The War of the Parasites

Hey all: Firstly, sorry for the large absence since my last post. Interestingly, I had my own parasite (pneumonia), which put me out of action for a while.

Anyway, to the post!

Hosts often have multiple parasites at a time, and what is good for one parasite may not be good for another parasite.

A real-life example.

The cestode parasite Schistocephalus solidus has a three host life cycle. It first infects copepods (Macrocyclops albidus). Then, three-spined sticklebacks (a fish) acquire the parasite through eating infected copepods. Finally, birds acquire the parasite by eating infected fish. The parasite reproduces inside the bird, releasing further parasites into the environment, which will infect copepods. The circle of life.

life cycle

Such a transmission cycle, in which the parasite moves through consumption of one species by another, is referred to as trophic transmission.

The predictions.

Trophically-transmitted parasites often manipulate the behavior of one of their earlier hosts to facilitate transmission to the final host (see, for example, Moore 2013, Poulin 2010). Normally, we think of these behavioral alterations as making hosts more susceptible to predation by the next host in the life cycle (e.g., Lafferty 1992). Parasites, however, often have development times inside their hosts. By development times, I mean that parasites are not typically infective to the next host in the life cycle immediately upon infecting the current host. Once in a host (say, the copepod), a parasite needs some time before it can successfully infect the next host (the fish). If the parasite’s host were to be eaten before the parasite was infective to its next host, then the parasite would die, even it were eaten by the correct next host. For this reason, many authors have hypothesized that before the parasite is infective, it should reduce its host’s susceptibility to predation (e.g., by increasing host hiding behaviors). Other authors have further noted that if a host is infected by parasites at different developmental stages (parasitized by both infective and uninfective stages), then the different parasites may attempt to sabotage the manipulation of the other. For example, the infective parasite may prevent the uninfective parasite from reducing the host’s susceptibility to predation. Crazy (cool), right?

Hypothesis Tested

Hafer and Milinski (2015) tested this idea with S. solidus in its copepod host. Just ignoring all the details about statistics and experimental design (who cares about that anyway?), here are their results: Hafer and Milinski found that copepods infected with different stages of parasite experienced sabotage of behavioral manipulation. When copepods were infected with two parasites, which were both infective, then the copepods displayed reduced anti-predator behaviors. Likewise, when both parasites were uninfective, the copepods exhibited increased anti-predator behaviors. But when copepods were infected with one infective and one uninfective parasite, then the copepods still demonstrated reduced anti-predator behaviors. The uninfective parasites lost out! The manipulation by the infective parasites prevented that by the uninfective parasites.

only one


Hafer, N. and M. Milinski (2015). “When parasites disagree: Evidence for parasite‐induced sabotage of host manipulation.” Evolution.

Lafferty, K. D. (1992). Foraging on prey that are modified by parasites. American Naturalist.

Moore, J. 2013. An overview of parasite-induced behavioral alterations – and some lessons from bats. Journal of Experimental Biology.

Poulin, R. 2010. Parasite manipulation of host behavior: an update and frequently asked questions. Advances in the Study of Behavior.

Sex Matters

Because of the differences between male and female physiology, males and females can have different reactions to the same parasite. These variable responses can have important consequences for parasite-host interactions and the overall transmission of the parasite. Take, for example, a relatively recent study of great tits (Parus major, a bird).

Dunn et al. (2011) explored the effect of malarial infection on the behavior of male and female great tits. While the differences between males and females are obvious in many species, sex-specific differences in behavior due to infection are not often investigated. Anyway, in their study, the authors captured wild birds, and compared the behavior of infected and uninfected birds, as well as a number of physiological measurements. A quick summary of the results: Infected males were more likely to problem solve (in this case pulling a lever to release food) than uninfected males, but infected females were less likely to problem solve than their uninfected counterparts. Infected females were more exploratory (exploration of a novel room) than uninfected females, but infection did not influence male exploratory behavior. Infected males were more risk averse (retreating from a disturbance – noise caused by stick movement) than uninfected males, but infection did not affect females.

