In overwintering populations of Pseudocalanus acuspes (Giesbrecht, 1881) in Balsfjorden, northern Norway, as well as in adjoining subarctic seas, stage IV and V copepodites frequently have a sex ratio skewed in favor of males (Norrbin, 1992). This may be caused by a difference
in the optimal timing of diapause between juvenile males and females. A simple encounter model is used to show that mate competition between males may be severe, especially at the high population densities characteristic of late summer in high latitudes. By delaying maturation until spring
and entering diapause earlier than females in the fall, the juvenile males may avoid competition from older males. For females mate competition does not appear to be a big problem. In these areas, a combination of seasonal adversity of the habitat and intra-specific competition seems to have
selected for differences in reproductive and survival strategies for males and females.
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