272 DISCOVERY REPORTS 



testes have been active in a period other than the months, June to October, when all except two of the 

 samples were collected. Evidence from the mean diameters of testis tubules confirms that such a 

 sexual season does exist in the male sperm whale. 



In those male mammals which have a sexual cycle, the seminiferous tubules commonly exhibit a 

 cyclic variation in their diameters, which are greatest during the sexual season. Statistical analysis of 

 tubule diameters has already been used for whale investigations by Chittleborough (1955) in studies on 

 the male humpback whale. 



Tubule diameters were measured in all testis samples, mature and immature, of whales examined 

 at Horta. For each whale ten tubules were measured and the mean taken. Each section was searched 

 for tubules cut in true cross-section, and selecting circular outlines in this way helped to make the 



200' 



O 

 u 

 E 



3 



-O 



3 



150 



100- 



<v 



e 



D 



O 

 (U 



50 



•• • 



8 



o o 



o CALF , F 306 

 1 I 1 I 1 I 1 T 



IMMATURE o 13 

 MATURE • 80 



-j — 1 — 1 — 1 — 1 — 1 — -1 — 1 1 — 1 — 1 — 1 — 1 — 1 1 — 1 — 1 — 1 — 1 — 1 — 1 1 1 — 1 — 1 — r 



5 6 7 8 9 IO II 12 13 14 15 16 17 18 



Length in metres 



Fig. 8. Males. Length of body and diameter of testis tubules in whales examined 

 at Horta in 1949, 1953 and 1954. 



tubule samples random for size. The outlines of the tubules were first traced under a camera lucida and 

 afterwards each was measured across two diameters at right angles, using a card previously calibrated 

 from a graduated slide. Table 23 is a summary of all the measurements. 



In Fig. 8 the mean diameters of testis tubules are plotted against the lengths of whales. In 

 immature whales there seems to be a regular and gradual increase of tubule diameter with increasing 

 body length, although the plot for the proven calf, F306 (p. 275, Table 25) which had the smallest 

 tubules in the series, is somewhat apart from the main trend of the points. After sexual maturity 

 there is clearly no correlation between tubule diameter and body length, and this is to be 

 expected if a sexual cycle exists. A noteworthy feature of the diagram is the pronounced gap 

 between plots of the largest immature and the smallest mature tubules. This again is to be ex- 

 pected if a sexual cycle exists and the material has been collected outside the sexual season. If there 

 were no sexual season, or if there were and the material had been collected in the sexual season, one 

 might expect a few maturing whales, the plots of whose tubule diameters would bridge the gap in 

 Fig. 8. 



