|Abstract or Summary
- Reproductive characteristics of introduced eastern cottontail
rabbits, Sylvilagus floridanus (J. A. Allen), were determined from
486 rabbits collected between January 1, 1968 and June 30, 1969,
near Corvallis, Oregon.
Sex ratios were 1:1 for all rabbits collected and for all embryos
20 days or more gestation. On the basis of weights of testes and
amounts of sperm in the cauda epididymides, adult male cottontails
were considered to be in breeding condition December through
August, but were not considered to be in breeding condition during
September and October. Much variation existed in the age at
which juvenile male rabbits became sexually mature. Although
some juvenile males matured sexually during their first summer
of life (at about four months of age), others did not achieve breeding
condition until the December or January immediately preceding their first full breeding season.
On the basis of litters in utero, the 1968 breeding season was
considered to extend from mid-January to mid-September. Many
adult female rabbits apparently produced up to 39 young in up to nine
litters during 1968. Sizes of litters produced by adult female rabbits
varied with chronological sequence in 1968 and 1969. First and last
litters averaged 3.75 and 4.00 young per litter respectively, whereas
the remainder of the litters averaged 5.00 or more young per litter.
Weather apparently affected the onset and termination of breeding.
Cold, snowy weather during January and early February, 1969
appeared to delay the onset of breeding until about three weeks later
than the onset of breeding in 1968. On the basis of age composition
of rabbits collected January-June 1968 and 1969, it was concluded
that more young rabbits were produced during August and September
1968 than during the same interval in 1969. Since the summer of
1967 was the driest on record and since the summer of 1968 was one
of the wettest on record, termination of breeding in 1967 and 1968
possibly indicates the widest possible span of time in which this
phenomenon might be expected to occur.
Eleven (52 percent) of 21 juvenile females over 2.5 months of
age were reproductively active during 1968. Three of these rabbits
produced at least two litters during their first summer of life. Five
juvenile female rabbits produced an average of 3.40 young per litter. Total productivity of the population was not estimated because
the total contribution of young by juvenile rabbits could not be determined.
However, it was concluded that total annual production of
young by adult female eastern cottontail rabbits potentially was greater
in western Oregon than throughout most of the natural range of the