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PACIFIC SCIENCE, VoL XIII, July 1959 
on its belly or rolls over and lies on its back 
(Fig, 10). The latter position seems to be a 
favorite one. 
When the mother seal comes ashore to give 
birth, she is usually enormously fat (Fig. 11). 
As the nursing period approaches its end, the 
mother becomes quite gaunt. When she bends 
her body, huge wrinkles appear. While moth- 
ers were swimming with their pups we watched 
closely but saw no indication that they were 
attempting to find food. We found no indi- 
cations that the mothers left their pups at 
night to feed. Near the beach camp of David 
Woodside on Laysan Island in late June, 1957, 
a nursing mother remained ashore with her 
pup during the night. We conclude from 
these observations that the mother fasts dur- 
ing the entire nursing period. 
The maximum observed nursing period 
was 37 days. We would judge that the average 
pup is weaned in about 5 weeks at a weight 
of about 133 pounds (see growth). 
The mother of pup MS-12, less fat than 
other mothers when her pup was born, nursed 
it for only 20 days and deserted it when it 
weighed only 82 pounds, somewhat more 
than half the more usual or apparently nor- 
mal weaning weight. Pup MS-12 survived for 
at least 27 days after being deserted, but 
whether it ultimately survived, we do not 
know, since it disappeared from its usual 
hauling-out places. 
Since the pup’s instinct to follow its mother 
is strong, we presume that the mother, after 
her supply of milk has been exhausted, slips 
away and deserts her pup while it is sleeping. 
This was the case when the mother deserted 
MS-12. Mother and pup were seen sleeping 
on the beach at 0300. By noon when we 
visited the area, we found the sleeping pup 
alone and the mother was not subsequently 
seen. 
After it is deserted by its mother, the pup 
remains near the location of its birth. Pup 
MS- 10 was frequently observed as it played in 
the water or slept near its birth site until it 
was 50 days old, when it was found sleeping 
about a half-mile away. After this it was occa- 
sionally seen at various places along the 
shore of Eastern Island over a mile from its 
birth place. 
Swimming 
The pup is able to swim at birth. When pup 
MS- 12 was taken from its mother for tagging 
Fig. 10. A monk seal pup, satiated after a nursing period, sleeps on its back. After approximately 3 weeks of 
supplying milk for her pup, the mother here shows considerable loss of weight. Light hair, an indication of the 
postnatal molt, is visible on the pup’s muzzle, Midway Atoll, April 22, 1957 (KWK 57-16-3). 
