James J. Krupa

Sexual conflict and the evolutionary ecology of mating patterns: water striders as a model system.

Two core ideas in the study of mating systems and sexual selection are (1) the existence of a conflict between the sexes over mating decisions, and (2) that variation in ecological conditions drives the evolution of adaptive mating strategies and the diversification of mating systems. A recent burst of experimental studies of mating behavior and sexual selection in water striders has focused on the interaction of these ideas and led to new insights into the evolutionary ecology of mating systems and sexual selection.

Assortative mating by size: A meta-analysis of mating patterns in water striders

SummaryAssortative mating by size is a common mating pattern that can be generated by several different behavioural mechanisms, with different evolutionary implications. Assortative mating is typically associated with sexual selection and has been regarded as an attribute of populations, species, mating systems or even higher order taxa. In most animal groups, however, appropriate analyses of assortative mating at these different levels are lacking and the causes and forms of assortative mating are poorly understood. Here, we analyse 45 different population level estimates of assortative mating and non-random mating by size in seven confamiliar species of water striders that share a common mating system. A hierarchical comparative analysis shows that virtually all the variance within the clade occurs among samples within species. We then employ meta-analysis to estimate the overall strength of assortative mating, to determine the form of assortative mating and to further assess potential differences among species as well as the probable causes of assortative mating in this group of insects. We found overall weak but highly significant positive assortative mating. We show that analyses of the degree of heteroscedasticity in plots of male versus female size are critical, since the evolutionary implications of ‘true’ and ‘apparent’ assortative mating differ widely and conclude that the positive assortative mating observed in water striders was of the ‘true’ rather than the ‘apparent’ form. Further, within samples, mating individuals were significantly larger than non-mating individuals in both males and females. All of these non-random mating patterns were consistent among species and we conclude that weak positive assortative mating by size is a general characteristic of those water strider species that share this mating system. We use our results to illustrate the importance of distinguishing between different forms of assortative mating, to discriminate between various behavioural causes of assortative mating and to assess potential sources of interpopulational variance in estimates of assortative mating. Finally, we discuss the value of using meta-analytic techniques for detecting overall patterns in multiple studies of non-random mating.

Experimental studies on water strider mating dynamics: spatial variation in density and sex ratio

SummaryWe used field surveys, field experiments and experiments in artificial pools to study the effects of variation in sex ratio and density on mating dynamics of a stream water strider, Aquarius remigis. Our field survey documented the existence of “hot spots”, sites of higher than average total gerrid density, a male-biased sex ratio, and higher than average female mating activity. Female gerrids frequently changed sites, usually moving upstream, perhaps to spread their eggs among many sites. Male gerrids showed two movement strategies: some males frequently changed sites, while other males were stationary at hot spots. Surprisingly, smaller males tended to be stationary at hot spots. A field manipulation of the availability of refuges for females to avoid harassment by males supported the notion that males prefer hot spots because they are sites where a scarcity of refuge for females makes it relatively easy for males to intercept females. Experiments in plastic pools compared sites with 20 males: 5 females (simulating hot spots) to pools with 5 males: 5 females. The rate of male harassment of females was higher in 20:5 pools as compared to 5:5 pools. In response to increased male harassment, females reduced their activity on the water and increased their time spent out of the water and thus unable to forage. Males showed a large male mating advantage (LMMA) in 5:5 pools, but, surprisingly, not in 20: 5 pools. This pattern can explain the field observation that small males prefer hot spots. A behavioral mechanism that can explain the LMMA is as follows. Mating occurs when males overcome female resistance. Larger males have a mating advantage over smaller males if females resist heavily. Increased harassment (e.g., in 20:5 pools as compared to 5:5 pools) might result in reduced female resistance to males and thus a reduced LMMA. Females also showed some non-random mating by size that might reflect an interplay between male preference for large females and the avoidance of males by large females.

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Abstract Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators.

Predation risk, food deprivation and non-random mating by size in the stream water strider, Aquarius remigis

SummaryNon-random mating by size (NRMS) plays a central role in the study of sexual selection and the evolution of mating systems. Theory suggests that NRMS should be influenced by conflicting demands (e.g., predation risk, hunger); few experimental studies, however, have addressed these effects. We used a factorial experiment to examine the influence of predatory green sunfish and food deprivation on NRMS in male and female stream water striders, Aquarius remigis. As predicted by theory, food deprivation decreased the large-male mating advantage. The influence of predation risk, however, went against existing theory; that is, risk increased the large male mating advantage. The degree of large-male mating advantage was negatively related to a measure of the rate of male harassment of females. A behavioral mechanism that can explain these patterns emphasizes the contrasting effects of different competing demands on male harassment rates, female resistance and the role of male size in overcoming female resistance. Females usually resist male mating attempts. Successful mating occurs when males overcome female resistance. If harassment rates (of females by males) are low, larger males have a mating advantage over smaller males perhaps because females resist heavily and thus only larger males can overcome female resistance. If, however, male harassment rates are very high, female resistance might be swamped; mating should then be more random with respect to male size. Food deprivation increases gerrid activity and thus increases harassment rates which should then reduce NRMS. In contrast, risk decreases gerrid activity, thus decreasing harassment rates and increasing NRMS. Females did not show significant NRMS. Females did, however, show a pattern of change in NRMS that is consistent with male choice for larger females.

Avoidance of Male Giant Water Striders By Females

Shifts in behaviour and microhabitat preference were exhibited by female giant water striders (G. remigis) when males were experimentally introduced into and removed from wading pools containing females. In the presence of males, females became significantly less active, interacted with one another less, spent less time in open water, and spent more time out of the water away from females. Furthermore, they resumed their original behaviours once males were removed. It is potentially advantageous for females to avoid male harassment due to costs associated with such interactions.

