(1) Behavioral and ecological interactions between native Gryllus pennsylvanicus fall field crickets and introduced Velarifictorus micado Japanese burrowing crickets.
The fall field cricket Gryllus pennsylvanicus is found throughout the U.S. and Canada. Field crickets represent a model system for behavioral reproductive isolation and sexual selection via acoustic communication. The Japanese burrowing cricket Velarifictorus micado is a relatively recent introduction to North America and is spreading rapidly. V. micado is a potential competitor, occupying the same microhabitats and reproductive season as native G. pennsylvanicus. My lab is currently studying how the presence of introduced competitor V. micado affects native G. pennsylvanicus, and how anthropogenic changes to their shared environment affect trade-offs in each species. We have found that exposure to Roundup (glyphosate) herbicide exposes trade-offs between life history and behavioral traits in both species, but that each species invests resources differently, resulting in differences in traits important to fitness. We are currently exploring the effects of anthropogenic sound and introduced V. micado song on traits important to native G. pennsylvanicus fitness, including male song and female song preference.
(2) The effect of disturbance on prairie orthopterans
My lab is currently studying the effect of fire on the biodiversity of prairie orthopterans using bioacoustic monitoring. We are also examining the effect of fire on native G. pennsylvanicus and introduced V. micado. Both species of field cricket deposit their eggs within the soil in September-October. The eggs diapause (pause development) within the soil until they hatch in the Spring. Fire is part of the natural disturbance regime of prairies, and managed prairies simulate this effect through controlled burns. We have been tracking what happens to native and introduced field cricket eggs when the prairie burns. So far, we have found that native G. pennsylvanicus exposed to fire as eggs have a high hatching success, while introduced V. micado have a relatively low hatching success. We are currently examining the effect of fire on sublethal behavioral and life history traits.
(3) The immunological costs of mating
One potential cost of mating for both males and females can be a reduction in the ability to fight future immunological challenges: trade-offs between immunity and reproduction can influence the optimal number of times for each sex to mate. In my dissertation research I found that Gryllus vocalis vocal field cricket females suffered an immunological cost to mating, however males (who contribute only sperm packets during mating) did not (Gershman 2008 Behav Ecol 19:810-815). In my postdoctoral research with the decorated cricket Gryllodes sigillatus and the sagebrush cricket Cyphoderris strepitans, two species in which males contribute large courtship feedings to females during mating, males with increased reproductive effort suffered a decreased ability to combat future immunological challenges (Gershman et al. 2010 Heredity 105:282-289; Leman et al. 2009 J Evol Biol 22:163-171). As predicted by life history theory, in both G. vocalis and G. sigillatus, males have a reduced ability to mount an immune response as compared to females (Gershman et al. 2010 J Evol Biol 23:829-839). In additional work done in collaboration with Sandra Steiger and Anne-Katrin Eggert, we found that both male and female burying beetles upregulate immunity during their period of biparental care (Steiger et al. 2011 Funct Ecol 25:1368-1378).
Thus mating can have a negative effect on immunity. Further, there is a trade-off between reproductive effort and immunity that can be seen both between the sexes and among species with divergent patterns of reproductive investment. When males contribute little reproductive effort per mating, females suffer greater immunological costs than males, and these costs may limit the optimal mating rate for females. However, when males make a large investment in reproduction, they may similarly suffer an immunological cost to mating. Immunological costs may limit the optimal number of matings for these males and potentially upend the assumption that males gain more from more mating many times than females do.
Recently, my lab examined differences in immunity among male and female Brood X 17-year periodical cicadas. We found that males have poorer immunity than females and throughout their brief adult lives, male immunity drops more rapidly than female immunity (Hord et al. submitted).
Older Research
(1) Sexual selection and plasticity in Drosophila flies: male olfactory displays as a multivariate trait.
I am currently using quantitative genetic techniques, as well as gas chromatography to examine sexual selection in Drosophila serrata flies, in collaboration with Howard Rundle at the University of Ottawa. In Drosophila flies, males produce olfactory signals that attract females. Males create these complex displays by varying the production of different cuticular hydrocarbons (CHCs). Previous research has identified a specific multivariate combination of CHCs that is most attractive to females. As a signal of male quality that is costly for males to produce, it is predicted that males will produce CHCs in an efficient way: males should produce the most attractive CHCs when there is the highest probability of attracting a female. I have been studying male plasticity in CHC production, and female plasticity in preference for male CHCs (Gershman et al. J Evol Biol 27(6):1279-1286). So far, we have found that male CHC attractiveness is affected by time of day and social environment, with males becoming more attractive during the day than at night. The presence of females increases male attractiveness, while the presence of other males decreases male attractiveness. Further, females are least attractive to males during the times of day that males are the most attractive, suggesting a conflict of interests between males and females (Gershman et al. 2014 Proc R Soc B in press). Experimental results in prep indicate that social environment affects the development of male CHC attractiveness, and female accessibility mediates male CHC plasticity.
