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Male-Male Competition and Access to Females in the Swordtail Xiphophorus nigrensis

Molly R. Morris, Puja Batra and Michael J. Ryan
Copeia
Vol. 1992, No. 4 (Dec. 18, 1992), pp. 980-986
DOI: 10.2307/1446627
Stable URL: http://www.jstor.org/stable/1446627
Page Count: 7
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Male-Male Competition and Access to Females in the Swordtail Xiphophorus nigrensis
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Abstract

Xiphophorus nigrensis males exhibit three size classes (small, intermediate, and large) that are primarily derived from allelic variation at the Y-linked pituitary (P) locus. Previous studies of X. nigrensis demonstrated that large males have greater reproductive success than small males and suggest that female choice should provide large and intermediate courting males an advantage over small noncourting males. In this study, field and laboratory observations suggest that male-male competition, in addition to female choice, contributes to the greater reproductive success of large males over small males. In the field, large males had greater access to females than did small males, and large males excluded access of other males to females more often than did small males. In the laboratory, the larger male of a pair blocked access to the female more than the smaller male, irrespective of size class. Laboratory results suggest that the pattern of size-biased mating success due to male-male competition should be different than the pattern predicted by female choice.

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