Dissection
Following trapping, mice were euthanised in accordance with Home Office guidelines using increasing CO2 concentrations. Death was confirmed by exsanguination. External physiological variables taken were: (i) body weight, the mass of the mouse in grams, and (ii) snout-vent length, the distance from the tip of the nose to the anus. These values were then used to calculate Scaled Mass Index, a measure of relative body mass which scales against a curve for the population as a whole, to be used as a proxy for animal condition (Peig & Green, 2009). The sex of the animal was also confirmed at this point.
Blood samples were obtained by pipetting during exsanguination, allowed to clot at room temperature for 1 hour, then stored at 4°C for another hour. The blood was then centrifuged at 13,500 RPM for 5 minutes, and resulting clots were retained and stored in liquid nitrogen for microparasite diagnostics, then kept at -80°C upon return from the field. The gastrointestinal tract was removed and stored at room temperature in 80% ethanol.
During dissection, the eyes were removed and stored in 10% buffered formalin. Later, the eye lenses were dissected out, dried and weighed, and the resulting measurements used to estimate age (Rowe, Bradfield, Quy, & Swinney, 1985). Eye lens mass is commonly used to estimate age as it grows continuously throughout an animal’s lifespan (Augusteyn, 2008). When compared to body mass, body length, tail length, foot length and ear length in corn mice, eye lens mass proved the most reliable at discriminating age (Carreno, Brigada, Rosi, & Castro-Vázquez, 1990). Bodies were wrapped in paper roll until ectoparasite surveys could be conducted in order to prevent parasites from being lost.