258 



NORTH AMERICAN FAUNA 68 



data obtained in summer, fall, and early winter are not directly 

 comparable with data obtained in late winter and early spring. 

 Between 1 October and 11 December 1957, 48 percent of the re- 

 mains found were juveniles. The remains of most dead animals 

 were judged to have lain on beaches since the previous winter- 

 spring period. Only seven were judged to have died in September 

 or later. There is a greater probability of finding remains of adults 

 because they are more durable than remains of immature animals. 

 The skeletons of juveniles, unless in sheltered locations well above 

 tide line, suffer environmental attrition more rapidly than the 

 harder bones of adults. Six months or more after the mortality 

 period, remains of many juveniles had disintegrated and were not 

 among the bones still on beaches. This probably explains why 

 juveniles did not predominate. 



In 1963, when juveniles composed 94 percent of the dead 

 animals, the study period was 10 March to 12 April. The data 

 from 1962 indicate that mortality among adults may increase as 

 the period of high mortality advances (fig. 100). In January and 

 February 1962, before the height of the mortality period, adults 

 constituted 16 percent of the animals that died. During and after 

 the period when the mortality reached its peak in March, 39 percent 

 of animals that died were adults. This might indicate that adults 

 are better able to withstand environmental stresses than immature 

 animals recently separated from mothers. In 1963 the period of 

 greatest adult mortality may not have been included in the study 

 period, and a disproportionately high mortality of juveniles was 

 indicated by the available data. The 1959 data show, however, that 

 the mortality among adults may, in some years, be high throughout 

 the period of increased mortality (fig. 101). 



The 1959 and 1962 study periods are the only two which spanned 

 entire periods of greatest annual mortality (fig. 97). Therefore, 

 the proportion of juveniles (60 and 75 percent, respectively) to 

 adults is probably more representative of the usual mortality 

 pattern. 



SEX-SPECIFIG MORTALITY 



More males than females were found dead on beaches. Among 

 juveniles, the disparity was greater than among adults. Rausch 

 (1953) noted in his 1951 and 1952 investigations that "the heaviest 

 mortality occurred ... in subadult males." The raw data from the 

 remains I examined, however, tend to exaggerate this difference. 

 The accurate determination of sex by the examination of juve- 

 nile skulls or other bones is perhaps impossible. If a carcass was 



