Living fast and dying of infection: host life history drives interspecific variation in infection and disease risk

Johnson et al.. Ecology Letters 15(3): 235-242. Living fast and dying of infection: host life history drives interspecific variation in infection and disease risk

Use of a Billy Joel album cover does not imply any indorsement of Billy Joel by PEGE. Taken from billyjoel.com

Use of a Billy Joel album cover does not imply any indorsement of Billy Joel by PEGE. Taken from billyjoel.com


Jennifer Garbutt

Jennifer Garbutt

I picked this paper because it tackles some of the host-parasite ecology questions that I study (why do some hosts get sick while others stay healthy? How do different life history strategies affect infection risk? Do body size and growth rate matter?), but on an inter specific rather than intraspecific scale. The paper asks whether host “pace-of-life” affects infection risk and the authors’ approach was to compare life-histories and infection outcome across amphibian species.

The pace-of-life idea comes from the eco-immunology hypothesis that mounting an immune response is costly and therefore trades-off against growth and reproduction. Long-lived species should benefit most from defence and invest accordingly, whereas short-lived species should invest instead in growth and reproduction.

The authors infected members of 13 amphibian species with one of my favourite parasites, the developmentally-disruptive trematode Ribeiroia ondatrae. Infected hosts develop malformed, missing or extra limbs. There’s some evidence that these abnormal limbs put the host at greater risk of predation from birds, which is the next host in the trematode’s complex life cycle (Goodman and Johnston, 2011) – so you can see why I think these parasites are pretty cool!

The main finding of the study is that long-lived, slow-developing amphibian species (with a slow pace-of-life) suffered less mortality and abnormality, as well as having a reduced parasite load. The traits that contributed most to infection outcome were size at metamorphosis and time to metamorphosis. A slow pace of life could be beneficial either because slow-paced hosts (i) have a prolonged larval period and so have more time for the repair of parasite-induced damage, or (ii) are larger when inoculated and thus receive a smaller dose of parasite relative to their mass.

I really admire the interspecific comparative approach taken in this paper, but feel that this strategy is inherently problematic because Ribeiroia may have differing evolutionary associations with the amphibian species. The authors say that all the species “likely overlap with Ribeiroia in nature” but it is not easy to be sure that all the species are equally important natural hosts for the parasite. However, any difference in evolutionary host-parasite relationships should cause extra variation in the infection experiments that would, if anything, limit the power of the authors’ analyses, making their inferences conservative. It would be interesting to complement this study with intraspecific experiments in some of the amphibian species to see if variation in life history on this level also affects infection risk (without the confounding effect of host-parasite evolutionary relationships).

I also really liked the study because the most important life history measures to impact infection outcome were early growth characteristics. I like this result because I study the impact on disease of maternal effects, which are normally more pronounced early in life, so this study supports my idea that early life characteristics can be really important in determining infection risk.


Will Pearse

Will Pearse

This is a neat paper; species that live fast are more likely to die of infection, and the authors argue that makes sense because there are better things to worry about (breeding) than looking after yourself if you’re going to die soon anyway. I also think this paper wins the “Understated Methods” award for the rather easy-to-miss statement “experiments were conducted over a 13-year period”!

I was initially surprised that amphibians are this maleable; normally when talking about what I call ‘fire-and-forget’ life history strategies I talk about Drosophila vs. humans, and there are quite a few million years between us to cover all manner of evolution. However, looking at figure 1, I can see that some of these species have most recent common ancestors over 200 million years ago – that’s plenty of time for evolution to kick in. I was also surprised that investment in immunity isn’t more of a discrete trait; it seems there really is an advantage to be ever-so-slightly-more immune in the long-run than another species, which (perhaps based on my fire-and-forget ignorance) I wasn’t expecting.

I’m intrigued by the other pieces of these animals’ life histories, though. Are amphibians that are good at dealing with pathogens also excellent at dealing with environmental stressors? I’d expect extremely mobile (high dispersing) species to have worse immune systems (they’re running faster than their pathogens), and equally they might have much faster life histories because they breed faster to colonise. By that argument, perhaps there could even be a correlation between age of a species and its resistance to pathogens, mixing some taxon cycle ideas with Rickleffs’ pathogen-based speciation ideas. Maybe it really is pathogens all the way down…

Advertisements

About will.pearse
Ecology / evolutionary biologist

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: