A bird parasite “farms” its host

Cowbirds (genus Molothrus) are brood parasites in North and South America. As a brood parasite, they parasitize the nests of other birds. Instead of building their own nests and tending to their eggs themselves, cowbirds lay their eggs in the nests of other species, letting this host species care for the parasitic eggs. Understandably, cowbirds have to time their parasitism well. If the cowbirds lay their eggs before the host has laid any, then the host would obviously know something was wrong. But if the cowbirds lay their eggs too late after the host has laid its own, then the host eggs may hatch before the cowbird eggs, and the host may stop caring for the cowbird eggs, or outright destroy them. Fortunately – for the cowbird – if the host’s nest is destroyed, then it may start another nest if enough time remains in the season. Cowbirds apparently take advantage of this behavior, destroying too-developed host nests in the hope that the host will start a new nest, which the cowbird may then be able to parasitize.

A recent study by Swan et al. (2015) investigated this behavior in brown-headed cowbirds (Molthrus ater), hoping to discover whether cowbirds actually farm their hosts, or whether the cowbirds just appreciate wanton destruction. Swan et al. (2015) found that when they presented female cowbirds with a choice of young vs. old nests, females destroyed the old nests more than the young nests. Specifically, the females were equally likely to attack both nest types, but would only destroy one egg from young nests, and all eggs from old nests. The authors hypothesized that the cowbirds used the first egg to assess egg age, and if the egg was developed, then the cowbirds destroyed the rest of the eggs. The authors further found that cowbirds were more likely to attack nests with a greater number of eggs, as a greater number of eggs also indicates that the host has finished laying eggs (meaning the current eggs are highly developed or the host would not accept further eggs, such as those laid by a cowbird). Swan et al. (2015) also compared these lab results with field data collected over 10 years, which demonstrated a positive correlation between host nest age and cowbird attacks, that as host nest age increased, cowbird attacks increased.

Citations

Swan, D.C., Zanette, L.Y. & Clinchy, M. (2015). Brood parasites manipulate their hosts: experimental evidence for the farming hypothesis. Animal Behaviour, 105, 29-35.

Wasp Biological Warfare

Have you ever had another organism growing inside of you? Has that organism at some point burst out of you? Then, have you found yourself behaving in a zombie-like fashion – feet shuffling, eyes drooping, clumsy movements – protecting that organism with your very life?

If so, then take comfort in the knowledge that you are not alone. The spotted ladybug, Coleomegilla maculata, suffers from a tortuous form of parasitism by the wasp Dinocampus coccinellae. The wasp, upon finding a ladybug, injects it with an egg. This egg then grows up inside the unsuspecting ladybug. Eventually, the new wasp (still in larval form) “exits” the ladybug. Apparently, growing by eating the insides of the ladybug is not enough for the developing wasp, because once it exits its host, it forms a cocoon between the ladybug’s legs, using the ladybug as a shield against predators. The ladybug, ecstatic at such an opportunity to serve its master, does not try to abandon the cocoon, mostly remains motionless, and even occasionally wiggles its body to ward off predators. This behavioral manipulation of the ladybug is impressively specific, not starting until the wasp larva has exited the ladybug and begun making its cocoon.

lady bug

Until recently, no one knew how the wasp managed to manipulate the ladybug in such an extraordinary fashion. Dheilly et al. (2015) studied the wasp-bug system, and found that a virus transmitted by the wasp may engender the behavioral alterations. The authors found that these wasps carried a newly identified virus, called D. coccinellae paralysis virus, which is transmitted to the ladybug through the injected wasp egg. Initially, the virus or the larva repressed the ladybug immune response, allowing the virus to spread in the bug, including into nervous tissue. Then, when the ladybug’s immune response reasserted itself, as it fought the virus, it caused collateral damage to the nervous tissue. The authors discovered that this nerve tissue damage correlated with the induction of altered ladybug behavior (which the wasp larva used to turn the ladybug into a bodyguard), and clearance of the virus correlated with the resumption of normal behavior (miraculously, some ladybugs survive this entire monstrous process, and continue living their lives!). It is not yet clear whether larval exit triggers the resumption of the ladybug immune response, or if the larva merely times its exit appropriately. Either way, in a manner reminiscent of biological warfare, the wasp is apparently using a virus to create zombies! Glad I’m not a ladybug.

Citations

Dheilly, N.M., Maure, F., Ravallec, M., Galinier, R., Doyon, J., Duval, D. et al. (2015). Who is the puppet master? Replication of a parasitic wasp-associated virus correlates with host behaviour manipulation. Proceedings of the Royal Society Biological Sciences Series B, 282, Article No.: 20142773.

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

Citations

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.