Researchers collected 47 G. firmus and 39 G. pennsylvanicus females from allopatric Connecticut and Vermont populations in September 2020. They kept the field-mated females in individual cages with food, water and oviposition dishes for two additional months. All eggs from each female were incubated together, and surviving offspring were counted after a period at 6°C followed by incubation at 25°C.

Among 44 scored G. firmus broods, 11 females had male-like profiles and produced an average of 74.2 offspring, compared with 95.3 among 33 females with non-male-like profiles. In G. pennsylvanicus, 11 male-like females averaged 132 offspring, while 27 females with non-male-like profiles averaged 183. Statistical models associated the male-like category with fewer offspring in both species, with p-values below 0.001.

The analysis did not find strong evidence that hydrocarbon category was associated with whether a female produced any offspring (p = 0.09). Across categories, G. firmus females averaged 90 offspring per brood and G. pennsylvanicus averaged 168, a significant difference in the fitted models. This species contrast differed from earlier laboratory observations, but it does not establish a temporal change in fertility.

A separate analysis genotyped 29 G. firmus and 19 G. pennsylvanicus broods using seven microsatellite loci and inferred parentage from multilocus genotypes. It estimated a minimum number of contributing sires because males that fertilized no offspring, or only a single offspring, could not necessarily be detected.

For G. firmus, the estimated minimum sire count averaged 6 for male-like females and 5.5 for typical females. For G. pennsylvanicus, the corresponding means were 8.8 and 8. Neither within-species difference was statistically significant, with p = 0.6 in both cases. Broods from G. pennsylvanicus had a higher estimated minimum sire count than those from G. firmus, with means of 8.2 and 5.6, respectively.

The inferred paternity distribution departed strongly from a fair-raffle pattern in which each male would be expected to contribute similar numbers of offspring. Approximately 81% of individual broods rejected that expectation at the 0.05 significance level, and the combined test gave p approximately zero. The observed pattern was consistent with a loaded raffle, in which one or a few males accounted for a disproportionate share of paternity.

Because field mating was not experimentally controlled, the study could not determine why sperm use was unequal. The authors considered possible differences in sperm quantity or quality, repeated mating with the same males, and nonrandom mixing within the spermatheca, but did not establish which mechanism produced the pattern.

The results do not support male-like cuticular hydrocarbon profiles as indicators of greater fertility in the laboratory conditions tested or as signals of more contributing sires. The study did not measure field fertility, survival, access to food or offspring fitness, and its laboratory setting provided abundant food. The findings therefore do not exclude other fitness advantages or explain the mechanism behind the unequal paternity distribution.

Subject of Research: An animal study of cuticular hydrocarbon profiles, measured offspring production, and inferred paternity in two field cricket species

Article Title: Do favored cuticular hydrocarbon profiles signal fertility? Analysis of Gryllus firmus and Gryllus pennsylvanicus

Article References: Lee, E. H., Kim, J. Y., Lee, J., Cook, N., Heggeseth, B., & Maroja, L. S. (2026). Do favored cuticular hydrocarbon profiles signal fertility? Analysis of Gryllus firmus and Gryllus pennsylvanicus. Journal of Orthoptera Research, 35(1), 69-76. https://doi.org/10.3897/jor.35.161638

Image Credits: AI Generated

DOI: 10.3897/jor.35.161638

Keywords: Gryllus firmus, Gryllus pennsylvanicus, cuticular hydrocarbons, fertility, paternity analysis, sperm competition, field crickets, animal study, microsatellite genotyping, favored, cuticular, hydrocarbon

The more northern and broadly distributed G. pennsylvanicus occurs throughout much of the United States, whereas G. firmus is concentrated along the eastern coast and extends into Texas. The species, whose common ancestry dates to roughly 200,000 years ago, meet along the eastern front of the Appalachian Mountains and form a hybrid zone. They differ in several respects: G. firmus is often associated with sandy habitats, tends to be lighter in color, has longer ovipositors and is generally larger.

G. firmus females mated to G. pennsylvanicus males lay few eggs that do not hatch, an asymmetry consistent with the direction in which mitochondrial DNA moves between the species. These differences had previously been characterized in only a handful of locations, particularly Connecticut. That focus left open whether the same traits consistently identified the species across their full ranges.

