In Your Nest, Eating Your Food
When a Parasite Steals Your Work Instead of Your Nutrients
The main evolutionary goal of any living organism is to pass on its genetic material to its offspring and ensure that those offspring can do the same. Over millions of years, different strategies to best achieve this have been developed. The natural world is all about trade-offs. No organism can devote enough energy to everything that might benefit it. Throughout its lifespan, an individual will shift focus between the categories of growth, maintenance, and reproduction. The organism can only take in so much energy, which limits what it can then ‘spend’. This week, we’re going to be talking about parasitic behavior that allow its practitioners to circumvent many of the energy costs for one of those categories: reproduction.
Across different species, energy costs for reproduction can vary based on the species’ strategies, including varying levels of parental care. For some species, this care is extensive and costly, so a few have developed a way to reap the benefits of such care without expending the required energy investment. Instead, they have another species do the work for them. These organisms are known as brood parasites.
Some of the most well-known such parasites are a couple of bird families, including the cuckoos and cowbirds. These birds lay their eggs in the nests of other bird species and have the host parents care for their offspring. In some cases, this simply means extra energy that must be expended by the host, but sometimes the parasite chick will force the other eggs or hatched chicks out of the nest so it can monopolize the care received from its host parents.
How does the warbler in the above picture not recognize that the chick it is feeding is clearly not its species? There are a number of factors at play. The fact that the cuckoo is so much larger than its parent is accepted because larger offspring are generally preferable as they are more likely to survive. This instinct means that the host parent will often still care for a larger parasite offspring.
Furthermore, there is some evidence that certain brood parasite species will periodically visit the nests where they have lain their eggs. If an egg or hatchling is rejected by the host, the parasite will then destroy that nest and kill the remaining eggs or hatchlings. Referred to as the ‘mafia hypothesis’ (think along the lines of ‘nice nest you have there, it would be a shame if something happened to it), this provides an incentive for the host to care for the parasite’s young because there is still a chance for its own offspring to survive. Also, making a whole new nest and laying new eggs is even more energy intensive than caring for a chick that is not your own.
While birds are the most well known brood parasites, there are also several species of fish and insects that engage in such behavior. Such species often have cuckoo in their common name as a reference to the bird that their behavior is similar to. For example, the cuckoo catfish, Synodontis multipunctatis, is native to Lake Tanganyika in Africa and parasitises mouthbrooding cichlid fish (fish that hold their eggs in their mouth to protect them) by quickly laying eggs among the cichlid’s eggs before the host mother can gather them in her mouth. The eggs of the catfish hatch first and the fry (fish hatchlings) will eat the host eggs. This behavior saves energy for parental care for the catfish and allows it to breed more frequently.
Many insects do not exhibit a large amount of parental care. Therefore, brood parasitism is not a valid strategy for those species. The main exception are the Hymenopterans, social insects that include bees, wasps, and ants. These insects live in colonies and provide large amounts of parental care. Certain groups of these, known as cuckoo bees and cuckoo wasps, will lay their eggs in the nest cells of other bees or wasps. Some species will actually have their queen kill and replace the host queen and use the host workers to care for their own offspring. There is also a species of butterfly, the mountain Alcon blue, Phengaris rebeli, that takes advantage of ants by using chemical mimicry to trick them into bringing the butterfly larvae back to their colony and feed them.
Brood parasites are fascinating because they aren’t stealing nutrients or structure like many of the examples in the past two weeks of stories. Instead they are stealing work and effort. It also provides an interesting example of the ‘evolutionary arms race’ I mentioned in the parasitism introduction. Brood parasites deposit eggs to take advantage of host parental care. Hosts that better recognize parasite eggs can eject them and avoid caring for non-related offspring. Parasites that check the nests for such behavior can punish hosts by destroying nests and thus discouraging further policing of eggs. Each step causes an escalation in tactics by one side or the other.
Next week is the finale of Season 2. I hope you all have enjoyed our journey through the world of symbiosis. As before, I’ll be going on a one month break to outline the theme and topics for Season 3.