Robin M. Tinghitella

Flexible mate choice when mates are rare and time is short.

Female mate choice is much more dynamic than we once thought. Mating decisions depend on both intrinsic and extrinsic factors, and these two may interact with one another. In this study, we investigate how responses to the social mating environment (extrinsic) change as individuals age (intrinsic). We first conducted a field survey to examine the extent of natural variation in mate availability in a population of threespine sticklebacks. We then manipulated the sex ratio in the laboratory to determine the impact of variation in mate availability on sexual signaling, competition, and mating decisions that are made throughout life. Field surveys revealed within season heterogeneity in mate availability across breeding sites, providing evidence for the variation necessary for the evolution of plastic preferences. In our laboratory study, males from both female-biased and male-biased treatments invested most in sexual signaling late in life, although they competed most early in life. Females became more responsive to courtship over time, and those experiencing female-biased, but not male-biased sex ratios, relaxed their mating decisions late in life. Our results suggest that social experience and age interact to affect sexual signaling and female mating decisions. Flexible behavior could mediate the potentially negative effects of environmental change on population viability, allowing reproductive success even when preferred mates are rare.

Complex community and evolutionary responses to habitat fragmentation and habitat edges: what can we learn from insect science?

Habitat fragmentation is the primary factor leading to species extinction worldwide and understanding how species respond to habitat edges is critical for understanding the effects of fragmentation on insect diversity in both natural and managed landscapes. Most studies on insect responses to the habitat edge focus on bottom-up changes in resources. Only a few recent studies have examined multi-trophic responses to habitat edges; the results of these studies highlight the problem that we lack a conceptual framework to understand the complex results observed when a single species response to an edge cascades throughout the food web in ways that are currently not predictable. Recent research from insect systems suggests that habitat edge responses cascade both up and down multi-trophic foodwebs and these altered species interactions may affect evolutionary processes. Future studies that investigate the effects of habitat edges on both ecological and evolutionary dynamics can help to fill these knowledge gaps and we suggest that insects, with short generation times, present an ideal opportunity to do so.

Male and female crickets modulate their courtship behaviour depending on female experience with mate availability

Ecological factors like demography can shape competition for mates, altering the strength and direction of sexual selection via changes in courtship behaviour, choosiness and postcopulatory mechanisms. Demography is also quite dynamic, frequently changing within the life span of individual organisms. In this paper, I ask whether and how female experience with mate availability and female age interact to influence both male and female mating decisions. I housed female Acheta domesticus crickets individually at low or high density throughout their adult lives in screen cages that allowed low- and high-density crickets acoustic, visual and tactile contact with conspecifics. I assessed mating behaviour in courtship trials with naive (untreated) males at two time points, early and late in life. Females that experienced high and low density initiated courtship by approaching and contacting males more frequently than did females that were housed in isolation. Older females also initiated courtship more often than young females. Female mating decisions (whether the female mounted the male and her latency to mount), however, did not depend on experience with population density, age, or their interaction, perhaps because females remained virgin throughout the experiment. Naive males were able to assess females prior experience, perhaps through variation in how frequently females approached them, and preferentially courted females that experienced high density. They did not differentially court females depending on their age or the interaction of age and population density. My results suggest that males assess mate competition using sociosexual cues derived from potential mates and adjust their behaviour accordingly.

On the role of male competition in speciation: a review and research agenda

Little attention has been given to how males competing for mates can facilitate the evolution and persistence of new species. We expand the current framework for how new species evolve (speciation) to include male competition, drawing on recent research to show how male competition contributes to divergence between co-occurring or spatially isolated populations. We also identify interactions with female mate choice and environmental variation, and formulate a research program that will move this field forward.

Interspecific competition, hybridization, and reproductive isolation in secondary contact: missing perspectives on males and females

Abstract Research on sexual selection and hybridization has focused on female mate choice and male–male competition. While the evolutionary outcomes of interspecific female preference have been well explored, we are now gaining a better understanding of the processes by which male–male competition between species in secondary contact promotes reproductive isolation versus hybridization. What is relatively unexplored is the interaction between female choice and male competition, as they can oppose one another or align with similar outcomes for reproductive isolation. The role of female–female competition in hybridization is also not well understood, but could operate similarly to male–male competition in polyandrous and other systems where costs to heterospecific mating are low for females. Reproductive competition between either sex of sympatric species can cause the divergence and/or convergence of sexual signals and recognition, which in turn influences the likelihood for interspecific mating. Future work on species interactions in secondary contact should test the relative influences of both mate choice and competition for mates on hybridization outcomes, and should not ignore the possibilities that females can compete over mating resources, and males can exercise mate choice.

