That possum you just ran over? It might have saved you from Lyme disease

Posted on by

ResearchBlogging.orgGrowing up in suburban Pennsylvania, where the most hazardous wildlife not extirpated from our woods is the occasional crazed whitetail deer, there was really only one danger I associated with the outdoors — ticks. Specifically, ticks carrying Lyme disease, a not-very-pleasant bacterial infection that attacks the joints, heart, and nervous system if left untreated. According to a paper released online early in Proceedings of the Royal Society B, my risk of picking up Lyme disease on an excursion into the woods behind my parents’ house may have depended on the diversity of bird and mammal species in those woods [$-a].



Sure, it looks like a giant rat, but that opossum is a walking death trap for disease-carrying ticks. Photos by ricmcarthur and jkirkhart35.

In a way, the ticks that carry Lyme disease are a threat to humans precisely because they don’t rely on us as a regular source for blood. Instead, they feed on a variety of mammals and birds, which allows them to maintain population densities high enough that a human wondering into a woodlot stands a good chance of picking up one or two of the little buggers.

But it turns out that not all of these non-human hosts are equally hospitable for ticks. The new paper’s authors, Keesey et al., caught a range of tick hosts — white-footed mice, eastern chipmunks, gray squirrels, opossums, veeries, and catbirds — and experimentally infested them with ticks. They found a huge range of tick success across the six host species: almost half of all ticks introduced onto mice were able to feed, while only 3.5% of ticks introduced onto opossums were. Most ticks that failed to feed disappeared — they were probably eaten when the host groomed itself.

The authors’ field surveys of ticks carried by these animals in the wild make the difference even more pronounced. Wild-caught opossums carried an average of almost 200 ticks — if that’s 3.5% of the ticks that try to feed on a opossum, then that means each opossum had attracted, and eaten, up to 5,500 ticks!

But the real impact of this result comes into focus in a mathematical model the authors develop to determine the effects of removing each of the six hosts from a woodland ecosystem. Removing intermediately-useful hosts like veeries or catbirds doesn’t have much effect on tick density. On the other hand, if you remove very tick-friendly hosts like the white-footed mice, tick populations plummet. And if you remove opossums, they increase dramatically. This is important because, the authors say, larger mammal species are the first to leave as patches of woodland are reduced to make way for human development — so an early effect of woodland fragmentation may be to reduce or eliminate opossums in that woodland, and boost the density of disease-bearing ticks.

This result goes a long way to fulfilling a proposal the authors made in a 2006 review article, that the diversity of alternative hosts for disease vectors like mosquitoes and ticks may shape the risk they pose to human populations [$-a]. It shows that, even in the relatively tame landscapes of suburbia, the way we humans manage what wildlife remains may have real consequences for our own well-being.

References

Keesing, F., Holt, R., & Ostfeld, R. (2006). Effects of species diversity on disease risk Ecology Letters, 9 (4), 485-98 DOI: 10.1111/j.1461-0248.2006.00885.x

Keesing, F., Brunner, J., Duerr, S., Killilea, M., LoGiudice, K., Schmidt, K., Vuong, H., & Ostfeld, R. (2009). Hosts as ecological traps for the vector of Lyme disease Proc. R. Soc. B, (online early) DOI: 10.1098/rspb.2009.1159

Here’s an idea

Posted on by

Let’s try talking about the actual text of actual proposed bills for health insurance reform. Instead of, you know, whatever nonsense pops into Sarah Palin’s pretty little head. Only place I’ve seen anyone doing it? That’d be over at OpenCongress.

A helpful invasive species?

Posted on by

This post was chosen as an Editor's Selection for ResearchBlogging.orgIntroduced species can wreak havoc on the ecosystems they invade. But what happens after they’ve been established for centuries? A new study in the latest Proceedings of the Royal Society suggests that, in one case, an introduced species has actually become an important part of the native ecosystem — and helps protect native species from another invader [$-a].



Dingoes (above) control red
foxes, which is good for native
critters. Photos by ogwen and
HyperViper.

The introduced species in question is the Australian dingo, the wild descendant of domestic dogs [$-a] that moved Down Under with the first humans to settle the continent. Today, 5,000 years after their introduction, dingoes are the largest predator in much of Australia, and they were a prominent part of the ecosystem encountered by European settlers. Europeans, like previous waves of human arrivals, brought their own domestic and semi-domestic animals — including red foxes, which prey on small native mammals.

The new study’s authors hypothesized that because dingoes reduce red fox activity both through direct predation and through competition for larger prey species, dingoes should reduce fox predation on the smallest native mammals. At the same time, dingoes prey on kangaroos, the largest herbivore in the Australian bush — and reducing kangaroo populations should increase grass cover, providing more habitat for small native mammals. When the authors compared study sites with dingoes present to sites where dingoes had been excluded to protect livestock, this is what they found: increased grass cover, and greater diversity of small native mammals where dingoes were present.

