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The Eggs-traordinary tale of Brood Parasitism

How obligate parasites can cause co-evolutionary arms races.

Being a parent can be one of the most rewarding, but paradoxically, thankless jobs available. Besides a cheesy “World’s #1 Dad/Mom” mug, parents don’t always get as much appreciation as may be deserved. In the Animal Kingdom, some parents have found an expedient to exploit in which to make being a “parent” as easy as possible; these animals decide to let someone else do the parenting for them. Certain animals called brood parasites hide their egg(s) inside the brood of another animal. For instance, the European Cuckoo is famous for laying its eggs in the nests of other birds, even coevolving to make their phony eggs look almost identical compared to the real ones. However, don’t get this concept confused with adoption: Brood parasites hide their own eggs in with other eggs then the parasite’s eggs kill or starve out the actual brood. These 100+ species of brood parasites, often dubbed ‘Cuckoos’, have ignited an evolutionary arms race between themselves and their hosts1.

In a biological sense, a parasite is an animal that takes advantage of another animal (the host) while providing nothing in return. This can include more classic parasites like mosquitoes or tapeworms, or, in this case, brood parasites. The basic concept of a brood parasite is virtually the same in all species that utilize this form of parasitism. First, a parasitic mother lays an egg inside a preexisting brood1. Then, the parasite’s egg hatches before the rest of the eggs and the hatchling destroys the remaining brood1. With the nest empty, the parasite’s child will get 100% of the host mothers’ attention, allowing it to grow rapidly1.

Inside crowded lakes in Africa, raising young can be very challenging because of constant predator spectators waiting for an opportunity to gorge on an unexposed piscine brooding ground. However, certain fish called cichlids have developed a unique way of protecting their young from these predators. By keeping their eggs and young in their mouth, they can easily protect them from the outside world. Unfortunately, certain brood parasites have found a way of taking advantage of the unique way the cichlids raise their young2.


Mouth-Brooding Cichlid. Image from Wikipedia


When a cichlid must leave its eggs unattended to go and procure food for itself, the Cuckoo Catfish swoops into the nest and sabotages the brood. By eating some of the eggs than replacing the destroyed portion with her own eggs, the Cuckoo Catfish can fool the Cichlid into rearing and caring for the parasitic eggs2. Normally, cichlid eggs hatch at relatively similar times, but the catfish’s eggs hatch much sooner than the rest of the eggs. In an act of pure fratricide, the invader catfish hatchlings devour every other non-parasitic egg in the brood2. By doing this, not only do the catfish hatchlings get a nice meal, but they have guaranteed similar nice meals until adulthood2. The cichlid will give the few catfish’s offspring the same attention she would give an entire brood, and this allows the remaining babies to mature much faster normal2. And, after maturing, the Cuckoo Catfish leave the nest and start taking advantages of other vulnerable cichlids. However, Cuckoo Catfish are obligate parasites2. This means that they have completely lost the ability to raise their own young and their only way of nursing is to parasitize cichlids. This means if cichlid populations plummet due to over-parasitism, the Cuckoo Catfish follow soon after. This boom-and-bust pattern of catfish and cichlid population helps keeps Cuckoo Catfish’s parasitism in check2.

Brood parasitism is also found in insects and birds. But, in birds, Cuckoos have started an evolutionary arms race in which evolutionary progress on one side provokes a further response on the other side3. The host, the target of the Cuckoo, evolves to defend against brood parasitism, while the parasite evolves to counteract the defenses set in place by the host3. This has been dubbed ‘pairwise coevolution’4.


European Common Cuckoo. Image from British Broadcasting Corporation.


For instance, Reed Warblers are often parasitized by European Cuckoos4. As an evolutionary response, the host Reed Warblers started using a distinct alarm call whenever a Cuckoo was spotted. This made it almost impossible for the Cuckoos to lay eggs in the nests of Reed Warblers4. Eventually, the European Cuckoo was able to evolve to look like a European raptor, via Batesian mimicry4. When the European Cuckoo bird tried to parasitize, instead of setting off the distinct Cuckoo alarm, the Reed Warblers just flew away from the apparent bird or prey, but it was merely a Cuckoo in disguise4. This mimicry has allowed the Cuckoos to easily parasitize the Reed Warblers4.


REed warbler.jpg
Reed Warbler. Image from Wikipedia.


Another defense mechanism that birds use to defend against brood parasites is egg detection. For instance, the Pied Wagtail can keenly remember what their eggs look like by detecting patterns5. If the there are too many eggs in its nest when it returns from feeding, the Pied Wagtail will reject and eject the phony egg from its nest. However, many birds are not advanced enough to have such amazing egg detection skills5. Some birds like the Superb Fair Wren use a more unique approach: The wrens will teach their eggs a special chirp so only the eggs that have been with the wren the whole time will repeat the chirp5. This allows Superb Fair Wrens to teach their offspring complicated calls and if an offspring does not repeat it, it can know that is a cuckoo and will avoid feeding it5.

But if no defense system works, the native eggs and chicks are often terrorized by brood parasites. For instance, if the European Cuckoo is successful, the Cuckoo’s egg will hatch earlier and will push the unhatched eggs out of the nest3. In an even more gruesome manner, Honeyguides will lay their eggs in another bird’s nest but then peck the native eggs to death, ensuring that the only egg that could hatch is the invader3.

But even if a bird does reject an egg it finds to be counterfeit, sometimes the upset Cuckoo mother will come back and destroy the host’s nest if it realizes it has killed the egg3. In many instances, it is better just to raise the Cuckoo’s kid for the hope that one of its true children survives.

It is easy to take for granted how much effort parents and caregivers give to help ensure a good future for their children. However, it is important to note how much dedication they must possess while raising their children. Perhaps, you should tell your parents or caregivers, “thank you” or “I love you” more often; and if you really want to go the proverbial extra mile, maybe you could also give them a very stylish “World’s #½ Dad/Mom” mug.

Works Cited:

[1] I. Rothstein, S. K. Robinson, Parasitic Birds and Their Hosts: Studies in Coevolution. Oxford Univ. Press, 1998.

[2] Blažek, Radim, et al. “Success of Cuckoo Catfish Brood Parasitism Reflects Coevolutionary History and Individual Experience of Their Cichlid Hosts.” Science Advances, vol. 4, no. 5, 2018.

[3] Takasu, Fugo. “Modelling the Arms Race in Avian Brood Parasitism.” Evolutionary Ecology, vol. 12, no. 8, 1998, pp. 969–987.

[4] I. Rothstein “A model system for coevolution: avian brood parasitism”. Annual Review of Ecology and Systematics, vol 21, pp 481–508, 1990.

[5] Lyon, Bruce E. “Egg Recognition and Counting Reduce Costs of Avian Conspecific Brood Parasitism.” Nature, vol. 422, no. 6931, 2003, pp. 495–499.

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