Sexual selections in the northern fall field cricket, Gryllus pennsylvanicus: a manipulation of the adult sex ratio

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.

Male reproductive competition in the field crickets : gryllus veletis and G. pennsylvanicus

Sexual behavior in the field crickets, Gryllus veletis and G. pennsylvanicus , was studied in outdoor arenas (12 m2) at high and low levels of population density in 1983 and 1984. Crickets were weighed, individually marked, and observed from 2200 until 0800 hrs for at least 9 continuous nights. Calling was measured at 5 min intervals, and movement and matings were recorded hourly. Continuous 24 hr observations were also conducted,·and occurrences of aggressive and courtship songs were noted. The timing of males searching, calling, courting, and fighting for females should coincide with female movement and mating patterns. For most samples female movement and matings occurred at night in the 24 hr observations and were randomly distributed with time for both species in the 10 hr observations. Male movement for G. veletis high density only was enhanced at night in the 24 hr observations, however, males called more at night in both species at high and low densities. Male movement was randomly distributed with time in the 10 hr observations, and calling increased at dawn for the G. pennsylvanicus 1984 high density sample, but was randomly distributed in other samples. Most courtship and aggression songs in the 24 hr observations were too infrequent for statistical testing and generally did not coincide with matings. Assuming residual reproductive value, and costs attached to a male trait in terms of future reproductive success decline with age, males should behave in more costly ways with age; by calling and moving more with age. Consequently, mating rates should increase with age. Female behavior may not change with age. G. veletis , females moved more with age at both low density samples, however, crickets moved less with age at high density. G. pennsylvanicus females moved more with age in the 1984 low density sample, whereas crickets moved less with age in the 1983 high density sample. For both species males in the 1984 high density samples called less with age. For G. pennsylvanicus in 1983 calling and mating rates increased with age. Mating rates decreased with age for G. veletis males in the high density sample. Aging may not affect cricket behavior. As population density increases fewer calling sites become available, costs of territoriality increase, and matings resulting from non-calling behavior should increase. For both species the amount of calling and in G. veletis the distance travelled per night was not different between densities. G. pennsylvanicus males and females moved more at low density. At the same deneity levels there were no differences in calling, mating, and, movement rates in G. veletis , however, G. pennsylvanicus males moved more at high density in 1983 than 1984. There was a positive relationship between calling and mating for the G. pennsylvanicus low density sample only, and selection was acting directly to increase calling. For both species no relationships between movement and mating success was found, however, the selection gradient on movement in the G. veletis high density population was significant. The intensity of selection was not significant and was probably due to the inverse relationship between displacement and weight. Larger males should call more, mate more, and move less than smaller males. There were no correlations between calling and individual weight, and an inverse correlation between movement and size in the G. veletis high density population only. In G. pennsylvanicus , there was a positive correlation between individual weight and mating, but, some correlate of weight was under counter selection pressure and-prevented significance of the intensity of selection. In contrast, there was an inverse correlation in the G.·veletis low density B sample. Both measures of selection intensities were significant and showed that weight only was under selection pressures. An inverse correlation between calling and movement was found for G. veletis at low density only. Because males are territorial, females are predicted to move more than males, however, if movement is a mode of male-male reproductive competition then males may move more than females. G. pennsylvanicus males moved more than females in all samples, however, G. veletis males and females moved similar distances at all densities. The variation in relative mating success explained by calling scores, movement, and weight for both species and all samples were not significant In addition, for both species and all samples the intensity of selection never equalled the opportunity for selection.

The role of Wolbachia bacteria in reproductive incompatibilities and hybrid zones of Diabrotica beetles and Gryllus crickets

A rickettsial bacterium in the genus Wolbachia is the cause of a unidirectional reproductive incompatibility observed between two major beetle pests of maize, the western corn rootworm, Diabrotica virgifera virgifera, and the Mexican corn rootworm, D. v. zeae. These subspecies are allopatric except for two known regions of sympatry in Texas and Mexico. We demonstrate that populations of D. v. virgifera, with the exception of two populations in southern Arizona, are infected with a strain of Wolbachia. Populations of D. v. zeae are not infected. Treatment of D. v. virgifera with tetracycline eliminated the Wolbachia and removed the reproductive incompatibility. Similar patterns of reproductive incompatibility exist among taxa of the cricket genus Gryllus. Gryllus assimilis, G. integer, G. ovisopis, G. pennsylvanicus, and G. rubens are infected with Wolbachia whereas G. firmus is usually not. Populations of G. rubens and G. ovisopis carry the same Wolbachia strain, which is distinct from that of G. integer. G. pennsylvanicus is infected with two Wolbachia strains, that found in G. rubens and one unique to G. pennsylvanicus. Moreover, a proportion of G. pennsylvanicus individuals harbors both strains. Wolbachia may have influenced speciation in some members of the genus Gryllus by affecting the degree of hybridization between species. Given that Wolbachia infections are relatively common in insects, it is likely that other insect hybrid zones may be influenced by infections with Wolbachia.

