386 DISCOVERY REPORTS 



Sexual maturity in female fin whales is attained on average between baleen groups III and IV, with 

 small numbers maturing in groups II and V. Consequently, the frequency distribution of corpora 

 numbers in group V may be slightly truncated in the lower numbers, but will represent fairly 

 accurately the frequency distribution of corpora numbers within early age-groups (Text-fig. 27) and 

 can be compared with the frequency distribution of corpora at the attainment of physical maturity. 

 Hylen et al. (1955) point out that baleen group V probably includes animals from more than one year 

 class because it includes females with rather high corpora numbers. They suggest that 5-7-5% of 

 individuals placed in group V are certainly older than 5 years of age, although the participation of 

 older year classes may be greater than this. The mean number of corpora in baleen group V is 5-607 

 and, on the assumption that the average increment of corpora is 1-4-1-5 per year, the average age of 

 group V would seem to be some 4 years later than puberty, suggesting, if puberty usually occurs in 

 groups III and IV, that it includes several year classes. 



A similar conclusion is reached on comparing ear-plug ages with baleen groups (see Text-fig. 55, 

 p. 467). 



The number of corpora at the attainment of physical maturity 



Growth in body length is accomplished in mammals by intercalary growth of the vertebrae, which 

 occurs, as in other bones, at the zones of cartilage joining the epiphyses to the diaphyses. A whale is 

 said to have attained physical maturity when the epiphyses are fused to the centra along the entire 

 length of the vertebral column. When this happens linear growth of the vertebral column ceases, 

 although growth in other dimensions may continue and the skull may continue to grow slightly. It has 

 been shown that fusion of the epiphyses begins at both ends of the vertebral column and proceeds 

 inwards. This process is more rapid from the tail end than from the head end so that fusion is usually 

 completed in the anterior thoracic region, generally at the level of the fourth and fifth thoracic 

 vertebrae. 



Wheeler (1930, p. 411) showed that in female fin whales physical maturity is reached when about 

 15 corpora lutea and albicantia have accumulated in the ovaries. Peters (1939) came to a similar 

 conclusion, but Brinkmann (1948) and Nishiwaki (1950 a, 1952) concluded that the threshold of 

 physical maturity was correlated with the accumulation of respectively 13 and 11-5 corpora. These 

 discrepancies are probably the result of the application of different criteria of physical maturity, which 

 will be discussed presently. Chittleborough (19556, p. 321) suggested that physical maturity in the 

 humpback whale coincides with the accumulation of some 30 corpora in the ovaries, but his criteria 

 (Chittleborough, 1955 a) differ from those of other workers and his relative growth curve (19556, 

 fig. 3) suggests that linear growth ceases when about 15-20 corpora have accumulated. The material 

 of Symons and Weston (1958), although very sparse, suggests that physical maturity in the humpback 

 whale is attained when between 8 and 20 corpora have accumulated and the criteria of maturity which 

 they adopted are those used by the ' Discovery ' Committee and the National Institute of Oceano- 

 graphy. Laurie (1937, p. 236) found a similar correlation in blue whales, physical maturity being 

 attained at about 11-12 corpora. 



'This accumulation of such a regular number of corpora lutea at such a landmark as physical 

 maturity seems to leave no doubt not only that the corpora lutea persist up to and well beyond the 

 age at which physical maturity is reached but also that the accumulation takes place at a fairly steady 

 rate. It can also perhaps be argued that the ossification of the vertebral epiphyses and the accumulation 

 of corpora lutea could scarcely keep in step with one another in this way except in their relation to the 

 age of the whale, and that therefore the females normally become physically mature at a fixed age in 

 either species after the attainment of sexual maturity (Mackintosh, 1942, p. 221).' 



