224 
PACIFIC SCIENCE, VoL XIII, July 1959 
ally produces her first pup when 3 years of 
age. Because of the several characteristics 
which monk seals share with the animals 
mentioned above and in view of the rapid 
growth rate observed in captive monk seals, 
it seems possible that the monk seal might 
participate in reproductive activity in the third 
year of life. All seals which we observed ex- 
hibiting sexual behavior, and all females 
which bore pups, were fully grown animals. 
Reproductive Rate 
The reproductive rate (number of pups 
born per 100 animals older than pups) of the 
Hawaiian monk seal seems to be unusually 
low when compared with that of other mam- 
mals. The total count of pups and the total 
count of older seals at each atoll are presented 
in Table 4. From these data, certain repro- 
ductive rates have been calculated for the 
following localities: 
PER CENT 
Kure Atoll 21.9 
Midway Atoll 7.5 
Pearl and Hermes Reef 12.8 
Lisianski Island 6.2 
Laysan Island 8.9 
The count at Lisianski was made quite 
early in the pupping season, that at Laysan 
was not complete, and when the last count 
was made at Pearl and Hermes Reef, probably 
only about 60 per cent of the pups had been 
born. Therefore, the actual reproductive rate 
at these localities was undoubtedly higher 
than the figures indicate. The June 5 ground 
count at Kure Atoll is the most reliable and 
important figure. Considering that the total 
population numbered 67, remarkably few 
pups (5 including one prematurely born) 
were seen at Midway in 1957. Furthermore, 
there were only 6 yearling seals, indicating a 
small pup crop in 1956 as well. Blasting op- 
erations in the ship channel and other human 
disturbances may have depressed the birth 
rate. 
It is apparent that annually a large propor- 
tion of adult female monk seals fail to pro- 
duce a pup. The walrus {Odohenus rosmarus) is 
the only pinniped known (with fair certainty) 
to breed in alternate years, a trait correlated 
with a lactation period exceeding 1 year 
(Brooks, 1954). Do missed pregnancies occur 
more frequently in monk seals than in other 
pinnipeds.^ The bearded seal {Erignathus bar- 
batus) frequently misses a season between suc- 
cessive pregnancies (Chapsky and Kovalev, 
1938). About 16 per cent of sexually mature 
female Weddell seals, and perhaps 20 per cent 
of sexually mature female crabeater seals, fail 
to become pregnant each year (Bertram, 
1940). In a collection of 14 leopard seals 
{Hydrurga leptonyx), 21 per cent had missed 
pregnancy (Brown, 1957). Missed pregnancies 
occurred in 8 out of 66 sexually mature female 
southern elephant seals (Laws, 1956^). Sev- 
eral large samples of apparently mature fe- 
male northern fur seals from the Pribilof 
Islands have been studied. Preliminary find- 
ings indicated that about 32 per cent fail to 
reproduce annually (Taylor, Fujinaga, and 
Wilke, 1955). Representing a sample of the 
commercial kill, 12,762 female fur seals, aged 
4 years and older, were examined in 1956 and 
1957. Of these, 46 per cent had missed preg- 
nancy in the season studied (C. E. Abegglen 
and A. Y. Roppel, in lit,). 
Age Groups 
Pup and yearling monk seals are recogniz- 
able in the field by size. Between yearlings 
and adults, no distinct size classes were recog- 
nizable. Animals of intermediate size are 
lumped together as subadults. 
Age group composition estimates at Kure 
Atoll and at Midway Atoll are presented in 
Table 6. It was not feasible to classify age 
groups other than pups during aerial counts. 
The following figures, therefore, are based on 
ground counts. The pup counts are the most 
accurate, as pups stay on land or in shallow 
water near shore where they are easily ob- 
served. Our observations indicate that sub- 
adults, and yearlings especially, do not haul 
out as frequently as do adults. Therefore, the 
