Collins New Naturalist Library. H. Hewer R.

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СКАЧАТЬ of the fore flippers (‘flippering’). Eventually, some time in the third week after giving birth, the cow accepts the bull in mating. Mating is often repeated several times at intervals. The cows desert their pups and leave the rookeries. Whether they go immediately to sea or remain in nearby waters is not yet certain, although I am inclined to believe the former is more likely.

      The deserted pups moult their first or ‘puppy’ coat of white hair and appear in their ‘moulter’ coat which is similar to the adult pelage. In a healthy pup this process takes about a week; the fourth week of their life. When moulted the pups may, as in the south-west, immediately leave their natal beach, or as in the north and north-west remain on land and in fact move farther inland. Hunger will however eventually drive them to sea within a week or two.

      The bulls remain on station for five to six weeks before becoming exhausted. They then move away from the rookeries to sea and recommence feeding to restore the blubber they have used during their enforced fast.

      While this is the time-table of individual cows and bulls, it must be remembered that the time-table of the rookery is more extended. Thus a fortnight after the first pups have been born matings will begin, but this will probably be before the peak of the pupping. Consequently while the peaks of the various processes, pupping, mating, moulting, desertion by cows and leaving by bulls will be separated by periods corresponding to the normal time-table of individuals, this central period will be preceded by two or three weeks when arrivals of bulls and cows and some pupping are the only phenomena and will be followed by several weeks in which mating and desertions will predominate. The overall period will be about months.

      Now follows a period about which not so much is known. The nearby haul-out sites are used but not to a great extent. Probably the answer is that both bulls and cows are feeding as much as possible to replace their lost blubber, to fit themselves for the rigours of the winter. One of the most remarkable facts of this period, which covers about months, is that among the cows found on the haul-outs, non-pregnant ones predominate. A high proportion of these non-pregnant cows are comparatively young and virgin or nulliparous. The differential behaviour of pregnant and non-pregnant cows is very remarkable, the more so because very little development of the embryo takes place for the first hundred days after conception (about months). (See Chapter 4.) Towards the end of this period most of the cows go into the annual moult.

      I have not mentioned the actual dates through this breeding and post-breeding period because there are differences between the geographical groups and generalisation is therefore difficult. What makes precision even more difficult is that the full story is not known in detail for all of the groups. However, some attempt may be made to give an overall picture if we omit the Farne Islands group. In the south-western group the peak of pupping is probably in the first week in October, and by mid-November all the beaches are completely deserted. In the Hebrides and North Rona the dates are about 10–14 days later, although the much larger size of the rookeries results in more early and late puppings so that the season appears to be longer and a few adults may still be found in the breeding grounds in early December. In Orkney and Shetland a further 7–10 days should be allowed, so that mid-December would be a reasonable date to compare with mid-November in the south-west. January to March may be counted as the months of the cow moult, although few Orcadian cows will have begun in January and few, if any, Pembrokeshire ones will remain unmoulted in March.

      For the Farne Islands group all these dates must be delayed by about a further month. There the first pups are in late October and the peak about 3–4 weeks later in November, pupping continuing actively well into December.

      Our knowledge of the growth of the foetus is derived solely from northern material from Orkney. The mean date of the recommencement of the growth of the embryo is the middle of February. Thereafter the foetus grows apace in about 80% of the cows, the others are non-pregnant. So far as can be made out they continue to fish in the adjacent seas periodically coming ashore in fishing haul-outs for short periods at low tide. In the summer months the haul-outs must be both few and short because the numbers seen are very few indeed compared to those at other times of the year.

      We have left the bulls at sea after the breeding season and it is not until mid-February that we have any evidence that large numbers come ashore. Their annual moult appears to be timed about two months after that of the cows. It was first observed in April in Pembrokeshire, but by then many were completely moulted and few remained unmoulted early in May. Dr Backhouse and myself, having found the comparatively large moulting haul-outs of bulls in Pembrokeshire, were able to direct attention to this in other groups, such as the Farne Islands, where the same phenomenon was then seen by other observers. These moulting haul-outs of the bulls are very remarkable and often number thousands at a time in the large northern groups. The bulls, like the cows, largely disappear in the summer months from all the inshore sites and we can only conclude that they too are feeding at sea to prepare themselves for the breeding season.

      We can now turn to some of the distinguishing characteristics of this species, both in structure and habits.

      It is always extremely difficult, if not impossible, to estimate the length or size of seals in the field since there is rarely, if ever, any standard for comparison. Measurements therefore, while of use on a carcase, cannot really be used by an observer in the field. Any deductions based on claims to be able to ‘age’ seals in the field must therefore be regarded as highly dubious and probably very misleading. The pup is born at a length of about 33 ins. and 32–33 lbs. Bull pups appear to be very slightly heavier, but the difference is not truly significant having regard to the numbers which have been weighed. In any case it could only amount to a few ounces. It is very difficult indeed to speak about moulters since by that time all the differences in nutrition have taken effect and weights can vary between 100 lbs. or more and little more than the birth weight, if the moulter has been starved and is likely soon to die. The growth in length is quite small so that few moulters reach 40 inches until well after they have left their natal beaches however great their weight. Some of these very heavy moulters (over 100 lbs.) are so bloated with blubber that they can hardly turn their heads and movements are quite lethargic. There is some evidence that pups which do not reach the weight of 90 lbs. before being deserted by their mothers and moulting, have little or no chance of surviving their first year of life. Certainly all those below 60 lbs. appear to die before the first six months is out and all the recorded weights of grey seals in their first year of life are about 90 lbs. or less. Further, yearlings which have been weighed on the Farne Islands appear to fall more or less within the limits shown by the largest moulters. In other words there is little, if any, increase in weight in the first year of life.

      These conclusions appear to be confirmed by the known weights of older bulls and cows. 3–4 year old bulls only weigh 170–190 lbs. while the same age group of cows weigh 130–150 lbs.; 5–6 year old cows may reach 170–180 lbs. but records for bulls are very few, although they suggest that by that age they may attain 2 cwt. There are practically no weights of fully mature bulls or cows to justify generalisations. All that can be said is that probably the older well-established breeding cows are over 2 cwt. and the older territorial bulls probably top 3 cwt. or even reach 4 cwt. In any case the bulls vary much more than the cows and in both sexes changes in weight amounting to cwt. are usual during the yearly cycle.

      Much more data is available about lengths. Although little, if any, weight is acquired during the first year, the moulters grow considerably in length, an average of 55–60 ins. being normal at the end of the first year for both sexes. Thereafter some differences can be considered significant between the two sexes. The rate of growth is fairly steady until puberty СКАЧАТЬ