Direct and indirect effects of multiple enemies on water strider mating dynamics

Previous studies have shown that green sunfish, Lepomis cyanellus, have strong effects on the activity, habitat use, social interactions and mating dynamics of a stream-dwelling water strider, Aquarius remigis (family Gerridae, hence, gerrids). In nature, however, stream pools often contain not just sunfish and water striders, but also smaller fish such as minnows. Here, we used factorial experiments in seminatural streams to document the direct and indirect effects of sunfish and fathead minnows, Pimephales promelas, on water strider survival and behavior. Sunfish, minnows and gerrids all consume surface prey (here, crickets); thus these three species are potential food competitors. Sunfish eat minnows. Accordingly, the presence of sunfish caused minnows to increase their schooling behavior and shift their activity from the surface toward the bottom substrate. The presence of sunfish was also associated with an increase in the number of missing gerrids, whereas minnows caused relatively little gerrid disappearance. Most interestingly, the presence of minnows decreased the effect of sunfish on gerrid disappearance rates; that is, minnows apparently had an indirect positive effect on water strider survival. We suggest that this indirect positive effect reflects the fact that minnows are alternative prey for sunfish. The effects of sunfish and minnows on gerrid mortality explained the influence of these fish on gerrid behavior. Sunfish caused decreases in male gerrid activity, female availability, mating activity, mating frequency and mating duration. Larger males had a mating advantage over smaller males only in pools with sunfish and no minnows. Sunfish also caused a borderline significant decrease in the large female mating advantage. These results were all observed in previous studies and can be viewed as adaptive responses to predation risk. These patterns were not consistent with the expected effects of sunfish as food competitors with water striders. In contrast, minnows had relatively little influence on water strider behavior and the few significant effects were the opposite of those of sunfish. Minnows caused increases in female activity and in mating duration, a decrease in the large male mating advantage and an increase in the large female mating advantage. These patterns fit the view that minnows caused an increase in gerrid hunger, i.e., that minnows acted as food competitors with gerrids. Finally, planned contrasts against controls showed that, in the presence of both sunfish and minnows, water striders showed no significant behavioral responses to fish (i.e., gerrid behavior in pools with sunfish and minnows did not significantly differ from behavior in fishless pools). The most likely mechanism explaining this pattern is a dilution of sunfish predation risk due to the presence of minnows serving as alternative prey for sunfish.

Evidence for competition between carnivorous plants and spiders

Several studies have demonstrated that competition between disparate taxa can be important in determining community structure, yet surprisingly, to our knowledge, no quantitative studies have been conducted on competition between carnivorous plants and animals. To examine potential competition between these taxa, we studied dietary and microhabitat overlap between pink sundews (Drosera capillaris) and wolf spiders (Lycosidae) in the field, and conducted a laboratory experiment examining the effects of wolf spiders on sundew fitness. In the field, we found that sundews and spiders had a high dietary overlap with each other and with the available arthropod prey. Associations between sundews and spiders depended on spatial scale: both sundews and spiders were found more frequently in quadrats with more abundant prey, but within quadrats, spiders constructed larger webs and located them further away from sundews as the total sundew trapping area increased, presumably to reduce competition. Spiders also constructed larger webs when fewer prey were available. In the laboratory, our experiment revealed that spiders can significantly reduce sundew fitness. Our findings suggest that members of the plant and animal kingdoms can and do compete.

Invasion of the Meadow Vole (Microtus pennsylvanicus) in Southeastern Kentucky and Its Possible Impact on the Southern Bog Lemming (Synaptomys cooperi)

-Robinson Forest is a 4450-ha eastern deciduous forest located in southeastern Kentucky on the Cumberland Plateau. Historically, the southern bog lemming (Synaptomys coopen) was the dominant microtine rodent in forest clearings. However, within the last 15 yr, the meadow vole (Microtus pennsylvanicus) has expanded its range southward into southeastern Kentucky and has invaded clearings in Robinson Forest. Recent trapping efforts revealed that M. pennsylvanicus was the most abundant microtine in clearings, accounting for 48% of all small mammals caught. It invaded an isolated, grass-dominated clearing within 3 yr of tree removal. By contrast, S. cooperi was the most rarely caught small mammal in these clearings and tended to occupy herb-dominated clearings where fewer M. pennsylvanicus occurred. Synaptomys cooperi has become rarer since the arrival of M. pennsylvanicus, possibly due to competitive exclusion. Extensive deforestation, coupled with a 50-yr period of climatic cooling, may have led to the range expansion of M. pennsylvanicus.

How likely is male mate choice among anurans

Male mate choice has received considerable attention in recent years. This phenomenon rarely has been demonstrated in anuran mating systems, nonetheless, male choice has been suggested as having a significant impact on some anuran pairing patterns. Here I present a quantitative model that predicts the potential and prevalence of male mate choice in nine anuran species with data available in the literature. Males of only four of the nine species (Bufo americanus, B. bufo, B. exsul, and Rana sylvatica) are predicted to reject females under Normal conditions; in those cases only the smallest 1% to 5% of the females, respectively, should be rejected. By contrast, male Bufo conorus, B. cognatus, Hyla rosenbergi, Physalaemus pustulosus, and Rana catesbeiana are not expected to reject females of any size. Not until operational sex ratios became lm: lf and search times were reduced to 30%, on average, of normal values will reject and accept strategies become equally prevalent. Thus for these nine anuran species, the quantitative model presented here elucidates why the extent of male mate choice may be very limited, and why it is not expected to exert a strong influence on mating dynamics or pairing patterns.