Hungry hungry (hygienic) ibex

Alpine ibex (Capra ibex) is a wild goat species that lives in the European Alps. Like other ungulates, it eats plants, defecates, eats more plants, and continues on its merry way. The ibex, unlike contemporary humans, does not have access to a toilet, latrine, or other location designated specifically for feces. Some of us might cringe at the ibex’s corresponding lack of hygiene – depositing previously eaten food near where it is now eating food does not seem like a good idea. However, the ibex does have certain behaviors that make it more hygienic.

poop pictureA recent study by Brambilla et al. (2013) investigated the foraging behaviors of ibex in their natural habitat. The authors recorded foraging locations of marked individuals, and counted the fecal pellets in foraging areas vs. patches in which the ibex did not feed (“avoided” patches). “Avoided” patches were chosen as the midway point between two foraging patches. While admittedly arbitrary, such a measure seems practical, since one cannot ask an ibex why it fed in one spot versus another. The authors compared the number of fecal pellets in the first foraging patch to the “avoided” patch. The authors discovered that grazed plots had a lower density of fecal pellets than avoided plots, and that individuals consistently differed in their avoidance behavior, but that such differences did not correlate with individual age or infection status (infection status was assessed from feces obtained from marked individuals).

poop travels

These results are interesting for a variety of reasons. For example, while ibex do not have toilets, they do avoid feces, a behavior analogous to our own feces-avoidance behaviors. Further, individual ibex vary in their feces avoidance behaviors, just as humans vary in their hygiene. Some people, for instance, are more likely to eat food off the floor, or more regularly wash their hands. These differences can be attributed to personality. Someone’s hygiene can thought of as part of their personality. Likewise, ibex have hygiene personality too. Interestingly, such personality does not seem to correlate, in the ibex, with their infection status or age. Apparently infected or older ibex are no more or less cautious around feces than uninfected or younger ibex. Thus, just as for humans, how the consistent ibex behavioral differences arise remains unknown.

Citations

Brambilla, A., von Hardenberg, A., Kristo, O., Bassano, B. & Bogliani, G. (2013). Don’t spit in the soup: faecal avoidance in foraging wild Alpine ibex, Capra ibex. Animal Behaviour, 86, 153-158.

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The Coevolution Continues


A while back, I made a post about parasite-host coevolution.

Here’s a cartoon, because my posts are too often filled with text.

Image

Anyway, to continue on the topic. In the last article about coevolution, I covered the idea of host and parasite coevolution leading to continual and reciprocal evolutionary changes – the hosts and parasites are constantly running, but they stay in the same place relative to each other. The (excellent) cartoon makes sense now, right?

In the article about coevolution, I also touched on the issue of multiple parasites infecting multiple hosts, and how this can complicate host-parasite coevolution. Here, I’m going to further complicate matters by talking a little about environmental heterogeneity and other species that interact with host-parasite associations.

Harrison et al. (2013) dealt with the issue of environmental heterogeneity. They conducted an evolutionary experiment with bacteria and their viral parasites, varying the bacterial habitat over time. They found that varying the habitat prevents coevolutionary dynamics between the bacteria and their viral parasite.

Conversely, Friman and Buckling (2013) experimented with bacteria, their viral parasites, and protist predators of the bacteria. These authors found that the bacteria tend to coevolve to either the viruses or the predator, not both. The presence of the bacterial predator also constrained the coevolution between the bacterial and virus.

So where does this leave us? It seems that when we consider only single host-single parasite systems, we may find the treadmill coevolutionary pattern seen in the cartoon above, but the likelihood of this situation breaks down when we consider other factors like habitat heterogeneity and multiple natural enemies (i.e., predators and parasites). However, one could reasonably argue that the discussed experiments with bacteria are not relevant for all organisms, and systems in which parasites and hosts do engage in treadmill (aka arms race) coevolution could exist. Whether such coevolution does or does not occur, though, will likely be system specific.

Citations
Friman, V. P. and A. Buckling (2013). Effects of predation on real-time host-parasite coevolutionary dynamics. Ecology Letters 16(1): 39-46.

Harrison, E., A. L. Laine, M. Hietala and M. A. Brockhurst (2013). Rapidly fluctuating environments constrain coevolutionary arms races by impeding selective sweeps. Proceedings of the Royal Society B-Biological Sciences 280(1764).

When Sharing is Caring: A Parasite’s Perspecitve

We’ve all heard of Influenza A, or Flu. It’s a viral parasite that can and has caused massive pandemics in humans. However, Influenza A does not infect only humans. It also parasitizes birds, particularly waterfowl, and other mammals. Many parasites have this diverse infection ability, i.e., many parasites infect multiple host species.

For such parasites, questions regarding their transmission dynamics become more interesting (and complicated). Instead of considering the interaction between only one parasite and one host species, one has to consider how different host species contribute to the parasite’s population. One host species may be the parasite’s main host, adding more parasites to the parasite population than any other host species. On the opposite side of the spectrum, the parasite may have no main host; each host species may contribute equally to parasite numbers in the community at large. In all likelihood, most parasites probably fall somewhere in the middle of this range; the parasites do not have a single main host, but the host species also contribute unequally to the parasite population.

In the cases where a parasite has a few (or one) main hosts, those hosts are called reservoirs. By producing most of the parasite population, reservoirs maintain that population. Reservoirs are thus important research areas for parasite population control, and therefore wildlife and human health. For example, many parasites that infect humans have non-human animal reservoir hosts. These parasites, ones that spillover into humans from other species, are called enzootic parasites. If one can determine their specific wild reservoirs, then one can eliminate parasite transmission to humans in a number of ways, including: limiting human-reservoir contact and preventing successful transmission among the reservoir species members (which itself can be accomplished through different mechanisms such as: vaccination, culling, habitat modification). However, such tools and goals are not simple, even when managing only a few host species.

