Data collection

Daily inspection of traps occurred each morning, and each trap was reported as open, closed with an animal trapped, closed but no animal trapped, bait removed or stolen, or damaged. When baits were removed they were resupplied each morning. The number of non-target species caught was corrected for, along with sprung but empty traps, when computing capture success rates. Thus, trap success was used to estimate the relative rat abundance as follows: number of rats trapped x 100/(number of effective traps x number of nights), where the number of effective traps each night was given by the number of fully functional traps minus half the number of traps that were sprung but empty or contained non-target species [27]. Stolen and damaged traps were excluded from this calculation.

After euthanasia, the species, sex, mass, and body length were recorded. In females, sexual activity was characterized by the presence of placental scars, pregnancy (number of embryos was recorded), or evidence of lactation. Sexual maturity in males was determined by convoluted seminal vesicles [28]. Also, the presence or absence wounds to the skin was recorded.

The von Bertalanffy equation is often used with reference to growth curves to convert weight to age for mammalian and in particular rodent populations [29]. We converted the recorded weights of the animals to ages using the von Bertalanffy equation. The parameters for the von Bertalanffy were found by fitting data deduced from the growth curve for male rats presented by Calhoun [30]. Also, we estimated the body condition using a ‘Scaled mass index’ (Smi) based on mass and body length, whilst accounting for the effect of age [31].