Sexual Selection of Male Song in Free-Living Sagebrush Crickets, Cyphoderris Strepitans

Measures of lifetime mating success in the sagebrush cricket, Cyphoderris strepitans, have revealed that most males succeed in obtaining only 1 mating, while many males fail to attract a female at all and a small minority mate 2 to 4 times. Relative to their abundance in the population, virgin males have a greater likelihood of obtaining a mating than non-virgin males have of securing additional matings, a phenomenon known as the virgin male mating advantage. Previous studies of sagebrush crickets have focused primarily on determining the proximate mechanisms responsible for the virgin male mating advantage, but little work has been done to identify the factors that influence male attractiveness in the first place. Because song plays a central role in mate attraction, it’s likely that variability in song parameters among males could account for the observed differences in mating success. Song is an energetically costly signal to produce and could serve as an honest indicator of male quality. Consequently, males that are able to invest greater amounts of energy into singing should be more attractive to females. In a previous field season, we recorded and analyzed the songs of virgin and non-virgin males and indeed found some evidence that females prefer males which invest greater energy into calling. In the present study, we synthesized artificial sagebrush cricket songs and directly measured female song preference with an arena playback experiment. Females were shown to consistently prefer song characteristics that would require greater energy expenditure by males. Males that sing with long pulse duration, long train duration, and at an intermediate dominant frequency were found to be highly attractive to females.

Lovesick: Immunological Costs of Mating of Male Sagebrush Crickets

A growing body of evidence suggests that resources invested in sexual signals and other reproductive traits often come at the expense of the ability to mount an immune response. Male sagebrush crickets, Cyphoderris strepitans, offer an unusual nuptial food gift to females during mating: females chew on the tips of males' fleshy hind wings and ingest hemolymph seeping from the wounds they inflict. Previous research has shown that once a male has mated, his probability of obtaining an additional copulation is reduced relative to that of a virgin male seeking his first mating. One hypothesis to account for this effect is that wing wounding triggers an energetically costly immune response, such that non­ virgin males are unable to sustain the costly acoustical signaling needed to attract additional females. To test this hypothesis, we injected virgin males with lipopolysaccharides (LPS), a non-living component of bacterial cell walls that leads to upregulation of the insect immune system. Males were released in the field and recaptured over the course of the breeding season to monitor their mating success. Over two breeding seasons, LPS-injected males took significantly longer to secure matings than sham-injected virgin males. An encapsulation rate assay showed no difference in the encapsulation response of males of different mating status, but virgin males had significantly higher levels of phenoloxidase than non-virgin males. These results suggest that males trade off investment in reproduction and investment in immunity.

Immuniological Costs of Mating to Male Sagebrush Crickets

Male sagebrush crickets, Cyphoderris strepitans, offer an unusual nuptial food gift to females during mating: females chew on the ends of the males' fleshy hind wings and ingest hemolymph seeping from the wounds they inflict. Previous research has shown that once a male has mated, his probability of obtaining an additional copulation is reduced relative to that of a virgin male seeking to secure his first mating, a pattern known as the virgin-male mating advantage. One hypothesis that may explain this phenomenon is that mated males experience an energetically costly immune response via their wounds and therefore may be unable to sustain the costly acoustical signaling needed to attract additional females. To distinguish between the effects of mounting a costly immune response and the costs of producing a spermatophore, we mimicked a non­virgin mating status by injecting virgin males with bacterial lipopolysaccharides, a non-living elicitor of several immune pathways. After they had been treated, males were released in the field and recaptured over the course of the breeding season to monitor their mating success. Contrary to our prediction, LPS injected males did not take longer to secure matings than sham-injected virgin males. However, a companion study revealed that immunochallenged virgin males spent significantly less time calling (as assayed using time-lapse video photography) than sham­control virgin males. This confirms work in other cricket species showing a decline in mating effort following an immune challenge.

Male Hemolymph Loss During Nuptial Feeding as a Proximate Mechanisim Underlying the Virgin-Male Mating Advantage in Sagebrush Crickets

Male sagebrush crickets, Cyphoderris strepitans, offer an unusual nuptial food gift to females during mating: females chew on the ends of the males' fleshy hind wings and ingest hemolymph seeping from the wounds they inflict. Previous studies have shown that once a male had mated, his probability of obtaining an additional copulation is reduced relative to that of a virgin male seeking to secure his first mating, a pattern known as the virgin­male mating advantage. One hypothesis to account for the virgin-male mating advantage is that non­virgin males, having lost a substantial portion of their energy reserves at mating, may be unable to sustain the costly acoustical signaling activity required for the passive attraction of additional females. If the future mating prospects of non-virgin males are diminished because of sexual fatigue, this could stem either from the resources required to manufacture a new spermatophore or through the energy needed to replenish haemolymph lost through female wing­feeding. To distinguish between these two alternatives, we experimentally depleted virgin males of varying amounts haemolymph (0, 5 and 10 ul) in a way that mimicked hemolymph loss of non-virgin males, without the attendant costs of spermatophore production. After they had been treated, males were released in the field and recaptured over the course of the breeding season to monitor their mating success. Control males mated significantly sooner than did males depleted of 5 or 10 ul of hemolymph. We conclude, therefore, that the depletion of hemolymph that occurs through female wing feeding is sufficient by itself to diminish a non-virgin male's ability to secure another mating, acting as a brake on the operation of sexual selection in this species.

