Behavioral Ecology Blog

There is a pretty cool paper in a recent issue of PRSOL by Hutchings, et.al.

The summary:

1. Sheep eat grass, and would like to avoid eating parasite containing poo
2. The poo grass contains more nutrients that the clean grass
3. Sheep vary in their innate ability to stave off serious endoparasitic infection

So if you were to make a prediction, you would obviously predict that resistant sheep would take advantage of this resistance and feed on the nutrient rich poo grass… Unfortunately, these guys did the study, and found that the resistant sheep were actually MORE wary of the parasite infested poo grass than were parasite sensitive animals…. wierd, not only were certain animals more likely to be infected, but it was these sensitive animals that had increased behavioral risks.

Before you worry too much, the patterns of resistance and susceptibility are well studied with microarray based expression work, and experimental manipulation. For whatever reason, resistant sheep are behaviorally adapted to limit infection.

Turns out, the authors have this pretty well worked out, and all but rule out the chance that these 2 traits (susceptability and behavior) are linked. Instead, they suggest that this behavioral adaptation arose spuriously during the “natural conditions” part of their experiment where:

After the period of selection under experimental challenge, the Perendale lines experienced 13 years of selection (using the same criteria on each year’s lambs) based on FECs during mixed gastrointestinal nematode parasite challenge through grazing (i.e. natural challenge), during which time selection could have continued being not only for immune function but also for selective grazing (i.e. parasite avoidance; see flock history below). Both immune function and avoidance of parasites by hosts influence parasite excretion levels and may have been included in the selection programme under natural challenge.

Basically

It is at least theoretically possible that the genes associated with the grazing behaviour were close to those for immune function and were co-selected. It would seem more likely that behaviours enhancing the difference in FEC scores between the selection lines were included during the natural parasite challenge selection period (i.e. the selection acted on between individual variations in parasite avoidance, e.g. level of faecal avoidance).

So this is one of those examples of “I would’ve been wrong if I hadn’t done the experiment” types of paper. It’s one of the best reasons why people should continue to fund and participate in basic science research. These are all complex systems we deal with, and while theory is great; it can lead us astray.

I’d be interested in knowing about the differences in parasite load between behavioral adapted/resistant sheep versus behaviorally adapted/sensitive hosts… It’s not too unreasonable to guess that they are similar.