What does all this mean, exactly? Well, one of two things: One, males who get infected differ from females who get infected. Or two, infection changes the behavior of males and females differently. The first scenario argues that infection does not change behavior, that certain behaviors instead make individuals more or less likely to become infected. According to Dunn et al. (2011), for example, problem solving males should be more adept at using different food resources, and exposure to those different food resources should expose the birds to more parasites than males who are less capable at problem solving and therefore use fewer food resource types. The second scenario asserts that infection itself changes behavior, either as a host response to the parasite or direct manipulation by the parasite. While one can generally use information about the biological system to hypothesize whether behavior causes infection or vice versa, the truly definitive way to test this question would be to do controlled infections and monitor behavior before and after infection.

Such distinctions, as well as male vs. female behavioral differences, can be important for a number of reasons. They might, for instance, affect species management programs. If one had a species infected with a parasite, and one thought that bold individuals were more likely to acquire and spread that parasite, one might imagine a culling or vaccination program that focused on bold individuals to decrease future parasite transmission. However, if infection makes individuals bold, this program could be relatively pointless. If one of the two sexes, moreover, is predominantly responsible for transmission of the parasite, one might focus isolation, vaccination, or culling efforts on that sex. One could probably list many other examples in which sex differences and the direction of causality in behavior-infection connections matter.

Or, one could end the blog. To finish, then: Sex matters.


Dunn, J.C., Cole, E.F. & Quinn, J.L. (2011). Personality and parasites: sex-dependent associations between avian malaria infection and multiple behavioural traits. Behavioral Ecology and Sociobiology, 65, 1459-1471.

Parasites and the Benficial Predator?

Parasites and predators, generally speaking, negatively affect the organisms that they use (i.e., parasites hurt their hosts, and predators are obviously deleterious for prey). However, as I mentioned in the last blog, in ecology everything depends (on the details). Here, I will focus on some of the complications that can arise when one considers the effects of parasites and predators together.

First off, parasites can be eaten. They can be eaten when they are inside a host, and that consumption can put the parasites in the right or wrong host (the right host would be the one in which the parasite can complete its life cycle). For example, improperly cooked pork can contain parasites (e.g., Taenia solium). These parasites do not “want” to get into humans; they cannot complete their life cycles inside humans, and instead tend to cause huge problems. Parasites can also be eaten when they are outside the host. Many parasites have free-living stages in the environment, and they need to be consumed in order to continue their life cycle. Such consumption can also lead the parasite to the right or wrong host (Johnson et al. 2010).

Both of the above consumptive scenarios (a parasite being eaten when it is in or outside of the host) contribute to parasite population dynamics (how large is the parasite population, how many hosts are infected, what proportion of hosts is infected). One could thus imagine that predation could facilitate or hinder parasite transmission. For instance, if hosts acquire more energy from eating parasite-infected prey than they lose from the infection, hosts might do better if they ate parasite-infected prey, and therefore predation could increase parasite transmission. But if non-host predators eat parasites in the environment or prey with parasites, then these predators might reduce transmission to the host species (Johnson et al. 2010, Lafferty & Morris 1996).

Importantly, predators also affect their prey by more than just killing the prey. As any hunter can tell you, prey tend to run or fight back when encountered with a predator. If prey survive a predation attempt, they tend to alter their behavior to reduce predation risk (e.g., increase hiding time and reduce foraging time). Such behavioral changes can happen even if the prey never experienced a direct predator attack. Simply exposure to the smell/sight/etc. of a predator, or the smell/vision/etc. of other dead prey, can elicit behavioral changes in prey. These behavioral changes may also affect that prey’s exposure to parasites. If, for example, prey are more likely to encounter parasites by staying in one place and reducing activity, predators may indirectly increase parasitism among prey. The opposite may occur, however, if prey activity increases exposure to parasites.

