Two way ANOVA or ANCOVA for hibernation data I am looking at differences in torpor bout lengths (hibernation times) for bats and my main grouping factor is WNS (disease) status. My study area is used for one analysis, and I basically want to do a two way ANOVA or ANCOVA when comparing to outgroups.
In past published research, the outgroups, an ANCOVA was used with BMI (random), site identity (fixed), and sex (fixed) as covariates.
I have run Spearman's correlation in my study and did not find correlation between body mass of my bats and torpor length. Shouldn't that be excluded as a covariate if there is no linearity between the covariate and dependent variable? All of my bats are male so I am not including sex as a covariate.
I have done a two way ANOVA with WNS status and Location (could be site identity) as the two variables, and both were found significant in differing areas with a Tukey HSD. Is there a better way to answer how site and disease status affects torpor?
Thanks! I have taken biostatistics, but answering something like this on my own is daunting.

Yes, there is a better way: Don't use Spearman's correlation to do model selection. Instead use regression to model torpor bout length as a function of WNS status, Location and BMI. And as @mkt reminds us, since body mass index is a ratio of weight and height, it may help to include weight and height as covariates in place of BMI. You can avoid pre-specifying the shape of the relationship between the body size measurements and torpor bout length by using splines.
The step after that is to check that the model fits the data and revise, if necessary. For example, hibernation times are positive and maybe skewed, so plot the residuals against the fitted values to check the constant variance assumption of linear regression.
All the steps to fit and validate a regression model are described in great detail and with many examples in the Regression Modeling Strategies course notes by Prof. F. Harrell.