The distinction matters because body size can vary with climate and growing-season length, while ovipositor length may be associated with local soil conditions. A trait that appears diagnostic in one hybrid zone could therefore reflect adaptation or population history rather than a stable species difference. To address that problem, the researchers assembled published and unpublished measurements collected with consistent methods, supplemented them with molecular markers for a subset of crickets and compared patterns across broad geographic regions and within the Connecticut hybrid zone.

Among allopatric populations, the two species differed significantly in male and female body size, absolute ovipositor length and ovipositor length relative to body size. The distributions nevertheless overlapped. Ovipositor length provided the clearest separation and remained informative after being scaled relative to pronotum width.

In a small subset of males for which wing-cover color was measured by spectrophotometry, color alone misclassified 3% of individuals as the wrong species: one of 24 G. pennsylvanicus males and none of seven G. firmus males. Body size alone misclassified 23% of 358 males, with 56 of 268 G. pennsylvanicus and 27 of 90 G. firmus assigned incorrectly. For females, body size and relative ovipositor length together produced a 12% misclassification rate, with 15 of 189 G. pennsylvanicus and 17 of 90 G. firmus misclassified.

These results identify useful diagnostic combinations, but they do not show that any one trait cleanly separates every individual. Crickets from or near the hybrid zone overlapped extensively with the allopatric samples, yet the sympatric group was intermediate for several female traits. Its females differed from both allopatric species in body size, absolute ovipositor length and relative ovipositor length.

Male body size in sympatric populations was not significantly different from that of allopatric G. firmus, although it was greater than in G. pennsylvanicus. The findings indicate that populations in contact do not simply reproduce the morphology of either pure species. They also show why a broad geographic survey is necessary: measurements from a single focal area may capture a regional form rather than the full extent of species divergence.

Variation within each species was substantial. G. pennsylvanicus from the southern, southwestern and central United States were larger than populations in the northwest and west-north-central regions. G. firmus from the south were also generally larger than those from the Northeast, although the southern samples were not significantly different from either northeastern or southeastern populations.

Ovipositor length varied geographically within both species. Eastern G. pennsylvanicus had the shortest ovipositors and central populations had the longest, though the central sample was limited. Within G. firmus, relative ovipositor length was shortest in the south, intermediate in the Southeast and longest in the Northeast.

Southeastern females had nearly the same absolute ovipositor length as northeastern females but were larger, showing why absolute and body-size-adjusted measurements can tell different geographic stories. Southern G. firmus populations also differed sharply from one another. Florida animals were much larger than northeastern G. firmus but had only slightly longer ovipositors, producing shorter ovipositors relative to body size.

Texas G. firmus did not differ from northeastern G. firmus in overall size but had still shorter relative ovipositors. The magnitude of these differences was similar to that separating G. firmus from the closely related, recently described G. thinos. Earlier genetic analyses had also placed Texas and Florida G. firmus-like populations in distinct evolutionary clusters.

The combined morphological and genetic evidence supports the proposition that Texas G. firmus may be an undescribed cryptic species, but the study notes that a phylogeny including northeastern, Texas and Florida populations, along with tests of reproductive compatibility, is still needed to resolve their relationships. Genetic data from the hybrid zone connected morphology to ancestry. Among crickets with a mixture of nuclear DNA from the two species, highly admixed individuals generally had intermediate phenotypes.

The nuclear hybrid index, which estimates the proportion of alleles inherited from G. firmus, explained 47% of the variation in relative ovipositor length. Its association was also strong with female body size, male tegmina color and male body size, explaining 28%, 31% and 19% of their respective variation, respectively. The analysis also detected asymmetric mitochondrial introgression: crickets with predominantly G. firmus nuclear ancestry frequently carried G. pennsylvanicus mitochondrial DNA.

Environmental models identified associations but did not reveal a single mechanism accounting for morphology across the entire distribution. Across the species’ ranges, the best-supported model for ovipositor length included latitude, elevation, average soil clay content and minimum and maximum temperatures; the selected model for pronotum width included latitude, longitude, soil sand content and minimum temperature. Higher clay content and higher minimum and maximum temperatures were associated with longer ovipositors, while higher minimum temperatures were associated with larger body size.

