Effect of age on female choice in the field cricket, Gryllus interger

Female choice is an important element of sexual selection that may vary among females of the same species. Few researchers have investigated the causes of variation in selectivity with respect to potential mates and overall level of motivation toward a stimulus source representative of a mate. This study demonstrates that female age may be one cause of variation in female choice. Females of different ages may have different mate preferences. As females age, they have less time left to reproduce, and their residual reproductive value decreases. This should correspond to a higher reproductive effort which may be represented as increased motivation and/or decreased selectivity. The effect of age on mate choice in Gryllus integer was investigated by using a non-compensating treadmill, called the Kugel, to measure female phonotaxis. Artificially generated male calling songs of varying pulse rates were broadcast in either a singlestimulus or a three-stimulus experimental design. The pulse rates used in the calling song stimuli were 70, 64, 76, 55 and 85 pulses per second. These corresponded to the documented mean pulse rate for the species at the experimental temperature, one standard deviation below and above the mean, and 2.5 standard deviations below and above the mean, respectively. Test females were either 11-14 days or 25-28 days post-ecdysis. Trials usually were conducted two to seven hours into the scotophase. In the single-stimulus experiment, females were presented with stimuli with only one pulse rate. Older females achieved higher vector scores than younger females, indicating that older females are more motivated to mate. Both groups showed little phonotactic response towards 55 or 85 pIs, both of which lie outside the natural range of G. integer calling song at the experimental temperature. Neither group discriminated among the three pulse rates that fell within the natural range of calling song. In the three-stimulus experiment, females were presented with stimuli with one of three pulse rates, 64, 70 or 76 pIs, In alternation. Both age groups had reduced responsiveness in this experiment, perhaps due to an increase in perceived male density. Additionally, younger females responded significantly more to 64 and 70 pIs than to the higher pulse rate, indicating that they are selective with respect to mate choice. Older females did not discriminate among the three pulse rates. Therefore, it was concluded that selectivity decreases with age. A further study was conducted to determine that these effects were due to age and not due to the differing periods without a mating between the two age groups. Again, stimuli were presented in a three-stimulus experimental design. Age was held constant at 28 days and time since last mating varied from 11 to 25 days. Females varyIng in time since last mating did not differ in their responses to the calling song pulse rates. This indicated that the increased motivation and decreased selectivity exhibited In the initial experiments were due to age and not to time without a mating. Neither time of trial nor female weight had an effect upon female phonotaxis. Data are discussed in terms of mate choice, residual reproductive value, and costs of choice.

Heritability of multiple mating by female field crickets, Gryllus integer (Orthoptera: Gryllidae)

The heritability of multiple mating in female Gryllus integer crickets was studied. Two preliminary experiments were conducted to determine when females first mate following the post-imaginal moult and to ascertain whether constant exposure to males affects female mating rate. Female Q. integer first mated at an average age of 3.6 days (S.D. = 2.3, Range = 0-8 days) . Exposing female crickets to courting males 24 hr daily did not significantly alter mating rates from those females in contact with males for only 5 hr per day. A heritability value of 0.690 ± 0.283 was calculated for multiple mating behavior in female Q. integer using a parent-offspring regression approach. Parental females mated between land 30 times (x 9.8, S . D. = 6. 6 ) and offspring matings ranged from 0 to 26 times (x 7 .3, S.D. = 3.4). Multiple mating is probably a sexually selected trait which functions as a mechanism of female choice and increases reproductive success through increased offspring production. Classical theory suggests that traits intimately related with fitness should exhibit negligible heritable variation. However, this study has shown that multiple mating, a trait closely linked with reproductive fitness, exhibits substantial heritability. These results are in concordance with a growing body of empirical evidence suggesting many fitness traits in natural populations demonstrate heritabilities far removed from zero. Various mechanisms which may maintain heritable variation for female multiple mating in wild, outbred Q. integer populations are discussed.