In a recently published paper, a group of authors attempted to discern whether the combination of these factors – predators eating parasites and predator effects on host behavior – sum to positively or negatively affect parasite transmission to hosts. Orlofske et al. (2014) found that in the presence of a predator (a dragonfly larvae), tadpoles reduced activity and acquired more parasites. Other authors have previously demonstrated that active tadpoles acquire fewer parasites than anesthetized tadpoles, so activity was the key factor driving the increase in parasitism in this study. Interestingly however, the authors also discovered that the purported predator (damselfly larvae) of the free-living form of the parasite in question (a trematode) did not actually eat the parasite in a more realistic experimental setting (but it did in previous laboratory studies). Further, damselfly larvae caused the same reduction in activity among the tadpoles as the dragonfly larvae, even though the damselflies are not predators of the tadpoles.

\While this study did not reveal that parasite predators reduce infection intensity (contrary to expectation), it did validate the idea that host predators should be expected to increase parasitism when predator behavioral responses work against anti-parasite behaviors. It also highlights the notion that everything in ecology depends on the details, and concepts that bear fruit in lab settings may not do so in the field for a variety of reasons.



Orlofske, S.A., Jadin, R.C., Hoverman, J.T. & Johnson, P.T.J. (2014). Predation and disease: understanding the effects of predators at several trophic levels on pathogen transmission. Freshwater Biology 59(5): 1064–1075

Johnson, P.T.J., Dobson, A., Lafferty, K.D., Marcogliese, D.J., Memmott, J., Orlofske, S.A., Poulin, R., & Thieltges, D.W. (2010). When parasites become prey: ecological and epidemiological significance of eating parasites. Trends in Ecology and Evolution 25(6): 362-371

Lafferty, K.D. & Morris, A.K. (1996). Altered behavior of parasitized killifish increases susceptibility to predation by bird final hosts. Ecology 77:1390-1397.

Tradeoffs. Should you wash your hands?

 Last time, I wrote about the somewhat controversial topic of “borrowing” your co-worker’s food. The choice to steal involves costs and benefits: the energy you gain from that food vs. the costs may you suffer from stealing. This is a general example of a tradeoff: You cannot be both good at stealing your co-worker’s food and in your co-worker’s good graces.

Tradeoffs come in all forms. For example, I often hear the following proverbial tradeoff for humans: “Jack of all trades, master of none.” If you’re okay at everything, you are not likely going to be great at any one thing. At its most basic level, this tradeoff involves investment. How much energy should you invest in one activity over another? Many tradeoffs involve investment. Though humans can reproduce as teenagers, many wait to reproduce until after they have stopped developing (e.g., mid-twenties). Such a choice, while for many occurs for social reasons, also exemplifies the tradeoff between investment in growth or reproduction (should I reproduce now and risk being unable to take care of my children because I’m not big and strong, or wait until I can take care of them but risk losing the opportunity to reproduce?). Information gathering can also be a tradeoff. Do you buy products spur of the moment, or carefully research? In this situation, you are trading off time and energy. If you research, you are using your time and mental energies when they could be used elsewhere. If you don’t research, you might pay too much, buy the wrong item, or be more likely to return the item later.

While these examples focus on people, all organisms face tradeoffs, many of which markedly resemble the ones above. Many non-human animals, for instance, feed on multiple items. The animal can thus choose between specializing on a particular food item or generalizing to eat everything. If the animal specializes, presumably it will become better at getting that item or getting more energy from that item. If the animal generalizes, it will have access to a greater variety of food, but may not get as much energy out of that food. This is analogous to the “Jack of all trades, master of none,” idea above in that you can be excellent at eating one thing, or good at eating everything, but not excellent at everything. Animals may also experience tradeoffs with responses to natural enemies, where “natural enemies” refers to predators and parasites. For example, hosts are exposed to many types of parasites throughout their lives, and hosts may invoke different defenses against those parasites. Hosts may therefore experience tradeoffs between different defense mechanisms (investing in behavioral defenses may mean less energy for immune defenses) or tradeoffs between the different types of parasites (being good against parasite A may preclude defense against B).

