ABSTRACT: Reproductive success is an important variable in the dynamics of a re-established population of muskoxen (Ovibos moschatus) in the Arctic National Wildlife Refuge, Alaska, USA. From observations of marked female muskoxen between 1982 and 1999, I generated reproductive histories based on the presence or absence of young in late June. I determined age at first reproduction and maximum age at which females successfully reproduced and calculated rates of reproduction by age class and changes in mean reproductive intervals (years between successful reproductive events) over time. Age at first reproduction ranged from 2 to 5 years. Nine of 15 females first reproduced successfully at age 3 years. Thirteen of 17 females >10 years old reproduced successfully at 11-18 years of age; 4 of these had young at age 15-18 years. Two females ceased reproducing at age 6-8 years. Age-related differences in rates of reproduction were not apparent. Reproductive intervals varied within and among individuals. By 1991-1993, most female muskoxen in this population successfully reproduced at intervals of 2-3 years.
ALCES VOL. 37 (2): 403-410 (2001)
Key words: age at first reproduction, muskox, Ovibos moschatus, reproductive history, reproductive interval
Muskoxen (Ovibos moschatus) were re-established near the Arctic National Wildlife Refuge, Alaska, USA, in 1969 and 1970 after an absence of almost 100 years. In regions first occupied, numbers of muskoxen grew rapidly for over a decade (1976-1986), and then decreased and stabilized (1986-1995) (Reynolds 1998), following the classic trend of an introduced ungulate (Caughley 1970). By 1986, mixed-sex groups of muskoxen were dispersing into new regions and production of young declined over time (Reynolds 1998). Rates of production and survival are essential to understanding the dynamics of this re-established population. In ungulates, successful reproduction and survival of juveniles are related to physical condition of females (Cameron et al. 1993, Testa and Adams 1998, Keech et al. 2000). In this study, I wanted to identify critical measures of female reproduction: age at first and last reproduction; reproductive rates at different ages; and mean interval between successful reproductive events. I predicted that young and mature muskoxen had higher rates of reproduction than old females, and that mean reproductive interval (number of years between successful reproductive events) increased over time.
The study area is on the eastern Arctic Slope of Alaska, USA, between the Colville River and the Clarence River (Fig. 1). This 24,700-[km.sup.2] area encompasses the oil fields at Prudhoe Bay and includes the coastal plain of the Arctic National Wildlife Refuge. The area is underlain by continuous permafrost and is snow-covered for 8-9 months each year. Major landscape features include braided north-flowing rivers and flood plains, coastal plain and foothills, small areas of thaw-lakes, and rugged mountains on the southern border (Walker et al. 1983). Vegetation is arctic tundra. Shrubs (Salix spp., Dryas integrifolium) and forbs (Equisetum variegatum) grow on partially vegetated gravel bars of rivers. Tussock-shrub (Eriophorum vaginatum) and lowshrub (Salix planifolia, Betula nana) communities occur on the slopes of rolling hills, and wet-graminoid communities (Carex aquatilis, Eriophorum angustifolium)...
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