Age determination and reproduction of female Fin Whales Balaenoptera physalus (Linnaeus, 1758), with special regard to baleen plates and ovaries

1. From Antarctic female Fin Whales, Balaenoptera physalus, a collection, consisting of baleen plates and the parts of these plates embedded in the gum, ovaries, and a number of earplugs was examined, in order to determine the age of these animals and to get a better insight in some characteristics of their reproduction. Data concerning pregnancy and lactation were also recorded. The material was sampled from animals worked up on board the f.f. “Willem Barendsz” during the Antarctic seasons 1953/1954 to 1959/1960 inclusive. The possibilities and values of age determination for this species were considered, based on the results of the analyses of the above material. 2. In most seasons the length distribution of the female Fin Whales, from which the material for the age determination was sampled (“examined animals”), did not differ significantly from that of the total catch of females of the “Willem Barendsz” in the same seasons. Neither was there any significant difference in length distribution between the total number of females from which the material was sampled and that of the females in the catch of the “Willem Barendsz” in all seasons. However, the length distribution of the catch of the “Willem Barendsz” differed significantly from the total Antarctic pelagic catch. 3. Baleen records were made according to the method described by RUUD (1940). In contrast with his age determinations with the aid of the part of the baleen plate protruding from the gum also the part of the plate hidden in the gum was used. A method of reading this part was developed. The advantage of this method is that the total length of baleen plate examined is much longer, and that the part hidden in the gum is not yet worn. In addition, the use of the part hidden in the gum has the undeniable advantage that the end of the cortical layer at the base of the “root” of the baleen plate represents the only exact date: the date at which the animal was killed. 4. Apart from the large waves of the cortical layer of the baleen plate the so-called “relief” was also used to divide the record of the baleen plate into ”growth periods”. The large waving is probably caused by variation in growth resulting from migration and available food, whereas the “relief” is probably a reflection of certain physiological events in the life of the animal, e.g. the sexual cycle. The influence of the sexual cycle may account for the fact that the records of the sexually immature animal and the sexually mature male animal and female animal show obvious differences, the external circumstances of life being more or less equal for all animals. LAWS (1961) is of the opinion that there is a certain relation between the sexual cycle and migration. “It is suggested that, as in females at puberty, the biannual ovulatory periods are primarily related to the twice-yearly period of increasing day lengths associated with the north and south migrations.” If this is correct migration (day-light, feeding) might reinforce the influence of the sexual cycle on the growth of the cortical layer of the baleen plate. In that case the so-called “ovulation peaks” would become very prominent in the “relief” of the baleen plate, which they actually do. 5. With the aid of periodical configurations of peaks and hollows in the baleen records the curves were divided into “growth periods”, which were supposed to represent periods of one year each. One special configuration of peaks was regarded as an “ovulation peak”. This was confirmed by the condition of the animal with regard to pregnancy or the age of the foetus. 6. It was proved that there was a significant regularity in the succession and the heights of the peaks and hollows of the “relief” of the baleen plate according to FRIEDMANN’S method of m-ranking. This means that the “relief” is not formed in an arbitrary way but is a reflection of particular events in the life of the animal. Besides it was shown by VAN UTRECHT (1965) that each peak will be formed at a particular time and place in the “root” of the baleen plate. 7. The method of age determination described by the present author is to be regarded as an improvement of RUUD’S method since a. the entire baleen plate is used in the analyses as stated above sub 3., b. besides the large waving also the “relief” of the baleen plate is taken into account. Though in itself too time-consuming for routine examinations it already offers the possibility of c. determining the mean rate of ovulation by means of “ovulation peaks” and d. determining the moment of attainment of sexual maturity. 8. It is evident that as to the Fin Whale, age determination with baleen plates can generally be used up to and including the fourth year of life. In the records of the baleen plates of all these animals there are indications of their suckling or weaning periods. If the baleen plates contain more than four “growth periods” the part of the baleen plate formed during the weaning period is generally worn off and then only a minimum age can be determined. Consequently in those animals having more than four “growth periods” in their baleen plate records and showing indications of at least the weaning period (“double hump”), the baleen plate can also be used in age determinations. 