teeth for females ^6 years old and the errors 

 for the conference reading from sectioned 

 teeth for females > 7 years old could be used. 

 Intuitively, the errors for other readers could 

 be substituted if Reader A should stop de- 

 termining ages of fur seals, 



COMPARISON OF AGES ASSIGNED TO 



FUR SEALS BY JAPANESE AND U. S. 



BIOLOGISTS 



Dissimilar criteria and methods used by 

 biologists from Japan and the United States 

 to determine the ages of fur seals from 

 laminations on canine teeth could lead to 

 errors in interpreting biological data. Age 

 compositions and pregnancy rates of groups 

 of seals are examples of data that could be 

 affected. 



To test the possibility that the methods of 

 determining age have differed, biologists of 

 each country determined ages from the canine 

 teeth of seals collected at sea in 1964. Col- 

 lections of teeth were exchanged, and ages 

 determined by each country were compared. 

 The true ages of most of the seals in each 

 san-iple were unknown because only a few of 

 the animals had been tagged as pups. 



Sample of Teeth Collected and Prepared 

 Dy Japan 



Teeth collected from seals taken in the 

 western Pacific Ocean by Japan were pre- 

 pared by cutting a thin longitudinal section 

 from the center of each tooth. The surfaces 

 of each section were not polished. Thin sec- 

 tions allow laminations used to determine 

 age to appear as dark lines on a white field 

 when the tooth is viewed by transmitted light. 



Japanese biologists assigned a single age 

 to each tooth, and U.S. biologists assigned 

 ages on two occasions separated by several 

 months. The results from each reading by 

 the United States are compared with the single 

 reading provided by the Japanese, and the two 

 readings by the United States are compared. 

 The data for each sex are discussed sep- 

 arately. 



Males , --Japanese and U.S. readers dis- 

 agreed on an average of 3.8 percent of the 

 teeth from males in ages 1 to 4 (range--0-8.3 

 percent) for the first reading. The percentage 

 disagreement increased markedly for the small 

 sannple of 5-year-old males (25 percent error 

 for 12 seals tested). 



For the second reading, the two countries 

 disagreed on an average of 3,3 percent 

 (range--0-6,8 percent) of the males in ages 

 1 to 4, and percentage differences again in- 

 creased at age 5, 



Differences in determining ages were less 

 between the first and second readings of U,S, 



biologists than between readings made by 

 U,S. and Japanese biologists. Also, the dif- 

 ference between the first and second readings 

 of U.S. biologists was slight for 5-year-olds. 



Females . --An increase in differences was 

 apparent between ages 5 and 6 for the Japanese 

 and first U.S. readings (from 5.0 to 28.6 per- 

 cent). For the Japanese and second U.S. read- 

 ings, and the two U.S. readings, the increase 

 occurred between ages 6 and 7. 



For ages 8 and older, the disagreement was 

 40.0 percent for the Japanese and first U.S. 

 readings, 42.2 percent for the Japanese and 

 second U.S. readings, and 41.1 percent for 

 both U.S. readings. 



Sample of Teeth Collected and Prepared 

 by the United States 



Teeth collected from seals taken in the 

 eastern Pacific Ocean were preparedby cutting 

 each longitudinally but slightly off center fronn 

 the apex of the root to the tip of the crown, 

 ground to expose the root canal, and polished 

 on the cut surface. The exposed laminations 

 used to determine age appear as white inter- 

 vals with a dark space between each, 



U,S, readers determined ages for all the 

 teeth in the sample, Japanese biologists were 

 unable to read all the teeth from female 

 seals. Ages were determined for only 2,2 per- 

 cent of females age 9 years old and older by 

 Japan, Each sex is discussed separately. 



Males ■ - - Differences between the two coun- 

 tries of over 20 percent are indicated in 

 determining the ages of male seals, though 

 sample sizes were too small to prove con- 

 clusively that the differences are meaning- 

 ful. 



Females . --Japanese and U,S, biologists dif- 

 fered on 36,5 percent of the teeth from 

 females. Where differences occurred, ages 

 assigned by the United States were always 

 older than those assigned by the Japanese, 

 These differences affect calculated pregnancy 

 rates. For example, if ages determined by 

 Japanese biologists are used, the pregnancy 

 rate for the small sample of 5-year-olds 

 included in the collection was 76,0 percent. 

 On the basis of ages obtained for the sample 

 by U.S. biologists, the pregnancy rate was 

 45.5 percent (table 42). The reason for the 

 large disagreement between Japanese and 

 U,S, biologists for determining ages for 

 5-year-olds (68.0 percent) is not known. To 

 test possible differences in pregnancy rates 

 due to reader error, larger sample sizes of 

 canine teeth should be read twice by U.S. 

 biologists, first whole, then sectioned. The 

 same sample of teeth should then be sent to 

 Japan to be prepared and read by Japanese 

 biologists, because they section canine teeth 

 much thinner than do U,S, biologists. 



41 



