Disney’s Tarzan and The Jungle Book have one thing in common: abandoned children being raised by wild animals. Tarzan grew up alongside apes and Mowgli with wolves. While animals raising human children is not a typical sight in nature, some animals, known as brood parasites, abandon their young with other species.

What is Brood Parasitism?

Brood parasitism is a phenomenon in which certain animals—parasites—will lay their eggs in the nests of other animals (hosts), which can be of the same species or different ones. This is an evolutionary strategy for the parasites because it saves them the energy, time, and resources of raising their offspring. Rather than nesting, these animals look for food or lay even more eggs to expand their population. Brood parasitism has been observed in species of birds, fish, and insects. A brood parasite common to San Diego is the Brown-headed Cowbird. These birds lay their eggs in the nests of several songbird species, the Yellow Warbler being one example. An aquatic brood parasite is the cuckoo catfish: a species of fish that leaves its offspring to mouthbrooding cichlids, which are large and colorful freshwater fish like angelfish. Furthermore, lycaenid butterfly caterpillars (insect parasites) cocoon themselves in the nests of their host, a species of ant commonly known as the Japanese carpenter ant, and feed off of them while developing. As can be seen, brood parasitism exists across many different habitats and classes of organisms.

The Disadvantages and Advantages of Brood Parasitism in Ecosystems

This phenomenon has proven to be disadvantageous to host species. For example, researchers have documented that some of the hosts of the Brown-headed Cowbirds, like California songbirds Bell’s vireo and the willow flycatcher, have become endangered because of their parasitic relationship. Cowbirds introduce competition into the nests of their hosts. Because Cowbird eggs tend to hatch earlier than their hosts’ eggs, the young Cowbirds mature faster and are better able to compete for food (seeds and insects), thus indirectly killing many of the host chicks.

Additionally, brood parasitism has been found to reduce host clutch size (the number of eggs laid in a nest) and decrease host offspring hatching success. In a study on the Shiny cowbird’s effects on host Black-backed water tyrant, it was found that 54% of all the eggs in the 83 observed host nests were damaged or punctured by the Shiny cowbirds when they came to lay their eggs. As a result, 20 of these nests were abandoned by the Black-backed water tyrants, leaving any eventual hatchlings to die of either starvation or predation.

Despite the negatives of brood parasitism, it does have one clear benefit: it promotes evolution in both parasite and host species, which increases the biodiversity of ecosystems in the long term. Biological evolution is the process by which a population changes over time. The basis of these changes is in genetics. Differences in genes caused by mutations give certain population members an advantage in reproduction because they are better adapted to survive in their given environment. Thus, they live to reproduce and pass this gene off to their offspring, and over time, these advantageous genes will become highly prevalent in the population’s gene pool. These differences in genes that allow for evolution are known as genetic biodiversity. Moreover, evolution can result in the development of new species over time, which increases species biodiversity: the variety of organisms that are found in environments. Biodiversity is essential because it is necessary for the survival and proper functioning of all life on Earth. Without it, we would not have the water, food, and natural resources that are the basis of our society. This all relates to brood parasitism because research has shown that it results in the co-evolution of both the parasites and the hosts. For example, hosts have been found to reject parasite eggs in their nests, distinguishing them from their own by appearance. So, eggs that more closely resemble those of the hosts have survived, resulting in a change, or evolution, of the appearance of parasitic eggs throughout several generations. Hence, they are no longer rejected by the host. In response, host eggs have begun to evolve to avoid this mimicry. Another common host defense is parasite nestling recognition and ejection from the nest. These outsider chicks are usually determined via size, color, or vocalization.

Research has found that brood parasitism results in better evolutionary success for parasites than hosts because they have significantly lower extinction risks. However, brood parasitism may not be the evolutionary strategy it once was for parasites because of climate change. For example, changes in weather patterns cause the breeding patterns of some avian species to change. This is a problem for parasite species because if their reproductive timing is no longer in sync with their hosts, they will fail to reproduce. Climate change is also causing many host species to relocate and is threatening to extinct roughly two-thirds of North American birds. While parasites with several hosts like the Brown-headed Cowbird are at less risk because they have several host species, birds like the Channel-billed Cuckoo that depend on just a few are in trouble if their host species are unable to adapt fast enough in the wake of this rapidly-oncoming climate change. These extreme temperature fluctuations threaten the biodiversity of many ecosystems at extreme risk.

Brood parasitism is a natural phenomenon that is both helpful and harmful. Its ability to drive the co-evolution of both parasite and host diversity is vital because it increases an ecosystem’s overall biodiversity. But, when not left unchecked, brood parasitism can be extremely damaging. For example, extremely large Brown-headed Cowbird populations have pushed some of their host species near extinction, like the Kirtland’s Warblers and Southwestern Willow Flycatchers. Like everything in nature, brood parasitism requires a careful balance, which climate change is ruining. Thus, if we want to continue seeing biodiversity in the ecosystems that we depend on for life, we should prioritize taking action against climate change by practicing sustainability. However, substantial progress may only be achieved when our efforts are coupled with systematic policy-level changes.