reviewed the status and ecological effects of the world's largest carnivores and found that their decline—most are threatened or in retreat—releases herbivore populations and transforms vegetation, soils, and even watercourses across whole landscapes (Ripple et al., 2014). The carnivore's foraging decisions, aggregated over a population, thus become a force structuring the entire ecosystem, and their loss is felt well beyond the animals they once ate.

Worked Example

The prey-choice rule looks paradoxical—why should whether a predator eats a marginal prey type depend on how common a different prey type is?—until the time budget is written down. Consider a predator that encounters a profitable prey type 1 returning E₁ = 8 energy units with a handling time h₁ = 1, and a marginal type 2 returning E₂ = 3 units with handling time h₂ = 1. The profitability of each item is its energy over its handling time: 8 for type 1, and 3 for type 2.

If the predator specializes on type 1 alone, encountering it at rate λ₁ per unit of search time, its net intake rate is

R₁ = (λ₁·E₁) / (1 + λ₁·h₁).

The predator should add type 2 to its diet only when eating a type-2 item on encounter beats continuing to search for type 1—that is, only when E₂ / h₂ > R₁. Setting the two equal gives the threshold encounter rate for the profitable prey:

λ₁\ = E₂ / (h₁(E₁ − E₂ ·h₁/h*₂)) = 3 / (8 − 3) = 0.6.

Below λ₁ = 0.6 the predator should generalize and take both types; above it, it should specialize and ignore type 2. Put in numbers:

When profitable prey are scarce (λ₁ = 0.4), specializing returns only 2.29 units per unit time—less than the 3.0 a type-2 item delivers—so the predator should take type 2. When profitable prey are abundant (λ₁ = 1.0), specializing returns 4.0, and stopping to handle a type-2 item would only slow the predator down, so it should pass type 2 by. Nowhere did the abundance of type 2 enter the calculation: the decision to include the marginal prey is set entirely by how common the profitable prey is (MacArthur & Pianka, 1966; Charnov, 1976). The OptimalDietDemo above lets the reader vary the encounter rate of profitable prey and watch the optimal diet switch at the threshold.

Discussion

The forms of carnivory treated here share one logic: an organism eats animal tissue when the energy and nutrients returned exceed the costs of obtaining it, and the balance is always struck in a common currency of rate and reward. The prey-choice rule, the functional response, and the body-size threshold are three expressions of the same time-and-energy budget—what to take, how fast it can be taken, and what a given body can afford to hunt. Optimal-foraging theory is what unifies them, because it puts every one of these questions under a single economic account rather than treating each as a separate natural-history fact (MacArthur & Pianka, 1966; Pyke, 1984).

That the same cost–benefit account reaches all the way to plants is the strongest evidence that carnivory is a strategy rather than an essence. A bog plant and a big cat have nothing in common anatomically, yet both are carnivorous for the same reason: under their particular conditions, capturing and digesting animal tissue returns more than the alternatives (Givnish et al., 1984; Carbone et al., 1999). Read as a decision rather than a label, carnivory becomes predictable—switched on where the nutritional arithmetic favors it and absent where it does not.

Cognitive Implications

For cognitive science, carnivory is a compact case study in decision-making under time and energy constraints. A foraging predator is, in effect, solving an optimization problem in real time: it must assess the profitability of an encountered item, estimate the rate it could expect by continuing to search, and commit to attack or to pass. The zero–one rule is a decision threshold, and the empirical finding that animals' diets shift as that threshold predicts implies a cognitive machinery for tracking prey encounter rates and comparing expected payoffs—the same class of problem studied in human choice under uncertainty (MacArthur & Pianka, 1966; Pyke, 1984).

The marginal-value theorem poses a second decision problem of the same family: when to leave. A predator exploiting a patch of depleting prey must judge the moment at which the diminishing return from staying falls to the average return available elsewhere, a computation over experienced and expected reward that has direct analogues in models of how animals and people allocate time among options (Charnov, 1976). Reading carnivory as a sequence of such decisions—what to eat, how long to stay, when to move—is what reveals the information-processing problems the forager is continually solving.

