Ooh thanks a lot! I forgot the realization of a GP is an MvNormal 
I’m gonna try that and keep you posted 
Do you know if there is a way to specify the mean function without the custom class though, as I did for the cov func? This is for the port of chapter 14 of Rethinking 2, so I’d like the code to be as simple as possible.
M, G and B are respectively body mass, group size and brain size, and in the book only the covariance function is a function of the phylogenetic distance (Dist_mat) between species 