ECOLOGY AND BIOLOGY OF THE PAGIFIC WALRUS 



197 



Three implanted embryos, 5, 23, and 16 mm long, were found by Brooks 

 (1954) in females taken on 10, 15, and 16 June, respectively; J. J. Burns (unpub- 

 lished data) recovered a 7-mm implanted embryo from a female taken on 

 17 June (Fig. 117c). Another of Burns' specimens, taken on 1 June, showed local 

 swelling of one uterine horn, suggestive of implantation (cf. Wimsatt 1963), but 

 the embryo was not certainly identified. Krylov (1969) reported the finding of 

 implanted embryos 7 and 9 cm long in two females taken on 13 June. 



Belopol'skii (1939) and Collins (1940) found implanted embryos and fetuses in 

 several females during July but did not indicate whether they examined other 

 females without finding any embryos. Krylov (1969) reported six implanted 

 embryos 13 to 25 cm long (nose-tail length) from females taken on 2 and 20 July; 

 earlier (Krylov 1966Z7), he reported another female in July with an unimplanted 

 blastocyst. All but 1 of 102 pregnant females examined by Krylov (1969) in 

 August and September contained implanted fetuses. The one exception, taken on 

 31 August, contained a blastocyst comparable in size to that in an animal taken 

 on 2 June. 



The foregoing data indicate that nidation may begin as early as May or as late 

 as July. In most females, however, implantation of the blastocyst seems to take 

 place during June and July. The modal time of implantation probably is about 

 1 July, 4 to 5 months after the probable modal time of ovulation and fertil- 

 ization. During those months, the spherical, microscopic blastocyst evidently 

 remains in an unattached state of suspended development. 



Until recently, delayed implantation had been assumed not to take place in 

 walruses (Scheffer 1958; Asdell 1964), though the possibility of its occurrence 

 was not overlooked by Burns (1965) and Harrison (1969). By addition of this 4-to 

 5-month period of delayed implantation, the duration of pregnancy in the 

 walrus now appears to be at least 15 to 16 months, rather than 11 to 12 months as 

 previously believed. Such a long duration is unusual among mammals (Asdell 

 1964) and may be unique among pinnipeds, or matched only by the Australian 

 sea Hon, Neophoca cinerea (Ling and Walker 1978). 



Growth of Fetus 



The growth of the fetal Pacific walrus is depicted in Fig. 118, on the basis of 

 data compiled from various sources. Measurements of embryos and fetuses 

 reported in the literature have been expressed in several different forms: (1) 

 nose-tail length along the curvature of the back, with the animal in the normal 

 fetal position; (2) standard length from nose to tail (straight line), with the 

 animal lying on its back and with head and tail extended; and (3) crown-rump 

 length, in a straight line from top of head to base of tail, with the animal in the 

 normal fetal position (Fig. 119). Since crown-rump length usually has been 

 found most reliable for analysis of fetal growth (Harvey 1959; Evans and Sack 

 1973), I converted all of the available nose-tail and standard length 

 measurements to crown-rump length, based on comparative measurements of 16 

 specimens examined by Belopol'skii (1939) and me, as follows: 



Crown-rump L = 56% nose-tail L = 84% standard L 



The resultant estimated crown-rump lengths, although only approximate, are 

 comparable in magnitude to those from actual measurements of specimens 