Current Directions

The most active recent work on carnivory is the molecular dissection of how carnivorous plants actually trap and digest prey. Jennifer Böhm, Rainer Hedrich, and colleagues showed that the Venus flytrap counts: a single touch of a trigger hair primes the trap, a second within about twenty seconds snaps it shut, and further prey-induced action potentials are tallied to switch on the genes for digestive enzymes and sodium uptake, so the plant invests in digestion only once it has electrically confirmed live, struggling prey (Böhm et al., 2016). This is decision-making by an organism with no nervous system, implemented in membrane excitability.

Comparative genomics has since traced where the machinery came from. Gergo Palfalvi and colleagues sequenced the genomes of the Venus flytrap and its close relatives and found that plant carnivory was assembled by repurposing ancient defense and root genes—the same pathways that elsewhere fend off herbivores or absorb soil nutrients—rather than by inventing new ones (Palfalvi et al., 2020). Rainer Hedrich and Kenji Fukushima's review synthesizes this molecular physiology and evolution into a general account of how a plant comes to live on an animal diet (Hedrich & Fukushima, 2021). The open questions now are how often, and by what routes, these pathways were recruited across the independent origins of botanical carnivory.

Common Misconceptions

A carnivore is simply any animal that can eat meat.

Carnivory is better understood as a diet an animal adopts when it pays, not a fixed essence. Many animals are facultative, eating meat or plants as conditions change, and optimal-foraging theory predicts the switch from the energy returns and costs involved (MacArthur & Pianka, 1966; Pyke, 1984).

A predator eats whatever prey it encounters most often.

The zero–one rule says otherwise: whether a predator takes a low-value prey type depends on the abundance of the high-value type, not on how often the low-value type is met. The marginal prey's own abundance does not enter the decision (Charnov, 1976; Pyke, 1984).

More prey always means a predator eats proportionally more.

Handling time caps intake. Holling's functional response shows that the number of prey eaten per unit time saturates at high prey density, because time spent subduing and consuming prey cannot also be spent searching (Holling, 1959).

Only animals can be carnivorous.

More than a dozen plant lineages trap, kill, and digest animals. They evolved carnivory independently on sunny, moist, nutrient-poor soils, where the cost of building traps is repaid by scarce nitrogen and phosphorus—the same cost–benefit logic that governs animal carnivory (Givnish et al., 1984; Ellison & Gotelli, 2001).

Glossary

Carnassial teeth.

Blade-like cheek teeth adapted for slicing flesh, a feeding apparatus that has evolved convergently in the Carnivora.

Carnivorous plant.

A plant that attracts, traps, kills, and digests animals to absorb their nutrients, typically on nutrient-poor soils; an independent, convergent origin of carnivory.

Carnivory.

The habit of obtaining nutrients by consuming the tissue of other animals; in MeSH, a feeding behavior and a form of animal behavior.

Convergent evolution.

The independent evolution of similar traits in unrelated lineages under similar selection; carnivory in plants and in several animal groups is a leading example.

Disc equation.

Holling's expression for the functional response, in which intake rises with prey density but bends to a plateau as handling time comes to dominate the predator's budget.

Encounter rate.

The frequency with which a searching predator meets a given prey type; in optimal-foraging theory it is the abundance of profitable prey, not marginal prey, that sets diet choice.

Functional response.

The relationship between a predator's per-capita intake rate and prey density; the Type II form rises then saturates as handling time dominates.

Handling time.

The time a predator spends subduing, consuming, and digesting a captured prey item, during which it cannot search for more.

Hypercarnivore.

An animal whose diet is more than about 70% meat, often with specialized slicing dentition; such forms have evolved repeatedly in the Carnivora.

Marginal-value theorem.

Charnov's rule specifying when a forager should leave a depleting patch: when its intake rate there falls to the average rate available in the environment.

