1. Transmission is a key component of a pathogen’s life history, but it is often difficult to obtain reliable ecological estimates of the dominant mode(s) of transmission and how host traits, such as sex, influence transmission dynamics.
2. Here, we have developed a robust system pairing the use of semi-natural populations of house mice (Mus musculus) with modern molecular viral titering approaches to evaluate the transmission dynamics of Friend Virus Complex (FVC). 
3. Semi-natural populations were founded with female and male mice that were either ‘index’ (initially infected via intraperitoneal injection) or ‘contact’ (uninfected at the start of experiment). Three experimental population designs were implemented: female index only, male index only, and both female and male index.
4. Utilizing these enclosures, we found male-male transmission to be the predominant mode of transmission, with low rates of horizontal female transmission and no evidence of vertical transmission. Additionally, contact males that became infected harbor higher titers than females, while suffering a similar amount of spleen enlargement; this, paired with a strong correlation between titer and spleen size in males, and no correlation in females, indicates FVC viral dynamics differ between the sexes.
5. Lastly, natural transmission may be an impediment to FVC replication as infected contact animals have a lower viral titer than index animals. These results illuminate the complex life history of murine leukemia viruses and demonstrate the important role of host sex on pathogen transmission dynamics.

Cage Spleen Mass and Titer

Spleen mass in grams is used as a proxy for virulence of the pathogen in mice (each row is a mouse) of both sexes (Sex), while FMuLV proviral load is used to quantify viral fitness. SFFV Infection Status indicates the presence/absence of this viral particle.

The raw FMuLV proviral load data provided was log transformed and multiplied by spleen mass in order to get viral-load-per-spleen measurements for most analyses. Furthermore, only log transformed proviral load data was used to generate correlations with spleen mass. For more information, please see methods in the manuscript.

Enclosure Spleen Mass and Titer

Spleen mass and FMuLV titer are provided for mice (each row is a mouse) of both sexes (Sex), from seven independent populations (Population). Mice had initial infection statuses of either “contact” or “index”, and were incorporated into three population designs (female only index, male only index, or female and male index). SFFV Infection Status indicates the presence/absence of this viral particle. The raw FMuLV proviral load data provided was log transformed and multiplied by spleen mass in order to get viral-load-per-spleen measurements for most analyses. Furthermore, only log transformed proviral load data was used to generate correlations with spleen mass. For more information, please see methods in the manuscript.

FVC Transmission Dynamics

The count of contact mice, by sex, that were either infected or uninfected at the termination of the study from seven populations representing three population designs (female only index, male only index, or female and male index). Both viral genomes (FMuLV and SFFV) are represented in Virus column.