33 o CONTENTS 



The reproductive cycle page 394 



Introduction 394 



The sex ratio 395 



The breeding season 395 



The male reproductive cycle 395 



The follicular cycle in females . . . . . . . .401 



Pregnancy and foetal growth 401 



The pairing season and the season of parturition .... 403 



Sexual maturity 406 



The mean length at sexual maturity 406 



The age at sexual maturity 407 



Newly mature females 409 



The mammary gland 409 



The first pregnancy .......... 410 



Growth in length just after puberty 413 



The first ovulations .......... 416 



Puberty and its relation to the migratory cycle . . . . .421 



MULTIPAROUS FEMALES 425 



The pairing season and the calving season . . . . . .425 



Post-partum heat ........... 429 



Females simultaneously pregnant and lactating 429 



Evidence from the sizes of corpora albicantia ..... 430 



Anomalous corpora albicantia of lactating females .... 434 



Ovulation after abortion, stillbirth, or loss of calf .... 436 



Post-lactation heat 436 



The lactation period ^/\/\ 



The sexual cycle and its relation to the migratory cycle .... 450 



Variation of fertility with age 454 



Multiple ovulations 454 



Proportion of females in oestrus or pregnancy 455 



Variations in fertility with time 456 



Age-determination by means of the ovarian corpora .... 459 



The rate of accumulation of corpora albicantia ..... 459 



Types of corpora albicantia 460 



The sexual cycle 460 



Recovered whale marks 463 



Age-determination .......... 465 



Comparison with other methods 466 



Applications ........... 470 



Survival curves .......... 470 



Growth curves 475 



Summary 477 



References 482 



Plates IV-VII following p. 486 