9. So far the baleen plate method was only used to determine minimum ages. Now further analyses of baleen plates were made to gain an insight into the mean age at which the animals attain sexual maturity and into the mean rate of ovulation. In age determinations of mature females these results enable us to use the ovaries only. 10. It is necessary to know something of the number of “growth periods” hidden in the gum and of the way this number varies to be able to compare earlier determinations of the age at which the animals attain sexual maturity by means of baleen plates, with my results. 11. Of all examined animals the mean number of “growth periods” hidden in the gum appeared to be 1.79. In young animals (up to and including 4+ “growth periods”) this number varied from 0.5 to 2.0. This is contrary to RUUD’S idea that in animals younger than five years, only part of one “growth period” is present in the gum, which can be ignored. The maximum number of “growth periods” found in the part of the baleen plate hidden in the gum appeared to be 2.8. 12. The mean number of “growth periods” hidden in the gum increases with a. the increase in number of “growth periods” in the total baleen plate, b. the increase in number of “growth periods” in the outer baleen plate, c. the increase in length of the animals. In sexually mature animals the mean number of “growth periods” hidden in the gum does not increase with the increase in age. 13. If there is a small number of “growth periods” in the outer baleen plate they are significantly longer than those in baleen plates with a greater number of “growth periods”. The mean length of the “growth periods” decreases with the increase in age. 14. The number of “growth periods” in the baleen plate depends on the length of the baleen plate. In a longer plate the number of “growth periods” is significantly greater. Moreover, there is a linear correlation between the length of the baleen plate and the length of the animal. 15. Female Fin Whales in the Southern Hemisphere attain sexual maturity in their sixth, seventh or eighth year of life. Most of them will attain sexual maturity in their seventh year. In most of the earlier investigations it was accepted that the female Fin Whale attained sexual maturity in the fifth or sixth year of life. This difference may be accounted for by the fact that, as a mean, 1.79 “growth periods” are hidden in the gum instead of only part of one “growth period” as RUUD supposed. In investigations carried out by the Committee of Three Scientists appointed by the International Whaling Commission, it is commonly accepted that female Fin Whales attain sexual maturity in their fifth year of life. This age is probably too low. 16. The variation in age at which sexual maturity can be attained is probably caused by the existence of three so-called “constitution types”. These different types in the catch may also be responsible for the considerable variation in length of the baleen plates and the number and length of the “growth periods” in the baleen plates of the different types. 17. The mean rate of ovulation is 2.5 per period of two years. This calculation is based on the presence of the “ovulation peaks” in baleen plate records, and the lapse of time represented by that portion of the record between two such peaks. This is based on the assumption that each “growth period” represents a period of one year and that the growth of the baleen plate is, as a mean, regular. A correction factor is applied because multiple foetuses, accessory corpora lutea and several simultaneous ovulations are found. During the whole life of the female Fin Whale the mean rate of ovulation is constant. A decrease of fertility will not occur at all, or only very late in its life. 18. Most ovulations of the examined animals occurred during the southern winter (June to September inclusive), although a reasonable number of ovulations occurred in the southern summer. 19. If no pregnancy follows the interval between two ovulations is in most cases circ. 6 months, other possibilities are circ. 12, circ. 18 and circ. 24 months. In one case an interval of circ. 30 months was found. 20. It is proved and accepted that corpora albicantia persist during the whole of the whale’s life, although they do reduce to a mean diameter of 2.0 cm. This persistence makes it possible to make an estimation of the age of an individual. In a large sample such an estimation is sufficient to reach a conclusion about the age distribution. 21. The weight of the ovaries in immature females increases with the increase in length. To a certain extent the age of these young animals correlates with their length, so there is also a correlation between the weight of the ovaries and the age of the animal. 22. When the animals attain sexual maturity there is a significant increase in weight of the ovaries. 23. In mature females there is a correlation between the weight of the ovaries and the length of these animals. With regard to the age of the animals there is at first an increase and subsequently a decrease in weight of the ovary. This may be an indication of a decrease of the sexual activity in some species. However, this is not the case in the female Fin Whale. The mean rate of ovulation and the percentage of pregnant females remains constant as age increases. A plausible explanation for these facts is that there is a proceeding wave of ovulations, from the pole of the ovary directed to the bursa ovarica to the opposite pole. If the main ovulatory activity occurs in the first half of the ovary (i.e. the part adjacent to the bursa) both halves will increase in weight; if the ovulatory activity occurs mainly in the second half, the first half is mainly inactive and will decrease in weight. At a certain time the decrease in weight of the first half will be greater than the increase in weight of the second half, so that the total weight of the ovary will decrease while the mean rate of ovulation remains constant.