4 oo DISCOVERY REPORTS 



of a diatom film or the presence only of small patches on the jaw is characteristic of whales which have 



only recently entered the colder waters. 



Records of both the extent and heaviness of diatom infection and of the mean diameter of the 

 seminiferous tubules, are available for a number of fin whales from South Georgia and the pelagic 

 whaling grounds. When doubtful cases have been eliminated the sample for which precise information 

 is available is reduced to only 43 male fin whales. 



Table 9. Summary of information on the diameters of the testis tubules of 43 male fin zvhales 



classified by means of diatom infection 



Heavy diatom 



For 13 recent arrivals with little or no diatom infection, the mean tubule diameter is 173 /^; for 

 fourteen whales taken at South Georgia with heavy diatom infection which had, therefore, been in the 

 Antarctic for several weeks the mean tubule diameter is 152//. For 16 with heavy diatom infection, 

 taken pelagically much further south, which had presumably been in colder water even longer than 

 the last group, the mean tubule diameter is 140//. There is a similar range of variation in all three 

 samples (Table 9), suggesting that the rate of regression is fairly constant. 



These mean values plus or minus za and 2 s.e. are shown in Table 9 and Text-fig. 29 D-F, and 

 it will be seen that the mean values for the tubule diameter in recent arrivals and in animals which 

 have been south of the antarctic convergence for some time are significantly different. These values 

 may be compared in the same figure with the frequencies of tubule diameters in the larger unclassified 

 sample (Text-fig. 29 C). There are two modes in this frequency distribution, the higher of which is 

 now seen to correspond to recent arrivals in the Antarctic in which the tubules are still shrinking, and 

 the lower mode represents males in which the tubules are almost fully regressed. The mean diameter 

 of fully regressed tubules in mature fin whales is probably about 140 /i, for material fixed and embedded 

 in this way. The last phrase is an important qualification. 



These results may be compared with the mean diameter of the testis tubules of 13 immature fin 

 whales, which is 79 /i (range 47-113 //). This is almost identical with the findings of Chittleborough 

 (1955a) for the humpback whale and close to the figures given by Clarke (1956) for the sperm whale. 

 One specimen, taken in October, which appears to be approaching puberty has a mean tubule 

 diameter of 102// (open tubules 122 fi; closed 82//), which is again close to Chittleborough's figures. 

 It is difficult to reconcile this with the apparent extensive shrinkage of the tubules of mature hump- 

 back whales in rut, but whether this shrinkage is postulated or not, it does not affect the general 

 conclusions given below. 



This brief discussion of testis histology is sufficient to establish that there is an annual cycle of 

 testis activity in the male fin whale. The season when the majority are in active spermatogenesis 

 probably extends over the period April-September, with most activity in April, May, June and July. 

 It is in these months that the majority of conceptions should occur. Nevertheless, the presence of 

 some males in January, February and March which have testis tubules up to 200 ft in diameter 

 suggests that successful pairings can occur in almost any month. 



