A SECOND REPORT 



ON THE 



SOUTHERN SEA LION, 



OTARIA BYRONIA (DE BLAINVILLE) 



By J. E. Hamilton, M.SC. 



(Plates XXVI-XXXIII; Text-figs. 1-6) 



INTRODUCTION 



The present paper contains the results of a continuation of the research on Otaria 

 byronia in the Falkland Islands of which the first report was published in 1934. In 

 it mention was made of five points on which further work was required, namely, length 

 of life, rates of birth, death and increase, oestrous cycle and limits of breeding season, 

 migration and census of the herd in the Falkland Islands. 



My sincere thanks are due to the numerous friends in the Falkland Islands who have 

 done so much to facilitate the work, and to the Staff of the British Museum (Natural 

 History) for the numerous occasions on which assistance was given ; and, finally, I would 

 express my deep indebtedness to Dr A. S. Parkes, F.R.S., for the great help he has so 

 willingly given in spite of innumerable calls on his time. 



During the collection of my first series of sea lions it was observed, in post-mortem 

 examination of the animals killed, that of the three fifth-year cows killed in July although 

 one was pregnant, another had recently aborted and the third had ovulated without 

 becoming obviously pregnant. 



Besides this, a sixth year female, killed also in July, contained a large Graafian follicle 

 in one ovary. From these circumstances it was concluded that there must be some 

 sexual activity on the part of the female outside the obvious breeding season in the 

 southern summer. 



Since the majority of the cows of suitable age are pregnant in the interval between two 

 breeding seasons, there was reason to expect that if one could discriminate between the 

 gravid and non-gravid animals by inspection it would be possible to make a collection 

 of material from which to work out the sexual history of the species. 



I returned to the Falklands in 1933 with the intention of making a census of the herd 

 there and of pursuing the course suggested by the considerations set forth above. 



With certain unavoidable interruptions the work was continued until March 1937. 