Such tradeoffs may make it appear that organisms are inappropriately adapted to their environments. “Why does that host suffer so much damage from that parasite?” However, such judgments need to account for the overall environment of the organism in question. Perhaps an organism is poorly adapted to a parasite because the parasite does not infect that many hosts in the population, and food shortage is far more important for survival to the host? If that is the case, the host may favor food acquisition at the cost of parasite defense capability. Additionally, if a host displays poor immunity, perhaps it exhibits numerous behavioral defenses? Such tradeoffs and environmental details, importantly, may make it appear that a host is acting maladaptively (i.e., in a manner that negatively impacts its survival and reproduction). To illustrate, have you ever touched a doorknob? And then ate something without washing your hands? Someone might be able to argue that this was “risky,” or even maladaptive, behavior because it exposed you unnecessarily to potential infectious agents. However, it’s a tradeoff. How much time and energy would you be wasting by washing your hands, or how much would you be damaging your hands by exposing them to basic chemicals, vs. the likelihood that you will get sick if you do not wash your hands? To really evaluate this scenario, we need to know more about the costs and benefits of washing your hands. Maybe in your culture, hand washing is considered unattractive, and so you are more likely to find a partner if you do not wash your hands, making hand washing maladaptive. Thus, the take home point: As is often the case in ecology, everything depends (on the details).



Hands are hard to draw.

Should you steal your (sick) co-worker’s food?

Do you have a co-worker, friend, or roommate? If you do, have you ever been in a position to “borrow” that person’s food? Did you? Ethical dilemmas aside, did you consider whether said person was sick?

Let’s say I have a co-worker named Gunter. If I take Gunter’s lunch, I will benefit from acquiring a “free lunch.” However, I might suffer costs if Gunter discovers who took his lunch. When I decide whether to purloin Gunter’s food, therefore, I am conducting a cost-benefit analysis. An additional factor I might include in this analysis could be whether Gunter is sick. If Gunter is sick, for example, I might risk acquiring his infection by eating his food. Such infection would likely more than negate the benefits I obtained from eating Gunter’s food. Gunter’s sickness would consequently dissuade me from “borrowing” his food. But, what if from past experience I know that Gunter gets incredibly angry when anyone borrows his food – except when he is sick. Naturally, he is more displeased about losing his lunch when he is sick than when he is healthy, but when he is sick he doesn’t have the energy to confront anyone about the loss. And now I have a conundrum. I am likely to suffer negative consequences from a healthy Gunter if I am discovered with his lunch. I may also acquire an infection from Gunter if I take his lunch when he is sick. But I am less likely to suffer negative consequences from Gunter at that time, even if I am discovered. So should I steal Gunter’s lunch, and when?

Unfortunately, no simple answer exists for this situation. The answer all depends on the cost:benefit ratio – the benefits I get from Gunter’s food relative to the costs I may suffer from stealing. If benefits are high enough, it would be optimal for me to steal all the time, regardless of Gunter’s state. If benefits are high, but costs due to Gunter’s behavioral response are high, it might only be optimal for me to steal when Gunter is sick.

If you think this example is a little contrived, well, you’re right. But not so much as you might think. For instance, Bouwman and Hawley (2010) discovered that male house finches preferentially fed near males who were infected with a bacterial parasite (mycoplasma), even though feeding sites enabled high transmission rates of the parasite. Bouwman and Hawley also noted that infected males were less aggressive than uninfected males. The authors therefore concluded that males who fed near an infected male benefited from encountering less aggressive males (meaning the former males were more likely to be able to feed without incident). This benefit might or might not outweigh the costs of infection for the population as a whole, but at least at the individual level, benefits appeared to override the consequences.

This house finch result has interesting implications for the evolution of host-parasite interactions. We often think of hosts evolving to maximize fitness, which means reducing parasite costs to an absolute minimum, but this does not have to come from reducing parasite infection to a minimum. If you engage in behaviors that lead to infection, but you get more benefits from those behaviors than the costs of infection, selection will favor that behavior, even though it increases infection.



Bouwman, K. M., and D. M. Hawley. 2010. Sickness behaviour acting as an evolutionary trap? Male house finches preferentially feed near diseased conspecifics. Biology Letters 6:462-465.