Ola M. Fincke

Predation by odonates depresses mosquito abundance in water-filled tree holes in Panama

Abstract In the lowland moist forest of Barro Colorado Island (BCI), Panama, larvae of four common species of odonates, a mosquito, and a tadpole are the major predators in water-filled tree holes. Mosquito larvae are their most common prey. Holes colonized naturally by predators and prey had lower densities of mosquitoes if odonates were present than if they were absent. Using artificial tree holes placed in the field, we tested the effects of odonates on their mosquito prey while controlling for the quantity and species of predator, hole volume, and nutrient input. In large and small holes with low nutrient input, odonates depressed the number of mosquitoes present and the number that survived to pupation. Increasing nutrient input (and consequently, mosquito abundance) to abnormally high levels dampened the effect of predation when odonates were relatively small. However, the predators grew faster with higher nutrients, and large larvae in all three genera reduced the number of mosquitoes surviving to pupation, even though the abundance of mosquito larvae remained high. Size-selective predation by the odonates is a likely explanation for this result; large mosquito larvae were less abundant in the predator treatment than in the controls. Because species assemblages were similar between natural and artificial tree holes, our results suggest that odonates are keystone species in tree holes on BCI, where they are the most common large predators.

Lifetime reproductive success and the opportunity for selection in a nonterritorial damselfly (Odonata: Coenagrionidae)

Major components of male and female lifetime reproductive success (LRS) were quantified for a damselfly that exhibits “scramble competition” for mates. The opportunity for selection on male reproduction was potentially 2.9 times that for females. Differential fertility/clutch and survivorship each accounted for about half of the total variation in female reproductive success. Variation in fertilization efficiency accounted for 7% of the total opportunity for selection on males. Although differences in survivorship and mating efficiency each contributed to about a third of the total opportunity for selection on male reproduction, both components appeared to be influenced by random factors. Survivorship was age‐independent, and the mating distributions among males with equal mating opportunities were indistinguishable from those expected if matings were random with respect to male phenotype. Because the proportion of the standarized variance (I) in LRS that was attributed to sexual selection depended on the way the selective episodes were defined, the sample of individuals included in the partitioning analysis, and the degree of sexual selection on mated males that could be detected, my results caution against drawing conclusions about the dynamics of sexual selection on populations based on a superficial comparison of I values.

Polymorphic signals of harassed female odonates and the males that learn them support a novel frequency-dependent model

Abstract For mate-searching species, the learned mate recognition (LMR) hypothesis assumes that sexual harassment favours signal variation among females, which exploits the receiver ability of males. The model predicts that coevolving males have responded to the female sexual foil by learning to recognize female variants as potential mates. I translate the LMR hypothesis into the language of signal detection theory to explain its novelty as a dynamic, coevolutionary, negative frequency-dependent selection model. Due to gene–environment interactions, males cueing to the morph detected most often should generate positive but often asymmetrical, detection-dependent harassment towards females. Females are expected to sort to an ideal free distribution where harassment costs are equal. At equilibrium, morph fitness, but not necessarily morph frequency, is predicted to be equal. The LMR hypothesis is consistent with recent experimental data and the distribution of colour polymorphisms in the Odonata, predicts general conditions favouring variation in sexual signals, and provides a novel mechanism for speciation via sexual signalling.

Organization of predator assemblages in Neotropical tree holes: effects of abiotic factors and priority

Abstract. 1. Water‐filled tree holes in a lowland forest in Panama harbour an assemblage of large predators consisting of the larvae of five common species of Odonata, the mosquito Toxorhynchites theobaldi, and tadpoles of Dendrobates auratus. Odonate females oviposit in both large and small tree holes. However, the three largest species emerge from larger tree holes, on average, than do the two smallest species. Can assembly rules explain this and other patterns of predator distribution?

Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies

Insect mate recognition is often viewed as stereotypic, innate, and species-specific. However, male damselflies can learn to identify female-specific color morphs as potential mates. A suite of male mimicry hypotheses assume that heteromorphic females, which differ from males in color pattern, are more easily recognized as “female” and thus lack the inherent, anti-harassment advantage that the more male-like signal provides for andromorphs. Using two measures of male preference, we investigated whether naïve males have a preexisting sensory bias for a given morph color in Enallagma civile, a species that appeared to exhibit extreme plasticity in morph expression across generations within a breeding season. E. civile males raised in the absence of females exhibited no preference for either morph, whereas males raised with one female type exhibited a learned sensory bias for that morph. Male Enallagma also lacked a bias toward conspecific females over a congeneric sister species. In a naturally naïve population of Enallagma ebrium, males reacted sexually to both morphs of Enallagma hageni as often as they did to conspecific females, whose thoracic spectra were nearly identical with those of E. hageni. Moreover, despite the similar thoracic spectra of males and andromorphs, both of which reflected UV, males rarely reacted sexually to other males. Our results falsified implicit assumptions of male mimicry hypotheses, supported learned mate recognition, and suggested a scenario for speciation via sexual conflict.

The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?

Abstract We compiled data on the occurrence and frequency of distinct female variants among Holarctic Odonata and interpreted the data in light of harassment-based hypotheses. The major source of male confusion for male mimicry hypotheses is predicted to be signal similarity between andromorphs and male distractors; for the learned mate recognition hypothesis (LMR), it is predicted to be variation in female signals. Mapping morphism state onto molecular phylogenies of Ischnura and Enallagma failed to resolve the general ancestral female condition. However, it appeared that the andromorphic state may be ancestral in one case, and that blue structural colors were ancestral to orange and green pigmentations. Of the polymorphic species surveyed, 13% had more than two morphs, 4% had multiple heteromorphs but no andromorph, and 7% of ‘monomorphic’ congeners were functionally polymorphic because developmental variants mate. Such female signal variation lies beyond the scope of simple male mimicry, but nevertheless should exacerbate a males problem in searching for mates. Andromorphs were the majority morph in at least some populations of 17% of the species for which data were available. Andromorph frequencies of Enallagma species were generally higher than in Ischnura species, as expected if Ischnura andromorphs have higher signal apparency. Andromorph frequency varied significantly across habitats and species, as expected if per capita harassment and signal apparency vary among habitats. Quantification of signal apparency and per capita harassment across populations and among species is required to more rigorously test the extent to which variation in signal crypsis can explain observed variation in morph frequencies.

