298
CHARLES J. KREBS AND JUDITH H. MYERS
60
5 0
?
c
0
E 4 0 -
- x -
0
3 3 0 -
2
-
9)
WJ
0 2 0 -
Y
-
2 1 0 -
c
5
0
I
' " I
I
I
[
I
I
I
I
,
l
l
~
l
l
~
-
1964
-
-
a
-
-
-
-
0
-
1961
a o
O
1958
1962
-
0
1960
1963
1959
1
1
"
'
1
"
~
"
'
1
'
'
b - b -
0
3
6
S
12
15
18
1959
1960
1961
1962
Low
Peak
Decline
Low
FIG. 17. Median body weight at sexual maturity for brown lemming (Lemmus
trimucronatua) summer-born young, Baker Lake, Canada. Only the first summer
litter is included. (After Krebs, 1964%)
FIG. 16. Percentage of young Ckthrionomye rmtilw becoming sexually mature
in their first summer in relation to the population density. Young sampled from
June 1 5 J u l y 31 each year; density measured by snap-trap catch per 100 trap
nights in May. 0 = males;