Kleptoparasitism, meaning “parasitism by theft,” is an ecological interaction where one animal deliberately steals food, nest materials, or other valuable resources that another animal has already acquired. This behavior is a widespread feeding strategy across the animal kingdom, serving as a shortcut that allows the thief, or kleptoparasite, to bypass the energy and risk associated with hunting or foraging. It is found in species ranging from insects to seabirds and large mammals, demonstrating an evolutionary solution to the constant pressure of resource acquisition. The strategy is often adopted when the cost of stealing is lower than the cost of obtaining the resource directly.
Defining the Theft
Kleptoparasitism is distinct from traditional parasitism, which typically involves an organism exploiting a host’s body tissues or internal resources. The defining feature of this behavior is that the kleptoparasite targets resources external to the victim, such as captured prey, stored food caches, or construction materials like nest rocks or grass. This interaction is not a form of predation, because the thief takes the resource from another animal that has already expended the effort to secure it.
This theft can occur between members of the same species, known as intraspecific kleptoparasitism, or between different species, called interspecific kleptoparasitism. For example, juvenile African black oystercatchers steal mussels cracked open by adults because the young birds lack the strength and skill to open the shells themselves. The behavior is also separate from brood parasitism, where the victim is tricked into raising the parasite’s young. However, the cuckoo bee (genus Nomada) is a true kleptoparasite because its larva consumes the pollen and nectar provisions stored by a host bee for its own offspring.
Strategies of the Thief
Kleptoparasites employ a range of specialized tactics to secure their stolen goods, which can be categorized by their level of aggression and visibility. One common approach is overt theft, which relies on direct aggression, force, or intimidation. This often involves a high-speed chase or a physical confrontation with the victim to force them to abandon their catch.
A contrasting approach is clandestine theft, where the kleptoparasite relies on stealth to take a resource without alerting the victim. Certain species of flies in the subfamily Miltogramminae, for instance, sneak into a spider’s web to consume the captured, paralyzed prey while the spider is distracted. This method minimizes the risk of injury but requires precise timing and a low profile.
A complex strategy involves deception, particularly seen in the African fork-tailed drongo (Dicrurus adsimilis). This bird follows ground-foraging animals like meerkats. When the host captures a large prey item, the drongo emits a false alarm call, often mimicking the specific alarm of the victim species. The startled host flees to cover, dropping the food, which the drongo then quickly swoops in to retrieve. The drongo’s ability to switch between its own alarm call and the mimicked calls of up to 51 different species helps maintain the effectiveness of its deceptive strategy.
Notable Examples in the Animal Kingdom
The diversity of kleptoparasitism is evident across multiple classes of animals, with some species being opportunistic thieves and others being obligate specialists. In the avian world, the magnificent frigatebird (Fregata magnificens) harasses boobies and tropicbirds in mid-air until the victim regurgitates its meal. Similarly, skuas and jaegers persistently chase gulls and terns, forcing them to drop or vomit their fish, which the kleptoparasite catches before it hits the water.
Mammalian examples involve the stealing of large kills between competing carnivores, such as the dynamic between spotted hyenas and lions. While hyenas are known to steal from smaller predators, lions will also readily appropriate kills from hyenas, demonstrating a fluid, reciprocal form of kleptoparasitism. On the coast, the great black-backed gull (Larus marinus) targets diving birds like puffins, accessing deep-water prey that the gull lacks the ability to hunt itself.
In the arachnid world, the small, silver-colored dewdrop spiders (Argyrodes) live almost exclusively in the webs of larger, orb-weaving spiders. These tiny thieves survive by sneaking up on captured insects and cutting away pieces of the prey, or even consuming the silk-wrapped egg sacs of the host spider.
Costs and Benefits
Kleptoparasitism is an evolutionary trade-off for the thief, offering significant energy savings but carrying inherent risks. The primary benefit is the reduction in time and energy that would otherwise be spent on searching, pursuing, and subduing prey. For a facultative kleptoparasite, like the bald eagle, stealing a fish from an osprey is a much more efficient way to secure a meal than hunting for itself. For obligate species, like the pea crab, it is the only way to obtain food.
However, the strategy is metabolically expensive and dangerous, as the kleptoparasite must often engage in high-energy pursuits or risk injury from a victim defending its property. The victim species, or host, faces the cost of lost resources, which translates into an increased foraging effort and a potential reduction in overall survival or reproductive success. This pressure drives the evolution of counter-strategies, such as gulls learning to swallow their prey more quickly, or African wild dogs forming tighter groups to better defend their catches from hyenas.