The effect of social condition and diet on the frequency of male-male agonistic displays in the field cricket, gryllus bimaculatus /

The frequency and type of agonistic displays involved in male-male encounters should be significantly influenced by the presence of females. Discrete agonistic displays vary in energy expenditure and risk, and therefore should be dependent on available resources. The influence of live females and the scent of females, on the frequency of male agonistic displays was observed in a laboratory terrarium using the field cricket Gryllus bimaculatus. The effect of energy constraints on display frequency was also determined. Half the males were fed a diet high in protein and fet; the other males were fed a lower quality diet, for a 7-11 day period. The frequency of five individual displays and mating frequency were recorded using an Event Recorder and notebook. Each group of males was presented with three experimental conditions, over three days, involving the presence or absence of live females and female scent. The presence of females elicited an increase in all displays except antennation; female scent increased the frequency of antennations, mandible flares and grapples, but to a lesser extent than did live females. The frequency of grapples significantly increased for males fed the high quality diet; however diet did not influence the other displays. The combined influence of diet and condition was significant for mandible flare only. Mating frequency was not influenced by diet. However, the frequency ofthe displays were positively correlated with mating frequency for high quality fed males. Escalated displays involving high costs, such as grapple and mandible flare, increased in frequency when the benefits of winning contests were high in G.bimaculatus. Escalation to grapple behaviour was less evident for males fed the lower quality diet as this imposed energy constraints on high cost displays.

Body size in the field cricket Gryllus integer (Orthoptera : Gryllidae) :heritability and male mating success

The influence of male body weight on the aggressive and mating behaviour of male Gryllus integer was studied under laboratory conditions. The relationship between adult age and weight was first determined; female weight increased and male weight decreased with age. Virgin males that had been isolated since the adult molt were paired for similar age and a difference in weight of greater than 200 mg. Paired males and a virgin female were observed in a glass arena for 24 minutes or until a mating occurred. Larger males mated significantly more often than smaller males. Larger males attacked more often, were more successful in aggressive encounters and had more contact with the female. Males that did not mate had lower rates of courtship and mounts than males that mated. Females in trials that did not result in a mating were signifcantly heavier than females in trials that resulted in a mating. Larger males that mated were significantly closer in weight to the weight of the female than larger males in trials that did not result in a mating. Larger males in trials that did not result in a mating had higher rates of aggressive stridulation than larger males that mated. Male weight is therefore important in mating success; fitness traits should theoretically show low genetic variability. However, significant heritability values were found for live weight, dry weight, head width, pronotum width and length, hind femur length and forewing length when estimated from the regression of offspring on mid-parent values, offspring and female and male values separately and full-sib correlations. The heritability of hind femur width was significant when estimated from the regression of offspring on male parent and from full-sib correlations. Heritability estimates of forewing length were significantly higher when estimated from the regression of offspring on female parent than when estimated from the regression of offspring on male parent. High phenotypic, genetic and environmental correlations were found between all pairs of traits. Data on male mating success and the heritability of fitness traits were discussed in terms of the maintenance of genetic variability.

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.

Mating behaviours of the field cricket (Gryllus integer) at different levels of male density

The reproductive behaviour of the field cricket, Gryllus integer, was systematically observed in indoor arenas to determine the extent of female Choice and male-male competition at different sex ratios representing two male densities (12:6 and 6:6). The costs and benefits to males and females in those two densities were analyzed according to the theory of the evolution o£ leks. Observations were conducted during the dark hours when most calling occurred since hourly rates of courtship song and mating did not fluctuate significantly over a 24 h period. Female mating rates were not significantly different between densities, therefore males at high densities were not advantaged because of increased female tendencies to mate when social stimulation was increased. Mean rates of acoustical signalling (calling and courtin"g) did not differ significantly between densities. Mean rates of fighting by males at the high density were significantly greater than those of males at the low density. Mating benefits associated with callin~courting and fighting were measured. Mating rates did not vary with rates of calling at either density. Calling was not a prerequisite to mating. Courtship song preceded all matings. There was a significant power fit between male mating and courting rates, and male mating and fighting rates at the low, but not at the high, density. Density differences in the benefits associated with increased courting and fighting may relate, in part, to greater economic defensibility and monopoly of females due to reduced male competition at the low density. Dominant males may be preferentially chosen by females or better able to monopolize mating opportunities than subordinate males. Three criteria were used to determine whether dominant males were preferentially chosen by females. The number of matings by males who won fights (within 30 min of mating) was significantly greater than the number of matings by males who were defeated in such fights. Mating rates did not vary significantly with rates of winning at either density. There was a significant power fit between male mating rates and the percentage of fights a male won (irrespective of his fighting-frequency) at the low density. The mean duration a male guarded the female after mating did not vary significantly between densities. There was a significant linear relationship between the duration a spermatophore was retained and the duration a male guarded the female after mating. Courtship song apparently stimulated spermatophore removal. Male guarding involved inter-male aggression and reduced courtship attempts by other males. Males at the high density received no apparent reproductive benefits associated with increased social stimulation. Conclusive evidence for preferential choice of males by females, using the criteria examined here, is lacking. Males at the lower density had fewer competitors and could monopolize females more effectively.

