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.