Within Connecticut, minimum and maximum temperatures and soil organic matter were retained in the best-supported model for ovipositor length. However, the expected simple relationship between sandy soil and long ovipositors did not hold. Both species use their ovipositors to place eggs in soil, but ovipositor length does not necessarily dictate laying depth, and established soil preferences and egg-viability patterns are geographically localized.

The authors therefore emphasize that habitat associations are variable and require further investigation. Body size also varied in ways compatible with climatic and developmental constraints. Northern populations tended to be smaller, matching the pattern expected where a shorter growing season leaves less time for growth.

G. firmus additionally shows regional variation in seasonal emergence: in Virginia it emerges earlier than G. pennsylvanicus, creating temporal isolation, whereas the two species emerge simultaneously in Connecticut. Florida G. firmus may produce multiple generations annually yet are considerably larger than northern populations, leaving open whether its life cycle differs discontinuously from that of other G. firmus. The geographic study therefore establishes a need to test species boundaries across many populations and environments, but it does not demonstrate that one model of body-size evolution, habitat association or hybrid-zone structure can explain every population.

Its clearest conclusion is more limited and more useful: traits that distinguish two species in one region may shift elsewhere, so apparent divergence must be assessed with range-wide sampling, genetic evidence and explicit consideration of local conditions.

Subject of Research: Geographic variation in morphology, ancestry, and environmental associations in two hybridizing field cricket species

Article Title: Geographic variation in phenotypic divergence between two hybridizing field cricket species

Article References: Byerly, A. R., Jenck, C., Goetz, A. R. B., Weissman, D. B., Gray, D. A., Ross, C. L., Maroja, L. S., & Larson, E. L. (2023). Geographic variation in phenotypic divergence between two hybridizing field cricket species. Journal of Orthoptera Research, 32(2), 189-200. https://doi.org/10.3897/jor.32.90713

Image Credits: AI Generated

DOI: 10.3897/jor.32.90713

Keywords: field crickets, Gryllus firmus, Gryllus pennsylvanicus, phenotypic divergence, hybrid zones, speciation, cryptic species, local adaptation, ovipositor length, mitochondrial introgression, Geographic, variation

The research, conducted by Chang S. Han of Kyung Hee University and LMU Munich, Cristina Tuni of LMU Munich and the University of Turin, and Niels J. Dingemanse of the University of Turin, focused on male two-spotted field crickets, Gryllus bimaculatus, drawn from a pedigreed laboratory population. The pedigree is crucial. Because the relatedness of every individual in the population is known, the researchers could apply quantitative genetic techniques, including animal model analyses, to separate the genetic contribution to behaviour from environmental effects. This allowed them to ask not merely whether the correlation between aggression and exploration declines with age, but whether the genetic underpinning of that correlation does too, and if so, why.

The behaviours in question are staples of personality research. Aggression was measured through staged contests in which males fought one another, with researchers scoring the intensity and outcome of each interaction. Exploration was assessed by observing how readily individuals moved through and investigated novel environments. Both are labile traits, meaning they can change from moment to moment, yet individuals differ consistently from one another in their typical expression. In this population, more aggressive males also tended to be more exploratory, producing a positive among-individual correlation of the kind documented across fishes, birds, mammals and insects, and central to the pace-of-life syndrome framework, which links behavioural types to differences in growth, reproduction and lifespan.

The study’s central finding is that this positive correlation, robust across the nymphal and young adult stages, steadily eroded as males aged through adulthood. At first glance, this pattern might suggest a simple developmental story: perhaps the developmental processes that synchronise aggression and exploration early in life simply dissolve over time. But the quantitative genetic analysis revealed something more intricate. The genetic correlation between the two behaviours, an estimate of the extent to which the same genes influence both traits, followed a parallel trajectory, remaining strong in early stages and weakening significantly in older adults. Crucially, the researchers found no evidence that short-term permanent environmental correlations were responsible for the observed age-related change. The similarity between the among-individual and genetic patterns pointed instead to causes operating at the level of genes and selection.

The first such cause is survival selection, a form of natural selection in which an individual’s phenotype determines whether it lives long enough to appear in the older age classes. In this population, selection at the young adult stage tended to favour less explorative males, meaning that highly exploratory individuals were disproportionately likely to die before reaching later ages. Because exploration was genetically linked to aggression, the selective removal of certain exploratory genotypes dragged the aggression-exploration correlation along with it. As the composition of surviving genotypes shifted with age, the tight coupling between the two behaviours weakened. This is a mechanism familiar from evolutionary genetics: selection on one trait can reshape the genetic architecture of correlated traits, and when selection is age-specific, that reshaping unfolds along the lifespan.

