Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Thomas A. Nelson


Beavers (Castor canadensis) have been extensively studied throughout North America, but little research has been conducted on this species in Illinois and the Midwest. Beavers exhibit wide variations in colony composition and demographics over their broad geographic range, so regional research is important for sound management. The objectives of this study were to investigate the: (1) typical composition of beaver colonies in Illinois; (2) reproductive potential of female beavers in Illinois; (3) sex-age composition of beaver populations in Illinois; and (4) efficacy of night-vision surveys versus removal trapping for estimating colony size. This study was conducted during the 1999-00 and 2000-01 trapping seasons in central and southern Illinois. I harvested and aged 239 beavers (128 males:111 females). The average colony contained 5.6 beavers. Family groups consisting of a breeding pair and at least 1 offspring composed 86% of my colonies, while the other 14% consisted of only a breeding pair. An additional adult (≥2.5 years old) was found in 43% of the colonies. The sex-age composition of beavers taken from trapped-out colonies differed from that of beavers harvested by trappers. Trappers took significantly more yearling and 2.5 year olds, whereas more kits and older beavers were taken in trapped-out colonies. I did not find significant over-winter weight losses in kits, yearlings, or adults. The mean number of ova produced annually was 2.5, 3.6, and 4.4/female for 2.5 year olds, 3.5 year olds, and older females, respectively. Fetal rates were 3.0, 3.4, and 4.2/female for the 3 age-classes. In utero loss was estimated at 11%. Circumstantial evidence suggested that the presence of an older breeding adult female suppressed ovulation in subordinate females. I found no evidence that the presence of yearlings and subadults in a colony allows parents to raise more kits. Night vision surveys under-estimated the size of colonies, accounting for only 55% of the beaver present.