Sperm utilization patterns in Gryllus integer (orthoptera: gryllidae

Sperm competition is the competition for fertilizations between ejaculates, within a female, following multiple mating. There are four sperm utilization or precedence patterns: first male precedence, where the first male to mate fertilizes most of the eggs laid by a female; last male precedence, where the last male to mate fertilizes most of the eggs laid by a female; "all-or-none" pattern, where sperm from either male fertilizes all the eggs laid by a female but which male's sperm that is used is random; or sperm mixing, where sperm from each male is used equally in fertilizing eggs laid by a female. Intermediate utilization patterns are also possible. Sperm competition occurs in a wide variety of insect species as well as other animals. This study was undertaken to study sperm competition in the field cricket, Gryllus integer. Four experiments were conducted: a radiation and sterilization experiment, a diapause experiment, and 2 competition experiments. It was found that 7,000 rad of gamma radiation sterilized adult ~ integer males. There was no diapause in the laboratory in ~ integer eggs. In the first competition experiment, three groups of females were used: females mated with a normal male, then with a second normal male (NN group); females mated with a normal male, and then with a sterile male (NR group); and females mated with a sterile male, and then with a normal male (RN group). The results obtained from this experiment showed that the mean proportion of eggs hatched was significantly different between 3 groups of females, with the proportion hatched much greater in the NN group than in either the NR or RN groups. The pattern for the proportion of eggs hatched following a double mating most closely resembled a pattern expected if sperm mixing is occurring. Results obtained in the replicate competition experiment showed that the mean proportion of eggs hatched for the females in the NR group was significantly lower than the proportion hatched in the other two groups. This also supports a model of sperm mixing as a precedence pattern. Values calculated following Boorman and Parker (1976), for the proportion of eggs fertilized by the second male to mate following a double mating, were 0.57 in competition experiment 1 and 0.62 in the replicate. These values indicate that sperm mixing occurs in~ integer.

The effect of diet and parasite load on female phonotaxis in the Texas field cricket, Gryllus integer

Female crickets respond selectively to variations in species-specific male calling songs. This selectivity has been shown to be age-dependent; older females are less choosy. However, female quality should also affect female selectivity. The effect of female quality on mate choice was examined in Gryllus integer by comparing the phonotactic responses of females on different diets and with different parasite loads to various synthetic models of conspecific calling song. Test females were virgin, 11-14 days old, and had been maintained on one of five diets varying in protein and fat content. Phonotaxis was quantified using a non-compensating Kugel treadmill which generates vector scores incorporating the speed and direction of movement of each female. Test females were presented with four calling song models which differed in pulse rate, but were still within the natural range of the species for the experimental temperature. After testing, females were dissected and the number of gregarine parasites within the digestive tract counted. There were no significant effects of either diet or parasitism on female motivation to mate although the combined effects of these variables seem to have an effect with no apparent trend. Control females did not discriminate among song types, but there was a trend of female preferences for lower pulse rates which are closest to the mean pulse rate for the species. Heavily parasitized females did not discriminate among pulse rates altho~gh there was a similar trend of high vector scores for low pulse rates. Diet, however, affected selectivity with poorly-fed females showing significantly high vector scores for pulse rates near the species mean. Such findings raise interesting questions about energy allocation and costs and risks of phonotaxis and mate choice in acoustic Orthoptera. These results are discussed in terms of sexual selection and female mate choice.