Recently a news article in Nature discussed ragamuffin earth [$-a] — the idea that human interference in nature has so dramatically changed natural systems that it may often be impossible to restore “pristine” ecological communities. In these cases, some ecologists say, conservation efforts might be better focused on how to maintain and improve the diversity and productivity of the novel ecosystems we’ve inadvertently created. It looks as though the dingo could be a poster child for exactly this approach.

References

Letnic, M., Koch, F., Gordon, C., Crowther, M., & Dickman, C. (2009). Keystone effects of an alien top-predator stem extinctions of native mammals Proc. R. Soc. B, 276 (1671), 3249-3256 DOI: 10.1098/rspb.2009.0574

Marris, E. (2009). Ecology: Ragamuffin Earth Nature, 460 (7254), 450-3 DOI: 10.1038/460450a

Savolainen, P. (2004). A detailed picture of the origin of the Australian dingo, obtained from the study of mitochondrial DNA Proc. Nat. Acad. Sci. USA, 101 (33), 12387-90 DOI: 10.1073/pnas.0401814101

I could be wrong …

Posted on by

… but I think that over at slacktivist, Fred is building up to something. Quick, somebody find this man a cathedral door.

Munger on Milk

Posted on by

ResearchBlogging.org editor Dave Munger inaugurates his new column on the Seed Magazine website by drawing together RB-aggregated posts about milk — and giving very kind attention to my own recent post about a new study of lactation lactase persistence in European and African populations. (Thanks, Dave!) It looks like the new column will shape up to be another good way to keep abreast of the ever-expanding science blogosphere.

Nature blogging from the ivory tower

Posted on by

Over at the Nature Blogging Network, N8 discusses the network’s “academic” category, with examples including a very nice nod to D&T.

I thought the addition of the category made a lot of sense — when I joined NBN in March, the best match I could find was “ecosystems,” which is fine, but maybe not as fitting as it might be. I think of D&T not so much as a blog about nature as a blog by someone who studies nature, which lets me post about pretty much whatever catches my attention, not just peer-reviewed research or photos from my latest hike. Also, to be honest, some of the appeal in switching to the “academic” category is that it’s a smaller pond — albeit a pond I share with Greg Laden.

I despair for humanity

Posted on by

The New York Times has just released a poll of American attitudes toward the health insurance reforms pending in Congress. There’s a lot to be depressed about, but the worst might be this: 75% of respondents are “very” or “somewhat” concerned that the cost of their health care will go up if no action is taken; but 77% have the same concern that costs will rise if action is taken. If those are completely independent probabilities, that means almost 58% of the country thinks health care costs will go up no matter what happens. (If, instead, we minimize the number of people who think costs will go up under both scenarios, it’s 52%, still a majority.)

Those numbers tell me that my fellow Americans are just scared, and they may not even know why. They’re faced, on the one hand, with an economic downturn in which it’s good news if we only lose a quarter-million jobs in a given month and health insurance is becoming more expensive and less meaningful by the day; and, on the other, with irresponsible idiots telling them that government health insurance means mandatory euthanasia for Grandma. The system is broken, and we’re terrified of the fix. And that terrifies me.

I should’ve done this when I audited botany

Posted on by

This fall I’m going to be teaching the mammalogy lab. This is going to be something of a stretch, since I actually study plants and insects — and I’m already trying to get a grip on the material. I haven’t put in any real memorization-based studying in a long time, but I’ve got my coyote skull, and I’m going to learn the names of all the bits, dammit.

One thing that helps: Genius, a freeware flashcard-creator and study tool. I spent my first time with the skull creating a list of flashcard-type associations between the bones of the skull and their descriptions, and now the program can quiz me. Incredibly, this seems to be working: I know the zygomatic process of the maxilla from the condyloid process after just a few minutes-long sessions.

My only complaint about Genius, so far, is that it’s only for Macs, which prevents me from offering to share the flashcard files with my students.


Canis latrans skull
Photo by boneman_81.

How it does a body good: The selective advantage of drinking milk depends where you drink it

Posted on by

ResearchBlogging.orgI am a super-powered mutant. For a given value of “super-powered” and “mutant,” anyway: I am an adult human who can drink milk. This is unusual among mammals, but as those (in retrospect, somewhat creepy) PSAs that used to run during my Saturday morning cartoons said, milk has a variety of nutritional benefits, if you can digest it. Which of these is behind the evolution of adult milk digestion in humans? According to a new paper in this week’s PLoS ONE, the benefit you get from drinking milk depends on where you live.

Originally, every human on Earth was lactose intolerant, like most mammals. That is, they lost the ability to digest lactose, the major sugar in milk, when their bodies stopped producing the necessary enzyme lactase after weaning. Then, some populations of humans domesticated milk-producing animals, and this seems to have generated strong natural selection [PDF] for a form of the lactase gene that remains active in adults.


Photo by bensonkua.

