Female fluid collection
We obtained captive Peromyscus maniculatus bairdii, P. polionotus subgriseus, P. leucopus , P. eremicus , and P. californicus insignis from the Peromyscus Genetic Stock Center at the University of South Carolina, and P. gossypinus from Dr. Hopi Hoekstra at Harvard University. We housed the mice in same-sex cages at 22ÂșC on a 16L:8D cycle in accordance with guidelines established by the Institutional Animal Care and Use Committee at the University of Maryland (protocol # R-Jul-18-38). We sought samples from all available captive Peromyscus species and avoided wild-caught specimens to control for variation due to age, nutrition, and sexual experience. We collected fluid from females in estrus (identified by methods in Byers et al., 2012) to reduce variation associated with the estrous cycle (Hunter et al. 2011; Simons and Olson 2018). We euthanized all focal females via isoflurane overdose and cervical dislocation prior to removing the reproductive tract for fluid extraction.