The effect of gregarine parasite infections, age, and diet on calling song structure and mating behaviour in the Texas field cricket, Gryllus integer

In the past ten years, many researchers have focussed their attention on parasites regarding the role they may play in causing variations in male secondary sexual traits and subsequent effects on female choice. Male age has also been suggested to be an important factor in female choice if old age reflects superior genes. This study investigated the effects that gregarine gut parasites, age, and diet have on the calling and mating behaviour of the male Texas field cricket, Gryllus integer. Male calling songs were recorded in the laboratory using a Digital Signal Processing Network. The song parameters measured were: pulse rate, pulse width, burst duration, pulses per burst, interburst interval, and percent missing pulses. The effects of parasite load and age on the various calling song parameters was investigated in crickets that were fed two different diets varying in nutritional quality. None of the calling song parameters were affected by either parasite load or age in either diet grou p. Courtship behaviour was ob served and recorded using an Eventlog recorder on an IBM computer in the laboratory. Females mated equally with paras(tized and unparasitized males and with old and young males The total duration and proportion of time spent performing each of 9 courtship displays were recorded for males on each diet. Only one display was affected by parasite load. Highly parasitized males fed the nutritionally inferior diet juddered for a proportionately shorter time than males with low parasite loads. Also, older males performed juddering and shaking antennae proportionally longer and juddering and raising wings for longer durations than younger males. Males that successfully mated were observed for performance of 8 post-copulatory guarding behaviour displays. None of the guarding behaviours were affected by parasite load. However, one display was affected by age, with older males performing guard turning for shorter durations than younger males. Results are discuss,ed in terms of the influence of parasites and age on female choice.

Demographic and genetic attributes of dispersing and resident individuals of an enclosed Microtus pennsylvanicus population

A dispersal polymorphism may exist in emigrants from cyclic populations of Microtus '~nnsylvanicus biasing trap-revealed movements of unenclosed animals in favour of sedentary or colonizing individuals. The dispersal tendency of emigrants from an enclosed population was investigated by releasing animals via tubes into one of two adjacent enclosures, one vacant and one inhabited. Individuals from the enclosed population were monitored for age, sex, weight and electrophoretically detectable serum transferrin genotype in an intensive live-trapping program. In 1973 the minimum number alive in the introduced enclosed study population reached approximately l67/ha when breeding stopped in October. In 1974 intensive breeding increased the population density to 333/ha by mid-July when a long decline in numbers and breeding intensity began without an intervening plateau. An adjacent unenclosed area had a much lower density and longer breeding season in 1974. The growth rate of young males in the enclosed population tended to be lowest during the decline period in 1974. Survival of the enclosed population was high throughout but was lowest during the decline phase in both sexes, especially males. Low transferrin heterozygote survival during the decline coincided with a significant heterozygote deficiency in females whereas in males genotype frequencies did not depart from Hardy-Weinberg equilibrium values throughout th.e study. Twenty-nine suitable ani.mals were released during the decline in five periods from July to November 1974. The proportions of males and transferrin heterozygotes in the released graun were generally greater than in the source population~ In the test enclosures 21% of the released animals continued their movement through the vacant area while 41% (no significant difference) moved through the inhabited enclosure. In the vacant test area, females had a greater tendency than males to continue dispersal whereas no difference was noted in the inhabited area. Low frequency of captures in the tubes, predator disturbances and cold weather forced the termination of the study. The role of dispersal as a population regulating mechanism was further substantiated. The genetic differences between emigrant and resident animals lend support to Howard's hypothesis that a genetic polymorphism influences the tendency to disperse. Support is also given to Myers' and Krebs' contention that among dispersers an additional density dependent polymorphism influences the distance dispersed.

Cryptic species of Gryllus in the light of bioacoustic (Orthoptera: Gryllidae)

O gênero Gryllus destaca-se com um grande número de espécies descritas e representa um dos mais complexos gêneros na sistemática dos Orthoptera, caracterizado por um conjunto de espécies cosmopolitas e crípticas. Este trabalho compara os sons emitidos por espécimens de Gryllus coletados no campus da UNESP em Rio Claro (SP), bem como a morfologia e morfometria das pars stridens, com o objetivo de aplicar os resultados no reconhecimento de possíveis espécies crípticas e contribuir na discussão de processos de especiação. Três grupos de grilos foram discriminados por diferenças nas pars stridens e sons de chamado, caracterizados por diferentes ritmos, freqüências e composição das notas, indicando assim, a presença de três espécies no local analisado, morfologicamente pouco distintas. Diante dos resultados, sugere-se a utilização das características da pars stridens e do som de chamado como caracteres diagnósticos imprescindíveis na taxonomia dos Gryllus.

Morphological characterization of the proventriculus of Gryllus assimilis Fabricius (Orthoptera, Gryllidae)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)