Sexual dimorphism in the alpine butterflies Boloria pales and Boloria napaea: differences in movement and foraging behavior (Lepidoptera: Nymphalidae)

Sexual dimorphism is a widespread phenomenon in Lepidoptera. It is reflected in differences in life history, behavior and morphology. Analyses of differences in behavior are mostly difficult and time-consuming, especially in high mountain ecosystems. To enhance our knowledge on sexual dimorphisms of alpine butterflies, we performed a mark-release-recapture study on 2 species common in the Alps: Boloria pales and Boloria napaea. We analysed movement and foraging behavior to investigate differences between sexes. Both sexes were mostly sedentary and the movement distances of males and females similar. However, obvious differences in dispersal behavior between the sexes were found in the movement patterns. Three different patterns were distinguished. Most males showed intensive flight activity, but mostly flew only in a limited part of the entire habitat (i.e., their individual home range) searching for females, whereas females were less flight active and flew only to find places for oviposition or feeding. The third pattern, where individuals flew larger distances, was only observed in a small number of males, which always returned to their home range. Nearly all feeding was observed on Asteraceae. However, males preferred the genera Leontodon and Crepis, while females preferred Leontodon and Carduus. Apart from this sexual difference in foraging, individuals of both sexes were found to be more or less specialised on nectar sources. Flight activity was generally greater in males than females. Therefore, we think that sex-specific requirements in nectar ingredients exist, that is, sugar for the intensive flight activity of males and amino acids for egg production of females.

Specialized or opportunistic-how does the high mountain endemic butterfly Erebia nivalis survive in its extreme habitats?

High mountain ecosystems are a challenge for the survival of animal and plant species, which have to evolve specific adaptations to cope with the prevailing extreme conditions. The strategies to survive may reach from opportunistic to highly adapted traits. One species successfully surviving under these conditions is the here studied butterfly Erebia nivalis. In a mark-release-recapture study performed in the Hohe Tauern National Park (Austria) from 22 July to 26 August 2013, we marked 1386 individuals and recaptured 342 of these. For each capture event, we recorded the exact point of capture and various other traits (wing conditions, behavior, nectar sources). The population showed a partial protandrous demography with the minority of males emerging prior to the females, but the majority being synchronized with them. Males and females differed significantly in their behavior with males being more flight active and females nectaring and resting more. Both sexes showed preferences for the same plant species as nectar sources, but this specialization apparently is the result of a rapid individual adaptation to the locally available flowers. Estimates of the realized dispersal distances predicted a comparatively high amount of long-distance flights, especially for females. Therefore, the adaptation of Erebia nivalis to the unpredictable high mountain conditions might be a mixture of opportunism and specialized traits.

The importance of trans-generational effects in Lepidoptera

The importance of trans-generational effects in shaping an individuals’ phenotype and fitness, and consequently even impacting population dynamics is increasingly apparent. Most of the research on trans-generational effects still focuses on plants, mammals, and birds. In the past few years, however, increasing number of studies, especially on maternal effects, have highlighted their importance also in many insect systems. Lepidoptera, specifically butterflies, have been used as model systems for studying the role of phenotypic plasticity within generations. As ectotherms, they are highly sensitive to environmental variation, and indeed many butterflies show adaptive phenotypic plasticity in response to environmental conditions. Here, we synthesize what is known about trans-generational effects in Lepidoptera, compile evidence for different environmental cues that are important drivers of trans-generational effects, and point out which offspring traits are mainly impacted. Finally, we emphasize directions for future research that are needed for better understanding of the adaptive nature of trans-generational effects in Lepidoptera in particular, but potentially also in other organisms.

Nutrient acquisition across a dietary shift: fruit feeding butterflies crave amino acids, nectivores seek salt

Evolutionary dietary shifts have major ecological consequences. One likely consequence is a change in nutrient limitation-some nutrients become more abundant in the diet, others become more scarce. Individuals' behavior should change accordingly to match this new limitation regime: they should seek out nutrients that are deficient in the new diet. We investigated the relationship between diet and responses to nutrients using adult Costa Rican butterflies with contrasting feeding habits, testing the hypothesis that animals will respond more positively to nutrients that are scarcer in their diets. Via literature searches and our own data, we showed that nitrogen and sodium are both at lower concentration in nectar than in fruit. We therefore assessed butterflies' acceptance of sodium and four nitrogenous compounds that ranged in complexity from inorganic nitrogen (ammonium chloride) to protein (albumin). We captured wild butterflies, offered them aqueous solutions of each substance, and recorded whether they accepted (drank) or rejected each substance. Support for our hypothesis was mixed. Across the sexes, frugivores were four times more likely to accept amino acids (hydrolyzed casein) than nectivores, in opposition to expectation. In males, nectivores accepted sodium almost three times more frequently than frugivores, supporting expectations. Together, these results suggest that in butterflies, becoming frugivorous is associated with an increased receptivity to amino acids and decreased receptivity to sodium. Nectivory and frugivory are widespread feeding strategies in organisms as diverse as insects, birds, and bats; our results suggest that these feeding strategies may put different pressures on how animals fulfill their nutritional requirements.

