Mary J. Taitt

THE EFFECT OF EXTRA FOOD ON SMALL RODENT POPULATIONS: II. VOLES (MICROTUS TOWNSENDII)

SUMMARY (1) Voles on three areas were provided with different levels of extra food in the form of laboratory chow and oats for approximately one year. (2) On areas with intermediate and high extra food, population density increased to twice the control density. (3) Extra food increased immigration and reproduction, and shortened the winter non-breeding season. (4) Males and females had smaller home ranges on areas with extra food. This may have facilitated immigration to these populations. (5) Voles were removed from three other areas and re-colonization was measured. Immigration was related to the density of extra food; three times as many voles colonized a high food area compared with a control. (6) These results, combined with those of previous research which has shown that a viable surplus of voles exists in natural populations, suggest that the voles population size is limited by both behaviour and food.

MANIPULATION OF FEMALE BEHAVIOUR IN FIELD POPULATIONS OF MICROTUS TO WNSENDII

SUMMARY (1) The behaviour of female Microtus townsendii was manipulated in field populations during a spring decline in numbers. All females were implanted with testosterone to increase aggressiveness in one area, and force-fed the chemosterilant mestranol to reduce aggressiveness in another area. (2) Testosterone treatment resulted in wounding among females, increased size of female home ranges, a reduction in female survival, and increased female immigration. (3) Males in the population with testosterone-treated females had the same population dynamics as males in a control population and a population with no females. (4) Mestranol treatment had no effect on female dynamics. Males with mestranoltreated females survived better than control males. (5) In spite of the profound affect of testosterone on social strife, the spring decline in numbers was little affected by these experiments. Female spacing behaviour may operate in more subtle ways than overt aggression.