Résultats surprenants (Suprising results)

The last several weeks have been very busy with the first major experiment of PODARCIS (described in this earlier post). We are testing the effects of high-altitude hypoxia on lizard sprint speed, endurance, metabolic rates, and blood oxygen-carrying capacity. By brining lizards from low-elevation populations up to high-altitude at Pic du Midi, this experiment will allow us a first glance at the potential for within-individual plasticity to respond to lower levels of available oxygen. Our recently accepted paper from last summer examined a similar question, but with developing embryos. Experiments next year will test for variation in these traits among lizards from populations at different altitudes. Taken together, we hope to quantify the capacity for lizards to move up in elevation – through short-term physiological responses, developmental responses, or perhaps via standing genetic variation within populations.

Posing the importance of these experiments in this larger context offers a reminder of the bigger questions we are hoping to address. These are easy to forget sometimes while in the midst of the day-to-day grind of an experiment. Mahaut and I have been working tirelessly, both in Moulis and at Pic du Midi, at the sometimes-tedious task of data collection. This includes long days of running lizards down the racetrack (sometimes close to 200 runs a day!), collecting blood samples, measuring metabolic rates, conducting wet lab work, and traveling between laboratory sites. When this experiment is done, we will have a hefty data set to work with!

But who wants to wait until the end to look at the data? I’m glad to say that we’ve been able to work through some preliminary results as we go and so far they are…interesting! And surprising! Just as good biology should be.

First, I was quite intrigued by the results of the sprint trials. Unlike the clear decrease in speed we saw with our pilot study earlier this summer (described in a blog post here), lizards in the experiment do not seem to slow down over the repeated runs, either at low or high elevations. In fact, their top speed (the fastest 50 cm of the racetrack) seems amazing consistent for both treatment groups, at least on our first measurement day.

Results of sprint speed trials for first timepoint. Lizards were rested for 30 s between sprints. Data for each individual shown in different color; black line shows mean. Notice the apparent lack of decline in performance over time.

I’m unsure what to make of this, especially in contrast to what we saw earlier. While we may not have the data to examine the impacts of hypoxia on performance decrement, this will provide some very interesting data on the individual consistency (repeatability) of sprint speed as a trait both in the short-term (over several minutes) and long-term (over several weeks).

In line with some classic physiology papers (for example, see Vinegar and Hillyard, 1972), our lizards at high altitude are altering their blood chemistry related to oxygen consumption. For example, after three weeks at high-altitude, lizards in the hypoxia treatment have increased their haematocrit (Hct) relative to lizards kept at their native altitudes.

Results of haematocrit measures at the second timepoint (after 3 weeks). Haematocrit is significantly higher in the hypoxia treatment (p < 0.0001).

Haematocrit is the proportion of blood that is composed of red blood cells, which are responsible for oxygen transport. The obvious interpretation is that these lizards are adjusting their blood composition to facilitate oxygen transport. Interestingly, however, the results of our red blood cell counts show no differences between treatment groups, despite the difference in haematocrit.

Results of red blood cell counts at the second timepoint (after 3 weeks).

One possibility is that the animals aren’t increasing their red blood cell numbers, but perhaps their size. Once the experiment is done, we will analyze the haemoglobin concentration in the blood, which will provide more insight into exactly how the lizards are adjusting their blood for low oxygen availability. 

Once the data are collected, an important (and sometimes overlooked) job of the biologist is to put together a plausible story of what is happening in their particular system. Sometimes these stories come together like a Hemingway novel, other times a bit more like Faulkner. I’ll be taking the first pass at a narrative when I present a talk at the Colloque d'Écophysiologie Animale in Strasbourg the first week of November. I’ll be giving a talk titled, “Lizards on the peak: Hypoxia induces physiological responses and bears performance consequences in Common Wall Lizards transplanted to high elevations.” Mahaut and I will be attending and are very excited to discuss these results with other researchers, especially over a couple beers!