More than the practical and financial considerations in appreciably affecting even a few host species, the different types of reservoir hosts complicate management efforts. Not all reservoir hosts are so for the same reasons. Streicker et al. (2013) argue three main mechanisms create reservoir host species: 1) The host species can be super abundant and have high population density; 2) The host species can be very susceptible to infection; 3) The host species can be highly infectious, producing numerous new parasites when infected. In the first mechanism, the host maintains the parasite simply by being so numerous. While a small percentage of the host population is infected, the host population is so large that it produces many parasites and maintains the parasite population. In the second mechanism, because hosts are so susceptible, a very high proportion of the host population is infected and can maintain the parasites at relatively lower host population sizes. In the third mechanism, hosts are highly infectious, so even though few hosts are infected, a single infected host produces many new parasites and has a high chance of infecting another host. These three situations are not mutually exclusive, but Streicker et al. argue that which one mostly applies to a particular reservoir species should affect management practices regarding that species. For example, the authors analyzed the effect of targeted removal of infected hosts and random removal of hosts from populations where a host species was either only super abundant, only super susceptible, or only super infectious. For super abundant hosts, random removal of hosts did not efficiently decrease the host’s contribution to parasite population because there was a small chance of removing an infected host. Targeted removal of infected hosts resulted in a moderate reduction, however. For super susceptible hosts, where a large proportion of the host population is infected, both random and targeted removal of hosts efficiently reduced the host’s contribution to parasite numbers. Because so much of the host population is infected, the chance that a random removal gets an infected host is not much less than targeted removal of infected hosts. For super infectious hosts, random removal finds infected hosts in proportion to their frequency in the population, and since each infected host contributes a large parasite number, each removal of an infected host will noticeably decrease the host’s contribution to the parasite population. Targeted removal of infected infectious hosts will decrease the host contribution to parasite population much more than random, although. This occurs because when there are few infected individuals, and each produces a large number of parasites, specific removal of the infected hosts will remove a large proportion of the parasite population pool. These results indicate that – for removal of hosts as a management tool – the different mechanisms creating reservoir hosts affect the outcomes of management actions.

Citations

Streicker, D.G., A. Fenton, and A.B. Pedersen, Differential sources of host species heterogeneity influence the transmission and control of multihost parasites. Ecol Lett, 2013. 16(8): p. 975-84.

Parasite extinctions. A new hope?

So, first off, sorry that I did not post anything last month.

But let’s not dwell on the past. Instead, let’s focus on the future, and the role of current massive extinction rates on future biodiversity – and how parasites fit in to this picture.

Extinctions can occur through numerous mechanisms. Common ones include habitat loss, species invasion, overkill, climate change, and coextinction (Dunn et al. 2009). Regarding coextinction, it is easy to understand how the extinction of prey species could lead to the extinction of predators (for example, the extinction of krill could cause the extinction of baleen whale species). In a similar fashion, coextinction can apply to parasites. If a host species has species-specific parasites (and many hosts species do), then those parasites may go extinct along with the host species (Dunn et al. 2009). Furthermore, coextinction can include cascades of extinctions. If an insect species goes extinct, a bird species that uses that insect as prey may go extinct, and the parasites of that bird may then become extinct also. Whether such processes occur depends primarily on the host specificity of the parasite (Dunn et al. 2009). Host specificity, importantly, is very difficult to measure – for a variety of reasons, including that it is infeasible to expose all possible hosts to a particular parasite. In addition, because individuals within a parasite population likely have varying host specificities, the frequency of each host specificity level is probably more important than the average host specificity level (for example, knowing that 80% of the parasite population is highly general and 20% highly specific would be more important than knowing that the average parasite is moderately host specific) (Dunn et al. 2009). These considerations beg the question: If a host goes extinct, how likely is it that the parasite (or mutualist) will become extinct also or adapt to another available host? While studies often indicate that parasite are more general than previously assumed (Dunn et al. 2009), whether a parasite survives will depend highly on the specific parasite species in question; generalizations seem inadvisable.

But why should we care? Parasites are bad, right? Well, if we care about biodiversity for the sake of biodiversity, then we should protect parasites, which are often more diverse than their hosts (Dunn et al. 2009), implying that parasite species may outnumber host species. Furthermore, a common hypothesis for the evolution of diversity within sexual organisms relies upon parasites – to escape their parasites, hosts diversify. Without parasites, the force to diversify could thus decline, resulting in declines in host biodiversity (Dunn et al. 2009). Additionally, as stated above, host-specific parasites are more likely to become extinct than generalist parasites; generalist parasites are also often more pathogenic than host-specific parasites (Dunn et al. 2009). Zoonoses, parasites that infect humans but are maintained in the wild by non-human animal reservoirs, are by definition generalists. Therefore, as host-specific parasites become extinct, we should ask ourselves whether pathogenic zoonoses will become more common, filling the gaps left by the specific parasites, and whether such a result will negatively affect human health.

Literature Cited
Dunn, R. R., N. C. Harris, R. K. Colwell, L. P. Koh, and N. S. Sodhi. 2009. The sixth mass coextinction: are most endangered species parasites and mutualists? Proceedings of the Royal Society B-Biological Sciences 276:3037-3045.