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CHARLES J. KR.EBS AND JUDITH H. MYERS
year. He found that wounding was confined almost entirely to adult
males; females had few signs of wounds on the skin. Immature young
males were not subject to wounding either. Christian suggested that
when the age at sexual maturity is low, fighting will be more prevalent
because a larger proportion of the male population will be mature.
He therefore predicts more fighting a t low densities because of early
maturation, but thinks that the influence of the larger portion of
belligerent males is offset by the lower density.
Wounding was not greater in higher density populations of M .
californicus (Batzli and Pitelka, 1971). Again the amount of wounding
observed in males was greater than that in females.
Aggression, fighting and wounding may be greater in Clethrionomys
than in Microtus. In a year of low abundance of C. rytilus Koshkina
(1 965) observed badly wounded individuals, particularly among those
who were sexually mature. She judged that the wounding was sufficient
to kill the animals, and badly injured voles were not often recaptured.
There are two difficulties in interpreting data on skin wounds.
First, the amount of wounding can only be a very crude behavioral
index of aggressiveness. Many of the techniques by which animals
space themselves do not involve physical aggression (Lorenz, 1963).
Second, wounding indices confound the two variables of population
density and aggressiveness. We cannot decide from wounding data
whether (1) the average level of individual aggressiveness is constant
and independent of the cycle, and all the changes in wounding are a
result of changes in density and therefore more numerous interactions,
or (2) whether the behavior of individuals also changes in relation to
density.
We know little of the factors affecting contact rates of individual
voles or lemmings in the field. Pearson’s (1960) elegant photographic
work on runway usage should caution us that contact rates may not be
a simple function of population density. There are indications that
the area traversed by individuals is smaller when population densities
are high (Krebs, 1964a, 1966, and unpublished; Koshkina, 1965).
By restricting its area of activity as the population density increases,
a vole may be successful at maintaining an almost constant rate of
intraspecific interaction regardless of population density. Because we
cannot be certain of the social climate in field populations we need to
obtain behavioral measurements on individual animals, and to look at
some of the details of social organization in microtine rodents.
Techniques for measuring aggressiveness on live rodents have been
used in psychological research for a long time, but few attempts have
been made to apply these techniques to voles and lemmings. The
simplest procedure is to observe two individuals in a fighting arena.
CHARLES J. KR.EBS AND JUDITH H. MYERS
year. He found that wounding was confined almost entirely to adult
males; females had few signs of wounds on the skin. Immature young
males were not subject to wounding either. Christian suggested that
when the age at sexual maturity is low, fighting will be more prevalent
because a larger proportion of the male population will be mature.
He therefore predicts more fighting a t low densities because of early
maturation, but thinks that the influence of the larger portion of
belligerent males is offset by the lower density.
Wounding was not greater in higher density populations of M .
californicus (Batzli and Pitelka, 1971). Again the amount of wounding
observed in males was greater than that in females.
Aggression, fighting and wounding may be greater in Clethrionomys
than in Microtus. In a year of low abundance of C. rytilus Koshkina
(1 965) observed badly wounded individuals, particularly among those
who were sexually mature. She judged that the wounding was sufficient
to kill the animals, and badly injured voles were not often recaptured.
There are two difficulties in interpreting data on skin wounds.
First, the amount of wounding can only be a very crude behavioral
index of aggressiveness. Many of the techniques by which animals
space themselves do not involve physical aggression (Lorenz, 1963).
Second, wounding indices confound the two variables of population
density and aggressiveness. We cannot decide from wounding data
whether (1) the average level of individual aggressiveness is constant
and independent of the cycle, and all the changes in wounding are a
result of changes in density and therefore more numerous interactions,
or (2) whether the behavior of individuals also changes in relation to
density.
We know little of the factors affecting contact rates of individual
voles or lemmings in the field. Pearson’s (1960) elegant photographic
work on runway usage should caution us that contact rates may not be
a simple function of population density. There are indications that
the area traversed by individuals is smaller when population densities
are high (Krebs, 1964a, 1966, and unpublished; Koshkina, 1965).
By restricting its area of activity as the population density increases,
a vole may be successful at maintaining an almost constant rate of
intraspecific interaction regardless of population density. Because we
cannot be certain of the social climate in field populations we need to
obtain behavioral measurements on individual animals, and to look at
some of the details of social organization in microtine rodents.
Techniques for measuring aggressiveness on live rodents have been
used in psychological research for a long time, but few attempts have
been made to apply these techniques to voles and lemmings. The
simplest procedure is to observe two individuals in a fighting arena.