Introduction to the Role of the Parasite:
A parasite is an organism that lives in or on another organism at that organism’s expense. For most people, the thought of a parasite is usually in the form of leeches, tapeworms, or ticks. Within populations, parasites have the power to dictate the health of a population and which individuals survive. From an evolutionary standpoint, this can determine who in a population survives due to a parasites ability to kill off the defenseless, and therefore allow individuals with only specific traits to survive. Under the basic concept of natural selection: those who survive, pass on their traits, while the unfit individuals die and fail to pass their traits to the next generation. Parasites act as a controlling factor for which hosts and genes survive for reproduction. With this in mind, Charles R. Brown and Mary Bomberger Brown began a study to investigate the effect of parasitic cimicid bugs on the brood size of cliff swallows (Brown and Brown 2017).
The Size of an Organism:
‘Why don’t organisms keep getting bigger?’ This is a broad topic in the study of organism life history. (Blanckenhorn 2000) (Bonnet et al 2017). As a subset of this, Brown and Brown (2017) studies size limitations of cliff swallow growth regarding exposure to parasites.
The study assumes that larger individuals are also easier targets, while smaller individuals serve as easier targets for parasites. Prior to the Browns, a study claimed that larger birds were selected against due to their inability to escape predators and that individuals from larger broods were hungrier, meaning they were more conspicuous and easily preyed on by hawks (Götmark 2002). Later studies looked at the effects of parasites and claimed that parasites attacked larger individuals because they were both easier targets and a greater source of food (Mohr 1961) (Cable & Van Oosterjout 2007). Other works also say that depending on the life cycle of the parasites, larger or smaller broods will suffer more from parasitism than the intermediate brood size (Horak 1999) (Saino 2002). Smaller broods die from short lived parasites while large broods suffer more from longer lived parasites. Yet, none of these previous works provided direct evidence of whether parasitism itself truly drove stabilizing selection, which selects for the average phenotype individuals in a population. The Browns seeked to clarify this.
To study the effects of parasitism, the 30 year study used 60 colonies of American cliff swallows (Petrochelidon pyrrhonota), varying from 2-6000 nests each, at a biological station at Cedar Point, Nebraska. Nests in the area were mainly found on the sides of bridges and road culverts across varying counties near the biological station. With these 60 colonies available, the Browns created a controlled test by designating colonies as fumigated (mostly free of parasites) or non-fumigated. Parasites within the fumigated nests were never fully eliminated but the number of parasites decreased tremendously (Brown and Brown 2017).
The Effects of the Parasitic Load:
Several observations were made during this extremely long-term study. On average, parasite load increased with weight, even in fumigated nests because parasites were not wholly absent. In non-fumigated colonies, the average weight of nestlings shrunk in comparison to the fumigated colonies, providing evidence for selection against larger birds (Brown and Brown 2017). Also, within non-fumigated nests, the number of bugs vary more in nests with more individuals. Their observations showed a decrease in weight in only the non-fumigated nests, meaning this did not occur as a response to predation because predation would have affected both groups. Surprisingly, smaller birds have poor survival whether their nests were fumigated or non-fumigated. This likely means these swallows are not suited for survival in most circumstances and non-parasitic factors are the main reasons for poor fitness. For survival, non-fumigated nest survival is best for both middling bird size and middling brood size. Yet, this pattern in brood size is non existent in fumigated colonies and these individuals survived better with size.
Results from this study demonstrate that parasitism can act as a driver for stabilizing selection, which in this case is the selection for the middle or averaged sized individuals of a given population of cliff swallows.
Beyond the Study:
The study in general has not investigated the mechanisms of the patterns they observed, but it does show the importance of parasitism on population studies. As a next step in this research, I would like to see the authors delve into more specific questions regarding the effects of parasitism. For example, does nestling size truly increase in the absence of the parasites or is this just the result of birds being able to fully utilize their resources for growth? The paper mainly describes average nestling size in relation to presence of parasites, but what would happen if they examined individuals in generations after controls for parasitism are established in natural populations? This would help determine the inheritance for size in the absence of parasitism. We don’t know for sure if an increase of larger individuals is because nestlings are able to grow larger given less environmental pressures from parasites, or that genetically larger individuals avoid dying. Heritability allows for size to vary between generations, but the selection towards intermediate sizes has limited changes in size. Also, maybe resistance to fumigation will be relevant in a multigenerational study. What would happen if introduction to parasites occurred partway through the growth stages of the nestlings? Would selection effects primarily effect these populations earlier or later in their life cycle?
The Browns’ work supports stabilizing selection (selection for the average form of a trait) for nestling mass and brood size, but how far can this pattern extend in evolutionary trends in general? Will these patterns be as common in other species of bird, or even other animals? The paper mainly refers to studies on birds, but are there other studies, that look at the effects of parasitism and growth in other subjects? What traits can these patterns apply to? The tendency for host parasite interaction is the persistence of traits that allow an organism to best survive a parasite. Depending on the trait, the forms can either vary between generations due to phenomena such as the red queen effect, or in the case of this study cause stabilizing selection on nestling mass and brood size. Further research into this trend among other species will expand the understanding of evolutionary theories.
Further Reading:
Basic parasitism– A general overview for understanding how parasites interact with hosts.
Stabilizing selection– Read to for a basic definition and easy to understand examples of stabilizing selection.
Red Queen hypothesis– An overview of an evolutionary trend common in host parasite interactions.
References:
- Blanckenhorn, W. U. (2000). The Evolution of Body Size: What Keeps Organisms Small? The Quarterly Review of Biology,75(4), 385-407. doi:10.1086/393620
- Bonnet, T., Wandeler, P., Camenisch, G., & Postma, E. (2017). Bigger Is Fitter? Quantitative Genetic Decomposition of Selection Reveals an Adaptive Evolutionary Decline of Body Mass in a Wild Rodent Population. PLOS Biology,15(1). doi:10.1371/journal.pbio.1002592
- Brown, C. R., & Brown, M. B. (2017). Parasites favour intermediate nestling mass and brood size in cliff swallows. Journal of Evolutionary Biology,31(2), 254-266. doi:10.1111/jeb.13218
- Cable, J., & Van Oosterhout, C. (2007). The impact of parasites on the life history evolution of guppies (Poecilia reticulata): the effects of host size on parasite virulence. International journal for parasitology, 37(13), 1449-1458. doi:10.1016/j.ijpara.2007.04.013
- Götmark, F. (2002). Predation by sparrowhawks favours early breeding and small broods in great tits. Oecologia, 130(1), 25-32. doi:10.1007/s004420100769
- Hõrak, P., Tegelmann, L., Ots, I., & Møller, A. P. (1999). Immune function and survival of great tit nestlings in relation to growth conditions. Oecologia, 121(3), 316-322. doi:10.1007/s004420050934
- Mohr, C. O. (1961). Relation of ectoparasite load to host size and standard range. The Journal of parasitology, 47(6), 978-984. doi:10.2307/3275037
- Saino, N., Ferrari, R. P., Romano, M., Ambrosini, R., & Møller, A. (2002). Ectoparasites and reproductive trade-offs in the barn swallow (Hirundo rustica). Oecologia, 133(2), 139-145. doi:10.1007/s00442-002-1015-4