Ingestion of Male Hemolymph by Female Sagebrush Crickets: No Evidence of Male-Derived Anti-Aphrodisiacs

Male sagebrush crickets, Cyphoderris strepitans, offer an unusual nuptial food gift to females during copulation: females are permitted to feed on the hind wings of males and ingest hemolymph that flows from the resulting wounds. Johnson et al. (1999) showed that females experimentally precluded from wing feeding during initial copulations were more receptive to subsequent matings than females allowed to obtain a blood meal during initial copulations. The present study was designed to test the hypothesis that substances contained in the hemolymph of males and ingested by females during copulation function to decrease female receptivity to further matings, and thereby constitute male manipulation. To test this hypothesis, females were allowed to mate with an initial virgin male whose hind wings had been surgically removed to prevent wing feeding. Following these initial copulations, females were fed one of the following substances: male hemolymph, female hemolymph, or cricket Ringer's solution. Immediately following ingestion of the prescribed substances, females were given the opportunity to remate with novel virgin males whose hind wings had been left intact, and their latency to remating used as a measure of their sexual receptivity. There was no significant difference in latency to remating across treatments, suggesting no effect of hemolymph ingestion on female receptivity. The disparity between the results of the previous study (Johnson et al. 1999) and those of the present one suggest that the nutritional contribution derived through wing feeding, rather than male manipulation of female receptivity via substances in the hemolymph, affect remating propensity of female sagebrush crickets.

VIRGIN-MALE MATING ADVANTAGE IN SAGEBRUSH CRICKETS: DIFFERENTIAL MALE COMPETITIVENESS OR NON-INDEPENDENT FEMALE MATE CHOICE?

Female sagebrush crickets (Cyphoderris strepitans) feed on males' fleshy hind wings during copulation and ingest haemolymph oozing from the wounds they inflict. The wounds are not fatal and usually only a portion of the hind wings are eaten at any one mating, so that mated males are not precluded from mating again. However, based on their relative abundance in the population, virgin males have a higher mating success than non-virgin males. One explanation for this virgin-male mating advantage is that non-virgin males, having been depleted of their energy reserves through the wing-feeding behaviour of their mates, are unable to sustain the same level of acoustic signalling they produce prior to copulation. Previous assays of male signalling behaviour have provided some support to this hypothesis. However, an alternative explanation is that females actively seek out virgin males as mates because of the greater material resources they offer. If the acoustic structure of males' signals were systematically altered by the loss of hind-wing material underlying the sound-producing tegmina, females could potentially discriminate against mated males through reduced phonotaxis to their calls. We tested this hypothesis by experimentally removing one hind wing from virgin males, thereby simulating the non-virgin condition without the attendant costs of copulation. We compared the mating success of these 'asymmetrical' males with that of sham-operated virgin males when competing under natural conditions. In a companion laboratory study, we used time-lapse video recording to examine the possibility that female preferences are exerted only after pair formation has occurred. There was no significant difference in male mating success across treatments in either study. We conclude, therefore, that the virgin-male mating advantage does not stem from an acoustically mediated, non-independent female mating preference, but rather, from the differential competitiveness of males.

Male Manipulation of Female Receptivity in Sagebrush Crickets

Male sagebrush crickets, Cyphoderris strepitans, offer an unusual nuptial food gift to females during copulation: females are permitted to feed on the hind wings of males and ingest hemolymph that flows from the resulting wounds. A previous study of this species showed that females experimentally precluded from wing feeding during initial copulations were more receptive to subsequent matings than females allowed to obtain a full blood meal during initial copulations (Johnson et al. 1999). The present study was designed to test the hypothesis that hormonal substances contained in the hemolymph of males and ingested by females during copulation function to decrease female receptivity to further matings, and thereby constitute male manipulation. We tested this hypothesis by allowing females to ingest one of the following substances prior to experimental pairings: adult male hemolymph, adult female hemolymph, or insect Ringer's solution. Experimental females were given the opportunity to mate 24 hours after treatments were established and latency to mating was used as a measure of female receptivity. There was no significant difference in latency to mating across treatments. The lack of a difference could be due to a decay in the activity of putative hormonal substances over the 24-hour period separating hemolymph delivery and mating trials. In addition, the simultaneous act of wing-feeding combined with intitial copulation may provide a behavioral primer that triggers any effect on female receptivity.