4i6 DISCOVERY REPORTS 



and lactating at the same time. In one lactating whale ovulation without subsequent 

 fertilization had occurred. These pregnant and lactating, or ovulating and lactating 

 whales suggest either that lactation is sometimes unduly prolonged, or that post- 

 partum ovulation may sometimes take place. If the latter is the true explanation, the 

 interval between successive births in these whales is one year instead of two. 



In Fig. 4 the frequency graphs for six seasons at South Georgia can be compared. 

 In the two previous " Fin years " (1925-6 ; 1928-9) a grouping similar to that of 1929-30 

 can be recognized, although the peaks alter their position within the groups according to 

 the number of unsuccessful ovulations that was most frequent in that particular season. 



It will be noticed in the previous work on the frequency of the corpora lutea [Report, 

 1929, p. 451) that Group I included the present Groups I and II. The present Group I 

 with a maximum at one was not evident because, in two of the three seasons reported 

 upon, the early part of the season was missed, and it can be shown by the incidence of 

 pregnant whales with a single corpus luteum that the earliest pregnancies of the season 

 arrive early at South Georgia. During this season there were eight single corpus 

 luteum pregnancies in November (thirty-five mature whales), four in December (eighty- 

 one mature whales), five in January (seventy-six mature whales), one in February 

 (twenty-nine mature whales), and none in March. Last season there were four in 

 October (thirty-one mature), two in January (forty-two mature), and one in February 

 (fifty mature). From the single functional corpus luteum it is evident that these whales 

 had been fertilized at the first ovulation of the season. 



There is a noticeable difference between Group I and all the subsequent groups in 

 that the greatest frequency occurs at the beginning rather than near the middle; in 

 other words, that in the first sexual season the majority of whales become pregnant 

 at the first ovulation, while in subsequent seasons unsuccessful ovulations usually 

 precede pregnancy. A possible explanation is that whales nearing the time of maturity 

 tend to stay near the breeding areas, or, at any rate, do not make a long southward 

 migration, and they are thus the first to be impregnated, while the older whales arrive 

 from the south somewhat later, after one or more ovulations not usually fertilized. 



In Fig. 5 the figures for 393 mature Fin females — the total catch since 1924-5 — are 

 combined, and it can be seen that, notwithstanding the overlapping that must necessarily 

 occur, the grouping is still evident. 



While it is not suggested that the groups in themselves give more than an indication 

 of diff'erence in age of a number of whales, some idea of the amount of overlap between 

 the groups can be gained in the following way. There are certain whales whose age 

 can be ascertained on anatomical grounds. Thus whales pregnant with one functional 

 corpus luteum are naturally in their first year from sexual maturity, while lactating 

 whales with one corpus luteum must be in their second year. When more than one 

 corpus luteum is present the state of the mammary glands has to be taken into account ; 

 for if the gland is immature then unfertilized ovulations have occurred during the first 

 season with or without a later successful ovulation depending on whether the whale is 

 pregnant or not. 