In fact, milk-drinking populations in Europe and Africa have evolved “lactase persistence” independently [$-a]. This parallel evolution of a single trait motivates the new study by Gerbault et al. — drinking milk might have different advantages for African pastoralists and Northern European farmers. Milk has two major dietary benefits:

  • It’s generally nutritious as a source of protein and calories, and
  • Lactose can aid in calcium uptake in lieu of Vitamin D.

The main source of Vitamin D, for humans, is sun exposure — the UVB rays in natural sunlight stimulate production of the vitamin. In Africa, close to the equator, it’s easy to get plenty of direct sunlight; but in northern Europe, sunlight is less direct — so it’s harder to produce enough Vitamin D. (This is actually thought to be one reason for geographic differences in human skin color [$-a]: under lots of direct sunlight, dark skin is favored to minimize cancer risk; but under indirect sunlight, light skin is favored to allow more Vitamin D production.)

If the benefit of milk is calcium, not protein, then we would expect adult-active forms of the lactase-producing gene to be common in northern populations, and to decrease in frequency with decreasing latitude. This has been observed in a survey across Europe [$-a] — but while the north-south pattern supports the calcium-benefit hypothesis, it is not conclusive evidence. This is because the same pattern could arise without any natural selection acting on the gene — populations generally tend to be less genetically similar if they’re farther away from each other, a phenomenon called isolation by distance, or IBD [PDF]. In fact, Gerbault et al. find that the north-south pattern of genetic similarity is replicated in genes that probably aren’t under selection arising from life at high latitudes, suggesting that IBD, not selection, is responsible for the pattern in the lactase gene.


Photo by tricky.

For a more conclusive test, Gerbault et al. developed computer simulations of the evolution of early European communities. By simulating populations’ evolution with different strengths of selection acting on the lactase gene, they could estimate how probable a particular value of selection was given the present-day frequency of lactase persistence in the real population — but also take into account the population genetic forces that create IBD. They found that in southern Europe, no natural selection was necessary to explain the present frequency of lactase persistence — but in the north, selection coefficients as high as 1.8% were needed. That is, in northern Europe, lactase persistence is so common that the simulations only produced the observed frequency when people who could not drink milk as adults had, on average, 1.8% fewer children than those who could.

In contrast to Europe, African communities don’t show the same gradual transition from frequent to rare lactase persistence, so IBD is less likely to explain the observed patterns. To explain the frequency of lactase persistence in African populations, the authors compared it to the frequency of pastoralism — and, finding a strong positive correlation, they concluded that lactase persistence evolved in Africa because it allowed shepherds to derive more nutrition from the animals they kept.

In short, widespread lactase persistence evolved in Africa because milk is a good source of protein; but it seems to have evolved in Europe because milk is a good source of bone-building calcium. Human populations on separate continents arrived at the same evolutionary solution, but for slightly different reasons.

Update, 18 October 2009: I’ve submitted this post to the NESCent competition for a travel award for the ScienceOnline 2010 conference in Durham, NC, January 14‐17th, 2010.

Update, 15 December 2009: Ye gads. I won!

References

Diamond, J. (2005). Evolutionary biology: Geography and skin colour Nature, 435 (7040), 283-4 DOI: 10.1038/435283a

Gerbault, P., Moret, C., Currat, M., & Sanchez-Mazas, A. (2009). Impact of selection and demography on the diffusion of lactase persistence PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006369

Ingram, C., Mulcare, C., Itan, Y., Thomas, M., & Swallow, D. (2008). Lactose digestion and the evolutionary genetics of lactase persistence Human Genetics, 124 (6), 579-91 DOI: 10.1007/s00439-008-0593-6

Swallow, D. (2003). Genetics of lactase persistence and lactose intolerance Annual Review of Genetics, 37 (1), 197-219 DOI: 10.1146/annurev.genet.37.110801.143820

Tishkoff, S., Reed, F., Ranciaro, A., Voight, B., Babbitt, C., Silverman, J., Powell, K., Mortensen, H., Hirbo, J., Osman, M., Ibrahim, M., Omar, S., Lema, G., Nyambo, T., Ghori, J., Bumpstead, S., Pritchard, J., Wray, G., & Deloukas, P. (2006). Convergent adaptation of human lactase persistence in Africa and Europe Nature Genetics, 39 (1), 31-40 DOI: 10.1038/ng1946

Wright, S (1943). Isolation by distance Genetics, 28, 114-38 Other: http://www.genetics.org/cgi/reprint/28/2/114

Kindle curmudgeonry

Posted on by

Via the Slog: Nicholson Baker reviews Amazon’s Kindle e-book reader for the New Yorker. He is, to say the least, skeptical.

Yes, you can definitely read things on the Kindle.

Damning with faint praise? Actually, the tone of the review is more just damning. Especially when you get to the list of books not yet available.

About the only thing I might want to do with an e-book reader is read PDF documents, which are my format of choice for journal articles. For that, I need note-taking support and the ability to rapidly zoom around on the page — and the confidence that figures will be as clear as they would be in color. I’ll stick with my MacBook for the time being.