TABLES FOR ESTIMATING AGES AND BIRTH DATES 
OF COTTONTAIL RABBITS, With Suggestions For Handling Lenses 
Lord (1959) first presented the concept of estimating 
the ages of cottontail rabbits (Sylvilagus floridanus spp.) 
on the basis of the dry weight of the eye lens. He gave 
a graphic representation of the relationship between age 
(x) and the dry weight of the lens (y) for approximat- 
ing the ages of cottontails in days (Lord 1959:360). 
Dudzinski and Mykytowycz (1961:159), using Lord’s 
data (1959:359), reduced the relationship of dry lens 
weight to age for cottontails to the algebraic form 
68.7927 
log, y = 2.4890 — | 2 | 
Manipulation of this equation to the form 
re 68.7927 
(2.4890 — log,, y) 
allows the biologist to estimate the age of a cottontail 
in days. However, the equation is tedious to use with 
samples of appreciable size. Consequently, the tables 
presented in this paper were prepared to facilitate the 
compilation and processing of cottontail lens data. 
aA 
William R. Edwards 
Estimated ages in days were computed for lens 
weights of 11-210 mg (Table 1). Because of the rela- 
tively low precision of the estimating equation, as sug- 
gested by the work of Dudzinski and Mykytowycz (1961: 
158), there is little justification for using the equation 
for growth of cottontail lenses in estimating ages beyond 
the first-year class. When one knows the dry weight of 
a lens in milligrams, it is a simple matter to find that 
weight in one of the columns in Table 1 and to read 
the estimated age in days in the adjacent column. 
Statistics gathered at the Illinois Natural History 
Survey suggest a high degree of bilateral symmetry in 
weight between a rabbit’s lenses. We now believe that the 
difference in weight between lenses of a pair is primarily 
the result of sloughing off of tissue of one or both during 
handling. Thus, we suggest that only the weight of the 
heavier lens of each pair be used in estimating age even 
when both lenses are in apparently good condition. 
Table 2 was prepared to simplify determination of 
an estimated date of birth after an estimate of age has 
been obtained. In this table days of the year are num- 
TABLE 1.—Estimated ages of cottontails from dry weights of eye lenses. 
Lens Lens Lens Lens Lens Lens 
Weight Age Weight Age Weight Age Weight Age Weight Age Weight Age 
in mg in Days in mg in Days in mg in Days in mg in Days in mg in Days in mg in Days 
11 7 45 4] 79 75 112 115 145 169 178 247 
12 8 46 42 80 76 113 117 146 Lid 179 250 
13 9 47 43 81 Thy) 114 118 147 173 180 253 
14 10 48 44 82 79 RES, 120 148 175 181 256 
15 11 49 45 83 80 116 121 149 177 182 260 
16 12 50 46 84 81 117 iZ2 150 179 183 263 
17 14 a 47 85 82 118 124 151 181 184 266 
18 15 52 48 86 83 119 125 152 183 185 269 
19 16 DS 49 87 84 120 12, 153 185 186 2i2 
20 17 54 50 88 85 121 128 154 187 187 276 
21 18 5S Df 89 86 122 130 155 189 188 279 
Ze 19 56 52 90 88 123 131 156 191 189 283 
23 20 57, 53 91 89 124 133 157 194 190 286 
24 21 58 54 92 90 125 134 158 196 191 290 
25 7a be, 55 93 91 126 136 159 198 192 295 
26 23 60 56 94 92 Wa 138 160 200 193 297 
27 24 61 58 95 94 128 139 161 203 194 301 
28 25 62 58 96 95 Wa) 141 162 205 193 304 
29 26 63 59 97 96 130 142 163 208 196 309 
30 27 64 60 98 97 Lt 144 164 210 197 313 
31 28 65 61 99 98 132 146 165 pales 198 317 
32 29 66 62 100 100 133 147 166 215 199 321 
sf) 30 67 63 101 101 Loe 149 167 end 200 325 
34 31 68 64 102 102 135 151 168 220 201 ae9 
30 32 69 65 103 103 136 152 169 222 202 334 
36 33 70 66 104 105 137 154 170 225 203 338 
37 34 71 67 105 106 138 156 171 228 204 342 
38 35 72 68 106 107 139 158 172 230 205 347 
39 36 73 69 107 109 140 160 173 235 206 eh 
40 37 74 70 108 110 141 161 174 236 207 357 
41 38 75 71 109 111 142 163 175 239 208 362 
42 38 76 72 110 113 143 165 176 242 209 366 
43 39 off | 73 Lid 114 144 167 177 244 210 Sf 
44 40 78 74 
