312 /. /. Christian 



viewed by Bourliere (1951).] Kalela also determined that mature sperma- 

 tozoa are present in the testes of these voles when the testes are seven or 

 more millimeters long and defined maturity on this basis. Females which 

 were pregnant or parous were defined as fecund. 



During the first summer of Kalela's studies in 1954, moderate population 

 densities were achieved after a spring characterized by a very small popula- 

 tion which had overwintered from the preceding fall. Males and females 

 from the litters born early in the breeding season matured without excep- 

 tion. Four-fifths of the mature females of late summer were young of the 

 year. A large number of the young of the year which reached maturity had 

 two litters and some even had thi-ee during their first summer. The old 

 overwintering females had as many as four litters. The following spring, 

 1955, the population started with a much larger number of overwintering 

 animals. The rate of growth of this population at first was as high or higher 

 than the rate of growth of the population in the preceding year, but it 

 rapidly declined as the density of the population increased. Peak densities 

 were reached in this year. A good many of the males born in the first part 

 of the breeding season reached maturity, but males with enlarged testes 

 were rarely found in August, and there was an accumulation of immature 

 males weighing between 20 and 24 gm. at this time (mature red-backed 

 voles normally weigh more than 24 gm., usually between 30 and 40 gm.). 

 This weight range represents the maximum size achieved by male or fe- 

 male red-backed voles which fail to mature, irrespective of their age. There 

 was no accumulation of voles in this weight category during the preceding 

 year of low population densities. Female young of the year exhibited the 

 same cessation of growth and failure to mature, except for those born early 

 in the breeding season. Less than half of the mature females in late summer 

 were young of the year and there was a marked accumulation of immature 

 females in the 20-24 gm. M^eight class. Furthermore, the pregnancy rate fell 

 sharply and had fallen to well below the 1954 levels by mid-August. The 

 overwintering females had up to three litters, but none had four litters. 

 Those young of the year which reached maturity had no more than two 

 litters and only a few had this many. One area (Malla) had twice the 

 density of the others under study at this time, and none of the young of the 

 year, male or female, attained sexual maturity or grew beyond the 20-24 

 gm. category. 



The population had undergone a major decline in numbers by the spring 

 of 1956 and was well below the 1954 levels for the same period, and yet the 

 rate of growth of the population was extremely slow. The overwintering 

 animals, as well as the young of the year, were distinctly smaller than 

 normal. Reproductive performance was essentially identical to that of the 

 preceding summer of 1955 when the density of the population was extreme. 