The second mechanism is arguably more surprising: genetic variation in age-related behavioural plasticity, sometimes described through genotype-by-age interactions. Different genotypes, the researchers found, do not all change their exploratory behaviour at the same rate as they age. Some genotypes maintain high exploration into old age, while others decline earlier or follow entirely different trajectories. This heritable variation in the age-specific expression of exploration meant that the genetic relationships among individuals were not fixed across the lifespan. As genotypes diverged in their ageing patterns, the genetic correlation between aggression and exploration diminished, independently of whether any individual survived or died. In other words, the genetic architecture of behaviour is itself dynamic, and genes that bind two traits together at one age may loosen their grip at another.

The significance of these findings extends well beyond crickets. Behavioural syndromes are widely regarded as evolutionary constraints: when the same genes influence multiple traits, selection cannot freely optimise one trait without dragging the other along. This idea has been formalised in models showing that behavioural correlations can slow or redirect adaptive evolution, and empirical work in wild birds, marmots, fishes and insects has repeatedly documented heritable correlations among personality traits. Yet most such studies capture a snapshot, typically of adult animals of unspecified or unremarked age. The new results warn that such snapshots may be misleading. A genetic correlation measured in young adults may overstate the constraint operating in older individuals, and predictions of evolutionary response that ignore age structure may therefore be systematically wrong.

The study also connects to a broader literature on the evolutionary genetics of ageing. Research on wild passerine birds, swans, houbara bustards and other organisms has shown that genetic variances and covariances of traits can change with age, consistent with theoretical predictions from mutation-accumulation and antagonistic pleiotropy theories of senescence. Previous work, including studies reporting that strong genetic correlations underlying behavioural syndromes disappear during development through genotype-age interactions, hinted at the kind of dynamics now documented in crickets. What distinguishes the new research is its explicit attempt to weigh competing mechanisms against one another within a single pedigreed population. By jointly estimating genetic correlations, age-related plasticity and survival selection, the authors demonstrated that age-related change in behavioural architecture is a multi-causal phenomenon, produced by both the selective sorting of genotypes and the age-dependent expression of behaviour within genotypes.

For evolutionary biologists, the practical message is that age must enter the models. Quantitative geneticists have developed powerful tools, notably the animal model, to partition phenotypic variance into additive genetic and environmental components, and these tools can now be extended to ask how the entire genetic covariance matrix, often abbreviated as the G-matrix, transforms across the lifespan. The cricket results suggest that the G-matrix is not a static property of a population but a moving target, reshaped continuously by mortality and by the plastic, genotype-specific unfolding of behaviour over time. Studies of morphological integration and developmental modularity have made similar arguments for structural traits; this work brings labile behavioural traits squarely into that conversation.

For anyone who has watched a young animal grow calmer, slower or more predictable with age, the findings offer a mechanistic explanation grounded in genetics and selection. The personalities we observe are not engraved once at birth and fixed forever; they are the output of genes whose effects shift as organisms age, filtered by the unforgiving arithmetic of survival. As highly exploratory crickets are weeded out and as different genotypes age along different behavioural paths, the once-tight bonds between boldness and aggression loosen. What looks like the mellowing of old age is, at the genetic level, a population’s architecture being rewritten. Understanding that rewriting, the authors argue, is essential if we hope to predict how animal populations will respond to selection in a changing world, one behavioural correlation at a time.

Subject of Research: Age-related changes in genetic correlations between aggression and exploration in male field crickets

Article Title: Selection and genetic variation in age-related plasticity drive the erosion of among-individual behavioural correlations in later life

Article References: Han, C. S., Tuni, C., & Dingemanse, N. J. (2026). Selection and genetic variation in age-related plasticity drive the erosion of among-individual behavioural correlations in later life. Heredity. https://doi.org/10.1038/s41437-026-00884-z

Image Credits: AI Generated

DOI: 10.1038/s41437-026-00884-z

Keywords: behavioural syndromes, animal personality, genetic correlation, age-related plasticity, survival selection, genotype-by-age interactions, quantitative genetics, field crickets, Gryllus bimaculatus, pace-of-life syndrome, aggression, exploration