SAR an Was £09 ee hy Mevindesnteh eee s RIE IIS aoa. ete: ? zj 3 tee tee pene eet atnae me ew wnat . at OR OF me SiS ROS AAs a IT pe Te tae Ser es rte) ak aia ee > rey ~~ whee os peas os ee ett e , Seo + ‘ : - : . * . s ‘. = ‘ X ° . - ow é . = “ ‘ ¢ - a . - ? . . » de bey |\WURAL HISTORY SURVEY et - 4-795 Pe «| TABLES FOR ESTIMATING AGES AND BIRTH DATES OF COTTONTAIL RABBITS With Suggestions for Handling Lenses WILLIAM R. EDWARDS a ILLINOIS NATURAL HISTORY SURVEY Biological Notes No. 59 Urbana, Illinois « December, 1967 STATE OF ILLINOIS Department of Registration and Education SATURAL HISTOR SURVEY NATURAL HISTORY SURVEY DIVISION ANU 1968 LIBRARY 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 logy) y = 2.4890 — | ee | Manipulation of this equation to the form we 68.7927 (2.4890 — log,, y) 510 41 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. 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- TasLe 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 41 79 75 112 115 145 169 178 247 12 8 46 42 80 76 113 117 146 171 179 250 13 9 47 43 81 77 114 118 147 173 180 253 14 10 48 44 82 79 115 120 148 175 181 256 15 11 49 45 83 80 116 121 149 177 182 260 16 12 50 46 84 81 Wl?) 122 150 179 183 263 17 14 51 47 85 82 118 124 151 181 184 266 18 15 52 48 86 83 119 125 152 183 185 269 19 16 53 49 87 84 120 127 153 185 186 272 20 17 54 50 88 85 121 128 154 187 187 276 21 18 55 51 89 86 122 130 155 189 188 279 22 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 22 59 55 93 91 126 136 159 198 192 293 26 23 60 56 94 92 127 138 160 200 193 297 27 24 61 58 95 94 128 139 161 203 194 301 28 25 62 58 96 95 129 141 162 205 195 304 29 26 63 59 97 96 130 142 163 208 196 309 30 27 64 60 98 97 131 144 164 210 197 313 31 28 65 61 99 98 132 146 165 212 198 317 32 29 66 62 100 100 133 147 166 215 199 321 33 30 67 63 101 101 134 149 167 217 200 325 34 31 68 64 102 102 135 151 168 220 201 329 35 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 69 107 109 140 160 173 233 206 352 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 77 73 111 114 144 167 177 244 210 371 44 40 78 74 bered consecutively and arranged by month. For exam- ple, when one knows that a specimen was collected on December 12 (day 346) and was estimated to be 216 days old when collected, one can estimate the date of birth by subtracting 216 from 346, in this instance day 130, or May 10. Data on estimated dates of birth permit computation of a mean estimated birth date and its standard error and thereby facilitate testing of differences oo in these parameters among populations or comparison with a normal as a means of determining differences in age structure. SUGGESTIONS FOR HANDLING COTTONTAIL LENSES 1. Care must be taken to remove eyeballs intact (Fig. 1). When an eyeball is ruptured, frequently at the 6 7 8 Fig. 1. — Cottontail eye intact and eye with lens removed. The eyeball at left is in the proper intact condition after fixa- tion in a 10-percent formalin solution. The fixed lens is shown after removal but be- fore drying. TasB_e 2.—Figures for estimating birth dates of cottontails from the estimated age in days. Day of Month Jan. Feb. March April May June July Aug. Sept. Oct Nov. Dec. 1 1 32 60 91 121 152 182 213 244 274 305 335 2 2 33 61 92 122 153 183 214 245 275 306 336 3 3 34 62 93 123 154 184 215 246 276 307 337 ! 