Optimal-foraging theory.

The framework that models foragers as maximizing net energy intake rate, from which prey-choice and patch-use predictions follow.

Profitability.

The energy a prey item returns divided by its handling time; the quantity the diet model compares across prey types.

Trophic cascade.

An indirect effect in which predators, by controlling herbivores, shape the abundance of plants and other organisms further down the food web.

Zero–one rule.

The optimal-foraging prediction that a prey type should be either always taken on encounter or never taken, with the decision set by the abundance of more profitable prey.

Key Researchers

Chris Carbone

(Institute of Zoology, Zoological Society of London). Macroecologist whose work on the energetic constraints of carnivore diet established the body-size threshold above which predators must hunt large prey, and the rule linking carnivore density to prey biomass. ORCID

Aaron M. Ellison

(Harvard Forest, Harvard University). Ecologist whose synthesis with Gotelli framed botanical carnivory as a convergent, cost–benefit nutritional strategy. ORCID

Nicholas J. Gotelli

(University of Vermont). Community ecologist and co-author of the standard review of the evolutionary ecology of carnivorous plants. ORCID

C. S. Holling

(University of Florida; 1930–2019). Ecologist whose 1959 functional-response framework described how a predator's intake saturates with prey density, a foundation of predation theory.

Robert H. MacArthur

(Princeton University; 1930–1972). Evolutionary ecologist whose 1966 paper with Pianka founded optimal-foraging theory, the economic framework for diet choice.

William J. Ripple

(Oregon State University). Ecologist whose 2014 review documented the status and trophic-cascade effects of the world's largest carnivores. ORCID

Frequently Asked Questions

What is carnivory?

Carnivory is the feeding behavior of obtaining nutrients by consuming the tissue of other animals. The Medical Subject Headings vocabulary classifies it as a feeding behavior, a form of animal behavior, because for most animals it is one dietary option taken when it pays rather than a fixed, defining trait (MacArthur & Pianka, 1966).

How does an animal choose what to eat?

Optimal-foraging theory models a predator as maximizing its net rate of energy intake. On encountering a prey item it compares the energy that item returns per unit of handling time against the rate it could expect by ignoring the item and continuing to search for better prey (MacArthur & Pianka, 1966; Pyke, 1984).

Why do predators ignore prey they could easily catch?

Because of the zero–one rule: when profitable prey are abundant, stopping to handle a low-value item only slows the predator down, so passing it by yields a higher overall intake rate. The low-value prey is ignored even though it is easy to catch (Charnov, 1976; Pyke, 1984).

Does a predator eat more when prey become more abundant?

Up to a point. Holling's functional response shows intake rising with prey density but then leveling off, because each captured prey must be handled, and handling time caps how many can be eaten no matter how plentiful prey become (Holling, 1959).

Why are there no insect-eating animals the size of a lion?

Energetics forbids it. Above a body mass of about 21.5 kilograms, the energy returned by small prey can no longer cover the cost of a large body, so large carnivores are forced to hunt vertebrate prey near their own size (Carbone et al., 1999).

Why are large carnivores so rare?

Because a roughly fixed quantity of prey biomass supports a given mass of predator, the larger the carnivore the fewer individuals a landscape can sustain. Carbone and Gittleman found this scaling to be a common rule across carnivore species (Carbone & Gittleman, 2002).

How can a plant be carnivorous?

More than a dozen plant lineages independently evolved traps to catch and digest animals for their nitrogen and phosphorus. It pays only on sunny, moist, nutrient-poor soils, where the cost of the traps is repaid by otherwise unavailable nutrients (Givnish et al., 1984; Ellison & Gotelli, 2001).

Why does the loss of large carnivores matter for whole ecosystems?

Large carnivores control herbivores, and through them the plants, soils, and waterways below. When carnivores decline, herbivore populations are released and vegetation and landscapes change—a trophic cascade documented across the world's largest predators (Ripple et al., 2014).

References