Deer mice like the one above are small parts of a complex and interconnected world. When two pieces of their world work against them simultaneously, how are these mice affected? (Image Credit: USDA, CC BY 2.0).

Botfly infections impair the aerobic performance and survival of montane populations of deer mice, Peromyscus maniculatus rufinus (2019) Wilde et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13276

The Crux

Parasites are bad news for the organisms that host them. Some parasites are so bad, they can actually make the host kill itself. Despite these clear and obvious costs to infection, the common consensus is that parasites are not too big of a deal for the host, because of how rare parasitic infection is on average. For example, in my research system only one in ten animals have parasites.

But when these ill-effects of parasitism are combined with other detrimental factors, such as a harsh environment, an organism with parasites is forced to deal with not one but two stressors. The authors of today’s paper were interested in how these effects of parasites may change depending on the environment that the host lived in.



What They Did

Deer mice (Peromyscus maniculatus rufinus) are a model system for investigating the effects of parasites on a host organism, and they are commonly infected by botflies (Cuterebra spp.), the larvae of which are laid inside of the mouse. They feed off of and grow inside the host before emerging for their adult stage (look this up or don’t, but be warned that it is NASTY).

In addition to their practicality as a model system, deer mice also inhabit the broadest elevational range of ANY North American mammal, making them perfect for investigating the effects of parasites across an environmental gradient. Using two different elevational transects, the authors investigated how the parasites affected a variety of physiological parameters such as heamoglobin concentration (used to transport oxygen throughout the body) and metabolic rate, in addition to overwinter survival of the mice. The transects ranged in elevation from ~2300m to ~4000m, allowing the authors to measure these effects up to the limits of both deer mice and botfly population ranges.

Did You Know: Living at Elevation If you have ever been ~1 mile/1600 meters above sea level, then you may remember finding it a bit harder to catch your breath. Simple physical activities like walking up a flight of stairs suddenly leaves you gasping for breath, when at lower elevations you wouldn’t even notice an increase in your breathing rate. That’s because the air is thinner at these higher elevations, and as a result mammals that live there have evolved a suite of traits to tolerate this lower level of oxygen. This also happens in humans, as people living in high-elevation areas acclimate over time by increasing the amount of haemoglobin in their blood, meaning they can transport more oxygen throughout their bodies.

What They Found

Mice infected with botflies had a lower haemoglobin concentration than those without the botfly larvae, and this lower concentration was also evident in mice that had been previously infected but had recovered. Infected mice had a daily survival rate 15% lower than the mice without botflies, meaning that few (if any) infected mice would survive the winter. Interestingly, at one of the sites the effect of parasites on survival was much more severe for male mice than it was for the females.

Despite inhabiting a wide elevational range throughout the Rockies, no mouse above 2400m had botfly infections. Since the infected mice had lower haemoglobin concentrations, infected mice may not be able to acquire enough oxygen at these higher elevations, meaning that both they and their botfly parasites wouldn’t survive.

Problems?

The authors used mass as an explanatory variable in their statistical analysis, allowing them to test for an effect of the mass on the different metabolic processes that they measured. While this is interesting to ask, it seems irresponsible to do in the light of metabolic processes. Physiological reactions such as metabolic rate are to be affected by mass, and if they are using mass, instead of controlling and correcting for it, they run the risk of conflating the effects of mass with other interesting effects, and thus may not be seeing the “true” pattern.

So What?

The results of this study provide evidence that the selection pressures inherent to alpine environments like the Colorado Rocky Mountains may be mediated by biotic interactions with other organisms, in this case parasites. This adds to the body of work on the context-dependent costs that parasites impose on their hosts.

These context-dependent cost meant that the mice at higher elevations survived more than than those at lower elevations, running in direct contrast to the previous studies on small mammals. Studies like this one highlight the complexity of the natural world, adding yet another wrinkle to the puzzle that is ecology.