4 35 63 94 124 155 185 216 247 277 308 338 5 5 36 64 95 125 156 186 217 248 278 309 339 6 6 37 65 96 126 157 187 218 249 279 310 340 7 7 38 66 97 127 158 188 219 250 280 311 341 8 8 39 67 98 128 159 189 220 251 281 312 342 9 9 40 68 99 129 160 190 221 252 282 313 343 10 10 41 69 100 130 161 191 222 253 283 314 344 11 11 42 70 101 131 162 192 223 254 284 315 345 12 12 43 71 102 132 163 193 224 255 285 316 346 13 13 44 72 103 133 164 194 225 256 286 317 347 14 14 45 73 104 134 165 195 226 257 287 318 348 15 15 46 74 105 135 166 196 227 258 288 319 349 16 16 47 75 106 136 167 197 228 259 289 320 350 17 17 48 76 107 137 168 198 229 260 290 321 351 18 18 49 77 108 138 169 199 230 261 291 322 352 19 19 50 78 109 139 170 200 231 262 292 323 353 20 20 51 79 110 140 171 201 232 263 293 324 354 21 21 52 80 111 141 172 202 233 264 294 325 355 22 22 53 81 112 142 173 203 234 265 295 326 356 23 23 54 82 113 143 174 204 235 266 296 327 357 24 24 55 83 114 144 175 205 236 267 297 328 358 25 P45) 56 84 115 145 176 206 237 268 298 329 359 26 26 57 85 116 146 177 207 238 269 299 330 360 27 27 58 86 117 147 178 208 239 270 300 331 361 28 28 59 87 118 148 179 209 240 271 301 332 362 29 29 88 119 149 180 210 241 272 302 333 363 30 30 89 120 150 181 211 242 273 303 334 364 31 31 90 151 212 243 304 365 loti OTT Y PWSNG amatereas C060 Reseeen ds — __ FISHER SCIENTIFIC COMPANY connection to the optic nerve, the vitreous humor is usu- ally lost and the eyeball collapses around the lens. If this occurs, the outer fibers of the lens adhere to the inner coatings of the eyeball during fixation and are lost when the lens is removed. Discard damaged eyeballs. 2. Lenses should not be allowed to freeze prior to or during fixing. Freezing frequently results in lens tissue being sloughed off. 3. Lenses should be fixed in a buffered 10-percent formalin solution as soon as possible after the animal is collected. fixing lenses in the buffered 10-percent formalin solution. Our data suggest that no adverse effects occurred when eyeballs were left in the fixing solution as long as 120 days. 5. After fixing, lenses should be dried for about 1 week at 80° C. circulation of air. t. ‘Ten days should be allowed for in an oven equipped with a fan for For an unknown reason some lenses and batches of lenses do not fix and dry properly. These lenses differ in appearance from those properly fixed, and with a little experience “bad” lenses can be quickly recognized. Discard any lens which evidences sloughing off of tissue or appears atypical in color or shape. QE 14 Fig. 2—Mettler Gram-Atic balance. This type of balance is suggested for weighing cottontail lenses for reasons of speed and accuracy. 6. Because lenses are hygroscopic, they should be weighed immediately after removal from the drying oven or stored immediately in suitable airtight, moisture- free containers. 7. Analytical balances of the Mettler type (Fisher Scientific Company, Pittsburgh, Pa.) (Fig. 2) are prob- ably the easiest and most rapid to use and the most reliable now available for weighing lenses; Roller-Smith precision balances (Roller-Smith Company, Newark, N. J.) have also proved satisfactory. LITERATURE CITED Dupzinsx1, M. L., and R. Myxytowyez. 1961. The eye lens as an indicator of age in the wild rabbit in Australia. Com- monwealth Scientific and Industrial Research Organization Wildlife Research 6(2):156-159. Lorn, Rexrorp D., Jr. 1959. in cottontail rabbits. 23(3) :358-360. The lens as an indicator of age Journal of Wildlife Management, This paper is printed by authority of the State of Illinois, IRS Ch. 127, Par. 58.12. It is a contribution of Illinois Federal Aid Project W-66-R, the Illinois Department of Conservation, the United States Bureau of Sport Fisheries and Wildlife, and the Illinois Natural History Survey, cooperating. William R. Ed- wards is Associate Wildlife Specialist, Illinois Natural History Survey. (45591—8M—12-67) TERE . DS we tey® yo c Scrat gerd : WE eae AS GRR re te