A few captive raccoons of both sexes ranging in age 
from 2 to 17 months were castrated to permit study of 
the effects of castration on closure of the epiphyses in 
animals of each sex and on development of the bacula 
in males. 
The average date of birth calculated for raccoons in 
the wild was used to arbitrarily assign an age to a 
raccoon found to be juvenile by X-ray of epiphyseal 
cartilage where (1) the body weight of an animal was 
not available; (2) the animal weighed more than 7.0 
pounds; or (3) the animal was caught after November 
1. Actual or approximate birth dates were determined 
for 17 litters conceived in the wild in the northern half 
of Illinois during 1958, 1959, and 1960. Four of the 
17 litters were born in captivity. Potential birth dates 
for the other 13 litters were estimated by measuring 
embryos in females as described by Llewellyn (1953: 
$21); most of the females examined were found dead 
along highways. The average date of birth for raccoons 
in the northern half of Illinois was assumed to be the 
same as that calculated for these 17 known or esti- 
mated dates of birth. 
RESULTS 
Most raccoons in the northern half of Illinois are 
born during April, table 1. The mean birth date, actual 
or potential, for 17 litters conceived in the wild was 
April 16; the earliest birth date was March 9, and the 
latest June 10. 
The mean birth date for six litters conceived and 
born in captivity was April 26. The monthly distribu- 
tion of these birth dates is shown in table 1. The 
earliest birth date for a litter conceived in captivity was 
March 16, and the latest was June 3. 
Although 160 wild female raccoons were examined 
for pregnancy during January, and several were ex- 
amined during February, July, and August, table 1, 
only one pregnant female was observed during these 4 
months, and this one, when examined on February 25, 
appeared due to give birth in about 3 weeks. 
Age Estimation by Weight. — Although there were 
no significant weight differences between males and fe- 
males, birth weights and growth rates of young captive 
raccoons varied among individuals. The weight varia- 
tions were probably influenced by the number of young 
per litter, the mother’s capacity to produce milk, soil 
types of the habitats, the availability of food other than 
milk, and individual genetic differences. Nevertheless, 
weights were useful im estimating the ages of juvenile 
wild raccoons weighing up to 7 pounds. The growth 
rates of 33 captive raccoons, reared in central Illinois 
up to 2 months of age, table 2, were similar to those 
reported by Hamilton (1936:136). The average 
weights of 60 wild raccoons which were caught in live 
traps two or more times each are presented in table 3. 
The interval between weighings varied from 1 to 119 
days (mean = 30 days), and the data obtained encom- 
passed weight changes that ranged from a 2.7-pound loss 
in 63 days to a 6.8-pound gain in 116 days. The weights 
of these animals ranged from 1.4 to 7.0 pounds at the 
time of initial capture, which occurred between June 
28 and October 29. 
If body weight indicated that a raccoon was born 
prior to March 1 (as occasionally happened), March | 
was considered to be the birth date, because few, if any, 
raccoons are born prior to this date in the northern 
half of Illinois. As an example, one male weighed 4.2 
pounds when caught on July 10, indicating an age of 
4.5 months, fig. 2. The weight at date of capture would 
indicate February 15 as the date of birth. 
of February 15, the date of birth was recorded as March 
1 and the age as 4 months. 
Data in table 3 suggest that the growth curve of 
young raccoons in the wild rises slowly during July, 
August, and the first half of September in central Illi- 
nois when most of these animals are 121 to 180 days 
of age. Of 87 young animals retrapped one or more 
times each in the wild during this period, 9 (10 per 
cent) weighed the same as they did when first trapped, 
and 26 (30 per cent) had lost weight. 
The average growth rate of captive young of 2.54.5 
months of age appears to be somewhat greater than 
that of wild raccoons of similar ages. This difference 
in growth rates probably reflects the abundance of high 
quality food easily available to the captives. When cap- 
tives were weaned at 2 months of age or younger, 
weaning was followed by slow growth for about a month, 
probably reflecting adjustment to the new diet. 
Age Estimation by Epiphyseal Development. — 
X-ray photographs and/or direct examinations of radii, 
ulnae, and some humeri of 45 intact male raccoons, 
some wild and some captive, revealed that: (1) a 
broad plate of epiphyseal cartilage was present between 
the diaphysis, or shaft, and the epiphysis, or head, fig. 3, 
of each radius and ulna in 35 males ranging from 4 to 
15 months of age, 9 being 13-15 months of age, table 4; 
(2) thin epiphyses were present in 17 males ranging 
from 13 to 17 months of age, but only 3 males in this 
group had thin epiphyses after 19 months of age, and 
4 had thin epiphyses at 13 months of age; and (3) that 
the epiphyses closed in 12 males after they were 15 
months of age and before they reached 30 months of 
age. One male had closed epiphyses when he was first 
X-rayed at 14 months of age. Eight had epiphyses 
which closed some time after the animals reached 15 
months of age and before they reached 20 months of 
age; the epiphyses of 2 males were closed at the time 
Instead 
they were first examined, at 20 and 21 months of age, 
and the epiphyses of another male were not closed at 
27 months of age but were closed at 29 months of age. 
In this study, 11 of 19 males which were 16-22 months 
old had closed epiphyses; 8 had thin epiphyses. All 
males with closed epiphyses were 14 months of age or 
older. Although considerable 
variation, there appeared to be no significant difference 
there was individual 
in time of epiphyseal closure in wild and captive males. 
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