Mr M. David responded to our recent article in which we introduce the Pregnancy Compensation Hypothesis. Mr David writes to bring up the role of endogenous retroviruses (ERVs) in the evolution of placentation. While the mechanistic evolution of the machinery to build the placenta as an organ was out of the scope of our original article, we agree that the role of ERVs in evolution of the placenta and immune modulation is an interesting subject that may provide useful insights into the evolutionary mechanisms contributing to sex differences in gene expression, immune functions, and disease prevalence.