Intrasexual competition enhances reproductive isolation between locally adapted populations

Abstract During adaptation to different habitat types, both morphological and behavioral traits can undergo divergent selection. Males often fight for status in dominance hierarchies and rank positions predict reproductive success. Ecotypes with reduced fighting abilities should have low reproductive success when migrating into habitats that harbor ecotypes with superior fighting abilities. Livebearing fishes in the Poecilia mexicana-species complex inhabit not only regular freshwater environments, but also independently colonized sulfidic (H2S-containing) habitats in three river drainages. In the current study, we found fighting intensities in staged contests to be considerably lower in some but not all sulfidic surface ecotypes and the sulfidic cave ecotype compared with populations from non-sulfidic surface sites. This is perhaps due to selection imposed by H2S, which hampers oxygen uptake and transport, as well as cellular respiration. Furthermore, migrants from sulfidic habitats may lose fights even if they do not show overall reduced aggressiveness, as physiological performance is likely to be challenged in the non-sulfidic environment to which they are not adapted. To test this hypothesis, we simulated migration of H2S-adapted males into H2S-free waters, as well as H2S-adapted cave-dwelling males into sulfidic surface waters. We found that intruders established dominance less often than resident males, independent of whether or not they showed reduced aggressiveness overall. Our study shows that divergent evolution of male aggressive behavior may also contribute to the maintenance of genetic differentiation in this system and we call for more careful evaluation of male fighting abilities in studies on ecological speciation.

Females sample more males at high nesting densities, but ultimately obtain less attractive mates

BackgroundSexual selection is largely driven by the availability of mates. Theory predicts that male competition and female choice should be density-dependent, with males competing more intensely at relatively high density, and females becoming increasingly discriminating when there are more males from whom to choose. Evidence for flexible mating decisions is growing, but we do not understand how environmental variation is incorporated into mate sampling strategies. We mimicked threespine stickleback (Gasterosteus aculeatus) breeding conditions in pools with high and low densities of nesting males and allowed females to search for mates to determine whether 1) mate search strategies change with the density of breeding males and 2) pre-copulatory components of mate choice (signalling, competition, search patterns, and mating decisions) are modified in parallel.ResultsWhile females sampled more males at high male density, suggesting greater opportunity for sexual selection, the expanded search did not result in females choosing males with more attractive sexual signals. This is likely because red throat colouration was twice as great when half as many males competed. Instead, females chose similarly at high and low male density, using a relative strategy to compare male traits amongst potential suitors. Reduced throat colour could reflect a trade-off with costly male competition. However, we did not observe more intense competition at higher relative density. Density-dependent signalling appears largely responsible for females associating with males who have more attractive signals at low density. If we lacked knowledge of plasticity in signalling, we might have concluded that females are more discriminating at low male density.ConclusionsTo understand interactions between mate choice and population dynamics, we should consider how components of mate choice that precede the mating decision interact.

Color and behavior differently predict competitive outcomes for divergent stickleback color morphs

Abstract Our knowledge of how male competition contributes to speciation is dominated by investigations of competition between within-species morphs or closely related species that differ in conspicuous traits expressed during the breeding season (e.g. color, song). In such studies, it is important to consider the manner in which putatively sexually selected traits influence the outcome of competitive interactions within and between types because these traits can communicate information about competitor quality and may not be utilized by homotypic and heterotypic receivers in the same way. We studied the roles of breeding color and aggressive behaviors in competition within and between two divergent threespine stickleback Gasterosteus aculeatus color types. Our previous work in this system showed that the switch from red to black breeding coloration is associated with changes in male competition biases. Here, we find that red and black males also use different currencies in competition. Winners of both color types performed more aggressive behaviors than losers, regardless of whether the competitor was of the same or opposite color type. But breeding color differently predicted competitive outcomes for red and black males. Males who were redder at the start of competition were more likely to win when paired with homotypic competitors and less likely to win when paired with heterotypic competitors. In contrast, black color, though expressed in the breeding season and condition dependent, was unrelated to competitive outcomes. Placing questions about the role of male competition in speciation in a sexual signal evolution framework may provide insight into the “why and how” of aggression biases and asymmetries in competitive ability between closely related morphs and species.

The roles of inter- and intra-sexual selection in behavioral isolation between native and invasive pupfishes

Abstract Male–male competition and female mate choice may both play important roles in driving and maintaining reproductive isolation between species. When previously allopatric species come into secondary contact with each other due to introductions, they provide an opportunity to evaluate the identity and strength of reproductive isolating mechanisms. If reproductive isolation is not maintained, hybridization may occur. We examined how reproductive isolating mechanisms mediate hybridization between endemic populations of the Red River pupfish Cyprinodon rubrofluviatilis and the recently introduced sheepshead minnow C. variegatus. In lab-based dominance trials, males of both species won the same number of competitions. However, male C. rubrofluviatilis that won competitions were more aggressive than C. variegatus winners, and more aggression was needed to win against competitor C. variagatus than allopatric C. rubrofluviatilis. Duration of fights also differed based on the relatedness of the competitor. In dichotomous mate choice trials, there were no conspecific or heterospecific preferences expressed by females of either species. Our findings that male–male aggression differs between closely and distantly related groups, but female choice does not suggest that male–male competition may be the more likely mechanism to impede gene flow in this system.

No evidence for adjustment of maternal investment under alternative mate availability regimes

Using treatments that mimic high and low availability of reproductive males, it was found that female three-spined sticklebacks Gasterosteus aculeatus, previously shown to adjust their mate choices when male mates were rare, did not alter their reproductive investment strategies. These results suggest that plasticity in investment is perhaps limited by physiological requirements or dependent on relatively extreme mate availability regimes. The probability of becoming reproductive, number of clutches per season (per female), initial clutch size and mass and the timing of reproduction were all independent of the experience a female had with mate availability. This suggests that pre-copulatory plasticity in reproductive strategies may contribute more to variation in the strength and direction of sexual selection than reproductive investment in offspring.