TECHNIQUES FOR DETERMINING AGE OF RACCOONS* 
For more than a decade, it has been possible to sepa- 
rate both living and dead male raccoons (Procyon lotor) 
into adult and juvenile classes, from at least the begin- 
ning of the hunting season each year through the follow- 
ing January, by examination of the penis (Sanderson 
1950:395-6). Also, it has been possible to separate 
dead males into juvenile and adult classes by the penis 
bone alone (Sanderson 1950:392-5). A criterion for 
separating female raccoons into adult and juvenile 
classes has not been available until recently, when it 
was demonstrated (Sanderson 1961) that the lens of the 
eye can be used for age estimation in dead raccoons, 
both males and females. This criterion is especially 
useful for determining the month of birth of raccoons 
less than 1 year old. 
The primary purpose of the present paper is to 
report on one phase of extensive raccoon studies now 
being made in Illinois: new methods which have been 
established for separating males and females into ju- 
venile and adult age classes. In this paper, juvenile 
refers to an animal less than | year old and adult to an 
older animal. A secondary purpose of this paper is to 
report on progress toward developing methods for esti- 
mating the ages of raccoons of both sexes to such a 
degree that during the hunting and trapping season the 
animals may be separated into more than two age classes. 
ACKNOWLEDGMENTS 
Special acknowledgment is made to Dr. Thomas G. 
Scott of the Illinois Natural History Survey for his 
critical review of the manuscript. Thanks are due to 
several co-workers, Dr. Carl O. Mohr, Dr. Ralph E. 
Yeatter, and R. I. Smith for advice concerning the 
presentation of data and to B. J. Verts for live raccoons 
and parts of dead raccoons; and State Department of 
Conservation officers Conrad Foley and Loyd Skinner 
for many live raccoons. The manuscript was edited by 
James S. Ayars, Technical Editor of the Survey. 
MATERIALS AND METHODS 
Several raccoons of known ages, exact or approxi- 
mate, were kept in outdoor cages at Urbana, and vari- 
ous characteristics associated with age were studied. 
Some of the animals had been born in captivity; their 
exact ages were known. Others had been captured as 
juveniles; their ages at the time of capture were esti- 
mated. Most of the captured animals had been taken 
in Champaign, Edgar, or Piatt counties, in east-central 
_ “A contribution from Illinois Federal Aid Project No, 56-R, the 
Illinois Department of Conservation, the United States Bureau of Sport 
Fisheries and Wildlife, and the Illinois Natural History Survey, cooperating. 
+ Associate Wildlife Specialist, Ilinois Natural History Survey; for the 
greater part of this study Sanderson was employed by the Illinois Depart- 
ment of Conservation under terms of the Federal Aid in Wildlife Restora- 
tion Act and was assigned to the Illinois Natural History Survey for 
administrative and technical supervision. 
GLEN C. SANDERSONT 
Illinois; some had been taken in Carroll County, in 
northwestern Illinois. 
Studies made on captive animals were supplemented 
by observations made on raccoons living in the wild. 
Several juvenile raccoons were live trapped on the 
Robert Allerton Park study area near Monticello, Piatt 
County, in east-central Illinois. Their ages were esti- 
mated, and the animals were marked and released at 
the points of capture. When one of these marked ani- 
mals was killed by a hunter or trapper and made avail- 
able for examination, it was studied in order to see if 
there were significant differences between wild and 
captive raccoons in the characteristics under considera- 
tion for estimating ages. 
The principal guides used for aging raccoons were, 
for animals of both sexes, (1) body weight, (2) the de- 
gree of ossification of the epiphyseal cartilage of the 
radius and ulna, fig. 1, and, for the male, (3) the 
size and degree of ossification of the penis bone or 
os baculum. The mean date of birth for raccoons in the 
northern half of Illinois was used to assign an approxi- 
mate age to juvenile raccoons of both sexes that were 
not weighed or that were taken after November 1. 
Body weight, the first guide or criterion investigated 
for aging raccoons, was studied in both captive and 
wild individuals. Several young raccoons born in cap- 
tivity were weighed at irregular intervals until they were 
6 months of age, and their weights in relation to their 
ages were plotted on a graph, fig. 2. Each of these 
young born in captivity was removed from its mother 
at some time prior to 2 months of age. Weaning at an 
early age was made necessary by the requirements of 
another phase of the raccoon studies; young raccoons 
normally nurse for about 4 months. It is believed that 
captive and wild raccoons that are nursing show similar 
rates of gain in weight. The weights of captive young 
subsequent to weaning showed that the average weight 
gains were greater for captive than for wild young. 
Therefore, in the first step in estimating the ages of 
wild raccoons, only the weights of captives previous to 
weaning were used. 
A line was fitted by eye to the points on the graph 
representing the weights of the nursing raccoons born 
in captivity. The weights of these captives took the line 
on the chart up to 2 pounds. Points representing the 
weights of captured young raccoons born in the wild 
and weighing less than 2 pounds at initial capture 
were placed on the line established for the nursing 
raccoons. The age of each of the wild animals at 
capture was estimated by reference to the point at 
which a vertical line drawn through its weight point 
intersected the base line of the graph. These wild 
young were released at the places of capture after they 
