I5 S DISCOVERY REPORTS 



non-breeding cows which exist in small numbers, and although meticulous accuracy 

 may suggest that some correction is desirable for these as well as the dead pups, from 

 the practical point of view of the management of the herd it is satisfactory to realize 

 that in these matters the figures of the estimates err on the side of conservatism. 



The factors which have to be taken into account in forming estimates of the population 

 are these: 



i. The count of pups. 



2. The length of life. 



3. Rate of increase. 



4. Deaths from all causes. 



The number of pups is known to be approximately 80,555 and the length of life has been 

 discussed before (1934, p. 306), when twelve years was suggested as being a reasonable 

 length of life for males and eleven for females. Having regard, however, to the exceedingly 

 strenuous life of the bulls as compared with the cows, after renewed consideration I now 

 suggest that the latter, on average, may well attain the age of twelve years, and therefore 

 the period during which their numbers may be directly inferred from the count of pups 

 is seven years, although in fact the cows would be sexually active for eight years, that is, 

 from the age of four (p. 151). Since, then, males are adult at the age of six years and 

 cows pup for the first time at the age of five, it follows that the males have an actual 

 sexual life of six, and the females of seven years, and the sexes will naturally be repre- 

 sented in the proportion of six bulls to seven cows in the herd in its natural state. One- 

 sixth of the bulls and one-seventh of the cows will die annually, 16-66 and 14-29 per cent 

 respectively, and accessions of these proportions will be necessary in order to maintain 

 the herd in a state of equilibrium. From these figures and the number of the pups the 

 total deaths between birth and maturity can easily be calculated. 



The sexes are born in equal numbers, so that with a pup count of 80,555 there would 

 be 40,277-5 of each, but since one cannot properly consider 0-5 of a living animal I have 

 adopted 40,277 as the basal number. 



The number of immature animals is calculated by taking the number which must 

 die between birth and maturity, and making annual deductions of such dimensions that 

 the final figure satisfies the demands of the herd for its annual accessions. The death 

 rate will be heaviest in the first year (1934, p. 309), and I have therefore allotted to it 

 a death rate of 40 per cent. The remaining deaths which have to be accounted for are 

 distributed in the following manner, since it is but reasonable to expect that the annual 

 rate will decrease as the animals become more mature. A and B refer to the alternative 

 estimates mentioned above. 



It will be observed that B assumes lower death rates than does A. 



Estimate A. Count of pups 80,555 and, from that, parous cows, by inference, 80,555. 

 Since the bulls are present in a proportion of six to seven cows there will be 69,047 bulls 

 if there has been no artificial death rate, but in the Falklands a commercial sealing 

 company has been operating rather irregularly since 1928 and it has killed in six seasons 

 a maximum of 32,183. 



