Are you super?

Disease ecology often uses the term, “super-spreader,” which describes an individual responsible for a disproportionate amount of disease transmission. For example, in the diagram below, each circle or triangle represents an infected individual. Each circle infects four more circles, but each triangle infects only one. After only a few rounds of infection, there are far more circles than triangles, and the circles would be called super-spreaders.transmission

However, the spread of infection depends on more than the transmitter. Some individuals may be far more likely to receive an infection than others (these individuals could be called “super-receivers”).

super receivers

As one might guess, if an individual were both a super-spreader and a super-receiver, they would greatly impact disease spread in a population. Whether such individuals exist, however, remains an open question.

Adelman et al. (2015) recently investigated this question in house finches (Haemorhous mexicanus) and their bacterial parasite, Mycoplasma gallisepticum, which causes conjunctivitis. The authors performed two studies. In the first, they monitored the presence of tagged birds at feeders in order assess feeder use, aggressive interactions at feeders, and aspects of sociality (such as local bird group size, number of birds with which a focal bird usually feeds, and a few others). The authors then compared these measures with the presence of infection. Interestingly, the authors found limited connection between a bird’s aggressive or social interactions and whether that bird was infected. But feeder use did strongly associate with infection. Birds that spent more time foraging at feeders were more likely to be infected.

However, because this study did not manipulate infection, it was unclear whether spending more time at feeders led to infection, or whether infection led to increased time at feeders. Consequently, in a second study, Adelman et al. created artificial bird groups in captivity, where each bird had a known foraging propensity. Then, the authors introduced the infection to each group, but to a bird of different foraging propensity each time. In this experiment, the infection spread faster in groups in which the initial infection was on a bird that spent more time at feeders. Although, because groups for which the initial infection was on a “high time forager” also had higher average foraging times than groups for which the initial infection was on a “low time forager,” the result that the parasite spread faster in the “high time forager” groups could be due to the initial individual infected or the high average foraging of the group. Foraging clearly affects transmission of the disease, but it is still unclear whether foraging does so through super-spreaders, super-receivers, or both.


Adelman, J.S., Moyers, S.C., Farine, D.R. & Hawley, D.M. (2015). Feeder use predicts both acquisition and transmission of a contagious pathogen in a North American songbird. Proceedings. Biological sciences / The Royal Society, 282.

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.


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.


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.

Infectious disease and group living

Picture two scenes: 1) a small room, crowded with people; 2) the same room, but with many fewer people. Now, in both of your rooms, imagine that one person starts coughing. If we assume that the coughing indicates infection with something, perhaps pneumonia, in which room would you be most worried about transmission of the pneumonia, the crowded or the relatively less dense room? Probably, you are most concerned about the crowded room. Because it is more crowded, you are more likely to be in contact with someone, and then more likely to spread the infection yourself if you acquire it. This enhanced risk of parasite, or infectious disease, transmission is often considered a major cost of group living. But, fortunately for us and some other group living animals, the room analogy inaccurately represents reality.

crowded room            not crowded room

In the original rooms, I imagined the people standing around shoulder to shoulder, randomly moving. But people don’t move that way. They associate with some more than others. Family, friends, co-workers. We each have our own social spheres, which are much smaller than the 7ish billion people on the planet, or even the 100 other people who happen to work in my building. Many non-human animals that live in groups also follow this pattern as well. In a recent paper that analyzed previously published work across 43 species to find more general patterns, a meta-analysis, Nunn et al. (2015) found that as group size increased, the number of subgroups within the larger group increased (where subgroup refers to smaller groups within the larger group). These subgroups limit contact among individuals; individuals primarily contact only the limited number of individuals in their own subgroup, not everyone else in the larger group.


Via this effect on contact rates, subgrouping may affect parasite transmission. Nunn et al. (2015) tested this hypothesis with a model simulating disease transmission within groups with and without subgrouping. According to the model, as group size increased, the percentage of the group that was infected, or prevalence, increased. But, subgrouping reduced this increase in prevalence. An empirical study of bighorn sheep lambs (Ovis canadensis) and pneumonia epidemics supports the general theme of the model. Manlove et al. (2014) found that sheep populations were organized into subgroups, and that during pneumonia epidemics, not all subgroups became infected. Thus, because subgroups had limited contact rates with one another, their presence may have reduced the transmission of pneumonia in comparison to a situation without subgroups, which could not be directly tested in this system. However, infection prevalence did not show a direct relationship with group size, in line with the predictions of Nunn et al.



Manlove, K.R., Cassirer, E.F., Cross, P.C., Plowright, R.K. & Hudson, P.J. (2014). Costs and benefits of group living with disease: a case study of pneumonia in bighorn lambs (Ovis canadensis). Proceedings of the Royal Society Biological Sciences Series B, 281, Article No.: 20142331.

Nunn, C.L., Jordan, F., McCabe, C.M., Verdolin, J.L. & Fewell, J.H. (2015). Infectious disease and group size: more than just a numbers game. Philosophical Transactions of the Royal Society of London B Biological Sciences, 370.

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.


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.