Abstract:
The operational sex ratio has long been considered an important
constraint on the structure of mating systems. The effects of an
experimentally manipulated sex ratio on mating behavior and
selection were investigated in a polygynous species, Gryllus
pennsylvanicus, where the potential exists for spatial/temporal
fluctuations in sex ratio of field populations. Four different sex ratios
(males: females, 5:0, 5:2, 5:5, 5:10) were investigated. Observations
were conducted in late summer over two field seasons, from 2400 h ,
to 1000 h EST. Several male characters thought to be associated with
male reproduc.tive success were studied: calling duration, searching
distance, weight, fighting behavior, courtship frequency, and mating
success. Variance in male mating success was used as the indicator
for the opportunity for sexual selection. Total selection was estimated
as the univariate regression coefficient between relative fitness and
the character of interest, while direct selection was estimated as
standardized partial regression coefficients generated from a
multiple regression of relative fitness on each character. The
opportunity for sexual selection was highest at 5:2 and lowest at
5:10. The frequency of fighting behavior was highest at 5:2 and 5:5.
Fighting ability (% wins) was determined to be an important
correlate of male body weight. Direct selection for increased male
body weight was detected at 5:2, while total selection for body
weight was seen at 5:5. Selection on male body weight was not
detected at 5: 10. Calling duration decreased as sex ratio became more
female-biased. Total and direct selection were detected for increased
calling at 5:2, only total selection for calling was seen at 5:5, whereas
direct selection against calling was detected at 5: 10. Searching
distance also decreased as sex ratio became more female-biased,
however no form of selection was detected for searching at any of
the sex ratios. Data are discussed in terms of sexual selection on male
reproductive tactics, the mating system and maintenance of genetic
variation in male reproductive behavior.
