Brood Parasites

Nature is full of cheaters. Competition for key survival resources is fierce and costly, so it is no surprise to see scroungers and freeloaders attempting to subvert competition. Parental care provides significant survival advantage and is a valuable and costly resource. It is a trait that evolves in a population that can pay that cost. So, like any other resource, cheaters are trying to subvert the system. Parasites, for example, are an entire biological class of cheaters.

A parasite is a species that draws energy and other resources from its host. A mosquito takes a bloodmeal, potentially transmitting another parasite, the malaria-causing plasmodium to a human host. Tapeworms absorb digested nutrients from the host’s intestines. In all cases, the parasite is taking something of value from its host. A brood parasite is a parasite that steals parental care.

This week, research published in the journal Molecular Ecology has discovered evidence of brood parasitism in a rather unlikely species.

Researchers from UC Santa Cruz were studying populations of a coral reef fish in the genus Altrichthys when they discovered this unlikely phenomenon. They were observing a species which cares for it’s young, which is a rare trait, even within this genus. Only two species within this genus care for their young and others do not. By observing schools of fish being reared by parents, they realized that some fish did not look like the others. “That was a complete surprise to me, and we only found it by doing the genetics,” Giacomo Bernardi, the lead author on the paper, said. By analyzing the genes of fish in these broods, they were able to determine that some of these fish were not the same species as their parents. Brood parasitism is a likely explanation for these mixed broods.

Altrichthys parent guarding a brood. Parental care is rare in fish, and there is evidence of brood parasitism. Credit: Giacomo Bernardi

Many bird species exhibit high levels of parental care, so brood parasitism is most common and well understood in birds. Fish tend not to provide much parental care, so seeing evidence of brood parasitism is surprising. If this phenomenon is confirmed, then this will be the first documented case of brood parasitism in a fish species.

What is brood parasitism?

Imagine a hospital maternity ward. There are ten babies in ten cradles from ten different families. Each one of these babies needs to be fed, clothed, and sent to school. This is a lot of work and costs a lot of money. The parents who produced these children understand this and are willing to provide for them. They are willing to pay the cost of having children. During the night, a couple with two babies sneaks into the ward. Tiptoeing around the cradles, they carefully place their children into cradles already occupied by other babies. The next day, excited parents arrive at the maternity ward to pick up their babies. Upon arrival, they find out that instead of one child, they now have two. Surprise! Let’s pretend for a second that the unassuming parents don’t know any better. Let’s say they accept this story that they had produced twins and go about their lives raising two kids.

This is a ridiculous story – no human parents would be that gullible. In the animal kingdom, this type of behavior is more common.

Brood parasitism vs. Adoption

On hypothesis that must be considered is adoption. Bernardi himself admits to having witnessed “recruitment events” where massive schools of Damselfish are being hunted by predators and a few escape predation by seeking the protection of adoptive parents. However, he is not convinced that this accounts for all cases of mixed broods. “If it’s not brood parasitism, then a whole clutch of baby fish would have to swim over and get adopted. That seems unlikely because the nests are far apart, and the mortality is enormous for unprotected babies,” Bernardi said. By high mortality, Bernardi is referring to the unlikelihood of surviving past the larval stage in this genus. This is not to say that there is conclusive evidence against the adoption hypothesis.

Why is fish parental care weird?

Many species of fish, insects, and amphibians will lay hundreds of eggs at a time. The more eggs produced increases the chance that some will be successful. This is even if the parents put no energy into protecting the eggs or larvae. An ecologist would refer to this as an r-selected survival strategy. In these species, there is an incredibly high likelihood of larval mortality (often because of predation), but some lucky ones will make it through the gauntlet and carry on the species. The opposite strategy is that of K-selection, where a lot of energy is invested in protecting and rearing just a few offspring.

Parental care is a typical K-selection strategy, which makes it a bit of a rare occurrence in coral reef fish species. Altrichthys parents who are providing parental care for their own broods are also providing parental care for other species that are laying eggs in their nests or seeking their protection.

So how do these behaviors evolve and how are they maintained?

For this, we will need to look at a species where brood parasitism is more common and better understood. Parental care is common among bird species, meaning brood parasites can find a niche of their own exploiting other species’ parental care habits. Meet the brown-headed cowbird – a brood parasite extraordinaire. The cowbird is known to parasitize over 220 other bird species. 

Photo credit: Allaboutbirds.org

The cowbird does not build nests. Instead, it will find nests other birds have made. It waits for them to lay a clutch of eggs before laying its own in the nest when the parents aren’t looking. The parents of the parasitized clutch will then incubate and care for the cowbird nestlings.

Common Yellowthroat feeding Brown Headed Cowbirds. Credit: Bill Thompson

There are several ways this system is maintained. Once the cowbirds lay their eggs in a nest, they will repeatedly check the nest throughout the incubation and rearing period. To ensure the success of their offspring, the cowbirds (typically much larger than their various songbird babysitters) threaten to destroy their nests. If anything happens to the cowbird’s eggs, all eggs in the nest are destroyed by the cowbirds. In essence, think about this as mob rule. The parents being parasitized have to rear the cowbird nestlings to secure their safety.

This can result in some very specific coevolutionary forces on both species. A brood parasite will evolve to produce eggs that look very similar to the species it parasitizes. All eggs that don’t initially fool the incubating mother are kicked out of the nest. Second, the parents are selected for their willingness to feed and rear chicks which are not their own.

It will take more research before it is determined how exactly brood parasitism has evolved in Altrichthys fish. Parental care, for one, is a rarity in these species, making this a very unique relationship. But if this resource can evolve, expect to see forms of parasitism on that resource evolving quickly after. “In most taxonomic groups that have evolved parental care, some forms of reproductive parasitism also evolve,” said Bruce Lyon, a professor of ecology and evolutionary biology who is a collaborator on the Altryches study. “Given the extreme rarity of parental care in reef fish, it is very interesting that these species also show forms of reproductive parasitism like adoption and brood parasitism. When organisms evolve to produce some resource, if it’s cheatable, evolution will produce a cheat.”

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