Amino acids in nectar enhance longevity of female Culex quinquefasciatus mosquitoes

Culex mosquitoes feed on a wide range of nectars consisting of mostly carbohydrates and amino acids, however, little is known about the utilization and effects of these different carbohydrates and their accompanying amino acids on longevity. Culex quinquefasciatus larvae were reared on low- and high-quantity food diets to produce adults that were nutritionally representative of wild-caught and laboratory-reared mosquitoes. Emerging adults reared on low- or high-quantity food diets as larvae were then provided Lantana camara nectar mimics containing mixtures of carbohydrates and amino acids to evaluate effects of nectar amino acids on longevity. Carbohydrates (with or without amino acids) were a critical component of the adult diet, and in their absence, adult mosquitoes died within 3-5 days. The nectar mimic that contained both carbohydrates and amino acids did not increase adult longevity of males originating from either poorly or well-fed larvae. However, females receiving adult diets containing both carbohydrates and amino acids lived 5% longer than females fed adult diets with only sugar.

Does dietary restriction reduce life span in male fruit-feeding butterflies?

Male life history and resource allocation is not frequently studied in aging and life span research. Here, we verify that males of long-lived fruit-feeding butterfly species have reduced longevity on restricted diets [Beck, J., 2007. The importance of amino acids in the adult diet of male tropical rainforest butterflies. Oecologia 151, 741-747], in contrast to the common finding of longevity extension in dietary restriction experiments in Drosophila and some other organisms. Males of some of the most long-lived species of fruit-feeding butterflies were collected from Kibale Forest, Uganda, and kept on diets of either sugar or mashed banana. Seven out of eight species had non-significantly longer life spans on mashed banana diets. Data analysis using a time-varying Cox-model with species as covariate showed that males had reduced survival on the sugar diet during the first 35 days of captive life, but the effect was absent or reversed at more advanced ages. These results challenge the generality of dietary restriction as a way to extend life span in animals. We argue that such studies on males are promising tools for better understanding life history evolution and aging because males display a wider variety of tactics for obtaining reproductive success than females.

Adult nutrition and butterfly fitness: effects of diet quality on reproductive output, egg composition, and egg hatching success

BACKGROUND

In the Lepidoptera it was historically believed that adult butterflies rely primarily on larval-derived nutrients for reproduction and somatic maintenance. However, recent studies highlight the complex interactions between storage reserves and adult income, and that the latter may contribute significantly to reproduction. Effects of adult diet were commonly assessed by determining the number and/or size of the eggs produced, whilst its consequences for egg composition and offspring viability were largely neglected (as is generally true for insects). We here specifically focus on these latter issues by using the fruit-feeding tropical butterfly Bicyclus anynana, which is highly dependent on adult-derived carbohydrates for reproduction.

RESULTS

Adult diet of female B. anynana had pronounced effects on fecundity, egg composition and egg hatching success, with butterflies feeding on the complex nutrition of banana fruit performing best. Adding vitamins and minerals to a sucrose-based diet increased fecundity, but not offspring viability. All other groups (plain sucrose solution, sucrose solution enriched with lipids or yeast) had a substantially lower fecundity and egg hatching success compared to the banana group. Differences were particularly pronounced later in life, presumably indicating the depletion of essential nutrients in sucrose-fed females. Effects of adult diet on egg composition were not straightforward, indicating complex interactions among specific compounds. There was some evidence that total egg energy and water content were related to hatching success, while egg protein, lipid, glycogen and free carbohydrate content did not seem to limit successful development.

CONCLUSION

The patterns shown here exemplify the complexity of reproductive resource allocation in B. anynana, and the need to consider egg composition and offspring viability when trying to estimate the effects of adult nutrition on fitness in this butterfly and other insects.

Effects of adult-derived carbohydrates, amino acids and micronutrients on female reproduction in a fruit-feeding butterfly

It is generally believed that butterflies (and other holometabolous insects) rely primarily on reserves accumulated during the larval stage for reproduction, whereas the carbohydrate-rich adult diet is thought to mainly cover energy requirements. In at least some species though, realization of the full reproductive potential is extensively affected by post-eclosion nutrition. While the importance of carbohydrates is fairly well understood, the role of adult-derived amino acids and micronutrients is controversial and largely unknown, respectively. We here focus on the effects of different adult diets on female reproduction in the tropical, fruit-feeding butterfly Bicyclus anynana (Nymphalidae). Carbohydrates were the most important adult-derived nutrients affecting reproduction. Adding amino acids, vitamins or minerals to sucrose-based solutions did not yield a reproductive output equivalent to that of fruit-fed females, which showed the highest performance throughout. This suggests that either not yet identified compounds of fruit substantially contribute to reproduction, or that resource congruence (the use of nutrient types in a specified ratio) rather than any specific nutrient component is of key importance. Apart from adult income, realized fecundity depended on egg size and longevity, with the former dominating when dietary quality was low, but the latter when quality was high. Thus, the egg size-number trade-off seems to be affected by female nutrition.

Amino acids in nectar enhance butterfly fecundity: a long-awaited link

Thirty years ago, researchers discovered that flowers pollinated by butterflies are consistently rich in nectar amino acids, and more recent findings have shown that butterflies prefer nectar with high amino acid content. These observations led to speculation that amino acids in nectar enhance butterfly fitness and that butterflies have acted as agents of natural selection on nectar composition. Despite a number of experimental efforts over the years, convincing proof that nectar amino acids affect butterfly fitness has been lacking. Here, we provide the first evidence that amino acids in nectar have a positive effect on fecundity of one butterfly species, supporting the existence of a relationship between nectar preferences and fitness benefits. Map butterflies (Araschnia levana L.) raised under natural larval food conditions laid more eggs when they were fed nectar containing amino acids, whereas nectar amino acids had no effect on the number of eggs laid by butterflies raised on larval food rich in nitrogen. Uptake and utilization of nectar amino acids by map butterflies appear to be compensatory mechanisms enabling them to override impacts of poor larval food. These results provide strong support for the long-standing postulate that nectar amino acids benefit butterflies.