(2) Sexual conflict over courtship feeding gifts: spermatophylax composition as a multivariate trait
Males of many taxa are required to contribute courtship gifts to females for successful copulation and sperm transfer. In orthopterans, males often produce large nutritive courtship gifts that they transfer to females, in addition to small packets that contain sperm. In G. sigillatus, these gifts have not been found to help offspring success. Instead, the spermatophylax (edible courtship gift) prevents the female from removing the male sperm packet before most of his sperm has been transferred to her sperm storage organ. Further, due to accessory gland products in the spermatophylax, consumption of the spermatophylax causes females to delay mating again (Gordon et al. 2012 Anim Behav 83:369-375). Therefore the spermatophylax both serves as ejaculate protection, and reduces sperm competition for the male. Females, however, may gain genetic benefits for their offspring from mating with more than one male, thus consuming the spermatophylax may not be in her best interest.
In work with the Sakaluk lab at Illinois State University, I examined the amino acid composition of the spermatophylax using gas chromatography, to study relationships between different amino acids and female willingness to accept and consume a spermatophylax. We found that the amino acid composition of the spermatophylax is highly heritable. Further, female resistance to consuming spermatophylaxes is also highly heritable. This information suggests that there is the potential for natural and sexual selection to act on these traits. Using selection analysis to determine which multivariate combinations of amino acids are most attractive to females, we found that the fitness surface is saddle-shaped, indicating that there is more than one combination of amino acids that results in an attractive spermatophylax. This selection analysis also provides an index of “spermatophylax attractiveness” to females. The heritability of spermatophylax attractiveness is highly significant, and the genetic correlation between spermatophylax attractiveness and female resistance to consuming spermatophylaxes is significant, indicating potential sexual conflict between males and females over the amino acid composition of the spermatophylax (Gershman et al. 2012 Proc B 279:2531-2538; Gershman et al. 2013 J Evol Biol 26(4):693–704).
(3) Female choice for novel partners
In many taxa, including crickets, females may choose one male instead of another based on traits that indicate male quality. However, females can also gain benefits by choosing to mate with different males, independent of male quality: females that mate with different males have more opportunities to acquire combinations of genes that increase offspring success. In contrast to male preference for novel females, female preference for novel males has been studied in relatively few species. Further, female choice to bias sperm in favor of novel males (postcopulatory choice) had not been previously studied. As one component of my dissertation, I found that females preferred novel males to previous partners, and were faster to remove the sperm packet and terminate sperm transfer from previous mates (Gershman 2008 Evolution 63:67-72). In my postdoctoral work I found a similar pattern of female postcopulatory choice in Gryllodes sigillatus, despite the presence of an ejaculate-protecting courtship gift from the male (Gershman and Sakaluk 2010 Ethology 116:1113-1117). I also found that in G. vocalis, female sperm removal behavior caused previous mates to lose paternity to males that were novel to the female (Gershman 2008 Evolution 63:67-72). Thus although female preference for novel partners can improve female fitness by adding diversity to her offspring, it can have a negative effect on male fitness, causing a conflict between males and females.
Surprisingly, in G. sigillatus, males did not have a preference for novel female partners (Gershman and Sakaluk 2009 Ethology 115: 774-780), suggesting that males either lack the ability to identify past female partners or that female discrimination against novel males renders male preference unnecessary.
(4) Sexual conflict over the optimal number of times to mate
For my dissertation research, I studied the vocal field cricket Gryllus vocalis, a species of field cricket in which females mate many time, to determine whether mating large numbers of times was beneficial to female lifetime reproductive success, or beneficial to only males. I found that females mated more times than was beneficial to their reproductive success, however even mating at such high rates did not have survival costs for females (Gershman 2007 Ethology 113:1099-1106). Further, these reproductive benefits were mediated by diet quality, such that females reared on a low quality diet did not gain reproductive benefits from mating large numbers of times (Gershman 2008 Evol Ecol Res 10:269-280).