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Annual Cycle of Fur Seals, Arctocephalus forsteri (Lesson), on the Open Bay Islands, New Zealand

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Title:Annual Cycle of Fur Seals, Arctocephalus forsteri (Lesson), on the Open Bay Islands, New Zealand
Authors:Miller, Edward H.
Date Issued:Apr 1975
Publisher:University of Hawai'i Press
Citation:Miller EH. 1975. Annual cycle of fur seals, Arctocephalus forsteri (Lesson), on the Open Bay Islands, New Zealand. Pac Sci 29(2): 139-152.
Abstract:Fur seals, Arctocephalus forsteri (Lesson), were studied on the Open
Bay Islands, South Island, New Zealand in 1970-1971. Few adult males were
present at the colony site during the winter, but many arrived ashore in November
to vie for territories. Individual territorial males remained ashore and fasted for up
to 63 days before losing or abandoning their territories. A few adult males reappeared
briefly at the colony site a few weeks after abandoning their territories,
and left again. Subadult males were common at the colony and other parts of the
Open Bay Islands at the start of the breeding season, but their numbers declined
steadily throughout it. Adult females frequented the colony site throughout the
year. Some pregnant females appeared some weeks before parturition in areas
where they subsequently gave birth, then left to feed. The tendency of pregnant
females to feed heavily in the weeks prior to giving birth resulted in few females
being ashore in mid-November. Pregnant females landed ashore about 2.1 days
before parturition. After having given birth, they remained ashore with their pup
for about 8.8 more days before leaving to feed. Parturient females entered estrus
and copulated about 7.9 days postpartum; sexual receptivity was observed to last
up to 14 hrs. Parturient females were absent for about 4.4 days on their first feeding
trip after having given birth, and were ashore with their pups for about 2.8 days
immediately thereafter. Subsequent feeding periods at sea were longer. Mothers
nursed their pups for about one-third of the time that the former were ashore. The
fraction of time spent with mothers by pups on land changed little between December
and May, and the female-pup nutritional bond extended in some cases for
up to a year.
Nonbreeding adult (?) females increased in numbers near the colony as the
summer progressed, then declined near the end. Very young males and some older
subadult males were common at the colony site in May, but relatively few very
young females were then present. An estimated effective sex ratio of 6.1: 1.0
(females: males) prevailed in the colony during breeding. Sex ratios based on census
data consistently underestimated this figure. The annual cycle is characterized
by marked synchrony of births: about three-fourths of them fall in a 22-day period.
A temporal equivalent of McLaren's "marginal male effect" may selectively
favor a short period of pupping and copulation by females, helping to maintain a
brief breeding period in the face of ecological determinants of breeding synchrony
that are weaker for A. forsteri in New Zealand than for populations of some other
Appears in Collections: Pacific Science Volume 29, Number 2, 1975

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