Tests of the harassment-reduction function and frequency-dependent maintenance of a female-specific color polymorphism in a damselfly

Color polymorphisms have provided classical examples of how frequency-dependent selection maintains genetic variation in natural populations. Here we tested for the first time, the hypothesized adaptive function of a female-specific color polymorphism in odonates to lower male harassment towards females generally. Under conditions controlling for sex ratio, population density and morph frequency, we also tested two major frequency-dependent selection hypotheses for the maintenance of the polymorphism. Using groups of captive Enallagma hageni, whose females are either green or a male-like blue, we varied morph frequency at two sex ratios. We quantified sexual harassment towards females by visual observations, and by the presence of dust on females that was transferred from dusted males. Per capita harassment rate for the female-monomorphic treatments did not differ from that of the female-polymorphic treatments. At a male-biased sex ratio, per capita harassment rate towards blue, but not green females increased with morph frequency, providing partial support for frequency-dependent selection resulting from male learning of female morphs. Even at high frequency, green females were not harassed more than blue, contrary to the prediction that males should always recognize green females as mates. Moreover, frequency-dependent harassment towards blue females was not detectable using harassment measured with dust evidence, which greatly underestimated the incidence of sexual harassment. Our findings identified problems with the use of insectaries and the dusting technique to quantify male sexual harassment towards females, as well as with a past insectary experiment on Ischnura elegans that failed to demonstrate frequency-dependent harassment.

Tree hole odonates as environmental monitors: Non-invasive isolation of polymorphic microsatellites from the neotropical damselflyMegaloprepus caerulatus

Because of their complex mating behaviour and life cycle (alternating aquatic and terrestrial stages) odonates provide important model systems for environmental monitoring, evolutionary ecology, and conservation genetics. Many odonate species are endangered and call for the use of non-invasive molecular studies. In the neotropical damselfly Megaloprepus caerulatus we have identified polymorphic microsatellite loci by means of the randomly amplified microsatellite technique (RAMS; Ender et al. 1996). Using the DNA from each a single leg of three unrelated individuals we screened 63 RAPD primers for small size banding patterns. A total of 95 RAPD profiles was hybridized with digoxigenin labelled di- and trinucleotide repeats (GAn ,G T n ,C A n and AATn) and 36 RAPD fragments harbouring microsatellite motifs were isolated. Cloning and sequencing of positive fragments revealed five polymorphic microsatellite loci. Since Megaloprepus caerulatus is a viable bio-indicator for primary rainforests the microsatellite system can be used to study the effects of forest fragmenation on population viability.

Lost in the crowd or hidden in the grass: signal apparency of female polymorphic damselflies in alternative habitats

Animals must locate prey and mates in noisy sensory environments. Species that rely on visual cues, and which are prey of visual predators, consequently face trade-offs. Additionally, within species, sexual conflict over mating may impose pressures to avoid both predators and mates. Many studies have attempted to explain female-specific polymorphisms in damselflies, but without considering their actual conspicuousness under natural conditions. Using models of colour perception for damselflies and birds, we assessed the detectability of female coloration to conspecific males and potential predators. Alternative colour morphs reduce female apparency either through signal similarity with conspecific males (i.e. mimicry) or by matching the noise of the visual background. The colours of male-mimicking andromorphs that reduce their apparency among groups of males at breeding sites render them highly detectable to males as well as visual predators in offshore vegetation, where females occur when not reproducing. By presenting tethered female damselflies to free-flying males amidst vegetation, we demonstrated that, among flying females, males were able to detect andromorphs more easily than the more cryptic heteromorphs. Thus, when male density is low, cryptic heteromorphs may experience less harassment than andromorphs, suggesting a scenario of disruptive selection on female coloration driven by males as well as predators. Greater attention is warranted not only to the predation risks of female signals, but also to the effect of variation in the visual environments on encounters between males and unreceptive females.

Mistakes in sexual recognition among sympatric Zygoptera vary with time of day and color morphism (Odonata: Coenagrionidae)

Abstract In odonates, female specific color polymorphisms appear to be an evolutionary response to sexual harassment, but we know little about the decision rules males use when searching for variable females. For two sympatric species of Enallagma, we measured male responses to live female variants under field conditions, early and later in the day. In the morning, when the operational sex ratio was the most male-biased and female density the lowest, males of the polymorphic E. civile did not discriminate among conspecific female morphs, and reacted sexually to the andromorphic females of E. aspersum, a monomorphic species. ‘Then, male E. aspersum did not favor conspecific females over E. civile morphs. Both morph types were more confusing for males than were conspecific male signals. However, after 13:00 h, males of both species made few mistakes, and E. civile males reacted sexually relatively less often to conspecific andromorphs, the minority morph in this population. The changes in a males sexual response suggested that they cued to female-specific traits when females were scarce, increasing their detection of potential mates at the expense of making mistakes with heterospecific females. When females of both species were more abundant, a males behavior was consistent with cueing to morph-specific features. Analyses of comparative data suggested that for several genera, males of polymorphic species were more likely to mistake heterospecific females as mates than males of monomorphic congeners. Our results best support the learned mate recognition hypothesis for the evolution and maintenance of female-specific polymorphisms.