Lens Weights of Sharp-Tailed Grouse

Abstract

Left eye lenses of sharp-tailed grouse (Pedioecetes phasianellus) bagged in South Dakota were oven-dried and weighed to determine variations between sexes and within age-classes. The mean difference in lens weight was highly significant for 162 juvenile males and 190 juvenile females, 51 adult males and 83 adult females, juvenile males and adult females, and all juveniles tested against all adults. Males had lens weights exceeding those of females; adults had lens weights exceeding those of juveniles. Although considerable overlap exists between lens weights of juveniles and adults, an adjustment can be applied to separate age-classes in a sample of unknown age. Apparently little, if any, increase in lens weight occurs after the first year. Eyeballs were collected from sharp-tailed grouse to determine whether juveniles and adults could be separated on the basis of lens weight, and whether adult lens weights would show groupings indicative of yearly age-classes. Lens weights have been shown to be reliable indicators of age for several species of mammals. Results of lens-weight studies have been reported for cottontails (Sylvilagus floridanus) and gray fox (Urocyon cinereoargenteus) by Lord (1959, 1961); raccoon (Procyon lotor) by Sanderson (1961); fox squirrel (Sciurus niger) by Beale (1962); and pronghorn antelope (Antilocapra americana) by Kolenosky and Miller (1962). Campbell and Tomlinson (1962) applied the lens-weight technique and found it possible to separate adult and juvenile Turkish chukars (Alectoris graeca) but not adult year classes. Payne (1961) reported little success with aging by lens weight in a study of the house sparrow (Passer domesticus). MATERIALS AND METHODS Left eyeballs were taken at road check stations from sharp-tailed grouse bagged in western South Dakota on September 21 and 22, 1963. Sex determination was made using central-tail-feather pattern or gonadal examination. Age was determined by examination of the pattern of primary molt or probe of the bursa. Birds of questionable age or sex were excluded from the sample. Eyeballs were placed in containers of 10 percent formalin solution, labeled by sexand age-classes. Lenses were extracted from the eyeball after about 6 weeks, rolled gently in the fingers to remove traces of the ciliary attachment, and oven-dried at 80 C for at least 72 hours. The dried lenses were weighed on an electrical balance to 0.0001 gram. Although hygroscopic in nature, the lenses tested did not pick up measurable amounts of moisture from the air for a period of over 2 minutes, while the weighing process required about 30 seconds. RESULTS AND DISCUSSION Using the standard t test, lens weights of 162 juvenile males were significantly greater (t = 6.27, P < 0.01) than those of 190 juvenile females (Table 1). Similarly, lens weights of 51 adult males were significantly greater than those of 83 adult females (t = 6.69, P < 0.01). 1 This report is a contribution from Federal Aid in Wildlife Restoration Project in South Dakota W-75-R.