Does mercury exposure affect male fertility? Implications for human health and wildlife conservation.

Methods

Summary

We will assign birds to two groups: mercury-fed and control-diet. By mixing in a known concentration of mercury in to their regular food we can manipulate dietary exposure. The parents of all of the birds we are studying are fed these diets, and then we will study their male babies as they grow up. This ensures that all "mercury" birds have experienced a mercury diet their entire lives (even when they were an embryo in an egg).

Once these young males are old enough we will get sperm samples from each individual. Yes, it's not pretty, but it is possible to gently massage a male to elicit a sperm sample. There are no pictures posted here, just to keep everything PG-13.

Once we collect the sperm we can fix some samples in a formalin solution so we can look at their morphology and count the density of sperm cells, all under a microscope. For other samples, we will put the live sperm on to heated slides where we will immediately take videos of the sperm swimming. We will then analyze those videos to generate metrics of swimming performance.

Ultimately, we will let all of our males (mercury and control) have the opportunity to mate with control females, and we will see how many babies each type of male can produce through these pairings. 

Collectively, these different types of data will give us a robust test of whether dietary mercury affects male fertility. 

Challenges

Our main challenge is in getting the birds to mate with each other, in our final assay. In our experience, at least 80% of pairings will try to produce eggs. So as long as those numbers hold up we shouldn't have any problems. We have been working with these birds for more than 15 years and have confidence that we can put them in the mood for a little romance. 

Pre Analysis Plan

All of the data will be compared with derivations of linear models, where treatment (mercury vs control) is treated as a fixed factor. We might include several covariates in these models, including age and body mass. We hypothesize that fertility metrics will be influenced by the diet treatments and predict that the mercury-exposed birds will have lower sperm counts, a greater proportion of deformed sperm, and have fewer babies when paired with control females. 

If we don't find that the mercury diet influence male fertility then we will design further studies to examine female fertility. We already know that zebra finches have fewer babies when exposed to this level of mercury, but we don't know if this is due to male effects, female effects, or both.