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Quantifying the effects of species traits on predation risk in nature: a comparative study of butterfly wing damage

Citation

Molleman, Freerk et al. (2020), Quantifying the effects of species traits on predation risk in nature: a comparative study of butterfly wing damage, Dryad, Dataset, https://doi.org/10.5061/dryad.r2280gb8j

Abstract

1) Evading predators is a fundamental aspect of the ecology and evolution of all prey animals. In studying the influence of prey traits on predation risk, previous researchers have shown that crypsis reduces attack rates on resting prey, predation risk increases with increased prey activity, and rapid locomotion reduces attack rates and increases chances of surviving predator attacks. However, evidence for these conclusions is nearly always based on observations of selected species under artificial conditions. In nature, it remains unclear how defensive traits such as crypsis, activity levels, and speed influence realized predation risk across species in a community. Whereas direct observations of predator-prey interactions in nature are rare, insight can be gained by quantifying bodily damage caused by failed predator attacks. 2) We quantified how butterfly species traits affect predation risk in nature by determining how defensive traits correlate with wing damage caused by failed predation attempts, thereby providing the first robust multi-species comparative analysis of predator-induced bodily damage in wild animals. 3) For 34 species of fruit-feeding butterflies in an African forest, we recorded wing damage and quantified crypsis, activity levels, and flight speed. We then tested for correlations between damage parameters and species traits using comparative methods that account for measurement error. 4) We detected considerable differences in the extent, location, and symmetry of wing surface loss among species, with smaller differences between sexes. We found that males (but not females) of species that flew faster had substantially less wing surface loss. However, we found no correlation between cryptic colouration and symmetrical wing surface loss across species. In species in which males appeared to be more active than females, males had a lower proportion of symmetrical wing surface loss than females. 5) Our results provide evidence that activity greatly influences the probability of attacks and that flying rapidly is effective for escaping pursuing predators in the wild, but we did not find evidence that cryptic species are less likely to be attacked while at rest. 15-Oct-2019

Methods

This study was conducted near the Makerere University Biological Field Station in Kibale National Park, Western Uganda. Butterflies were captured in fruit-baited traps in two areas with selectively logged sub-montane tropical forest (Lowercamp and K31) and a forest regeneration site (Mikana). We used 22 trap locations in Lowercamp (Molleman et al. 2006), and 40 trap locations in the understory of forest compartment K31, and six in the Mikana area. In K31, traps were baited once each a week from January 2006 until February 2007, and butterflies were scored, marked, and released on four consecutive days between 10:00 and 16:00, replacing bait only when it was lost. In Lowercamp and Mikana, trapping was performed once every 4 weeks from May 2006 to June 2012. Since the traps accumulated butterflies over 24-hour time periods, any differences in diurnal activity could not bias trap catches.

In forest compartment K31, 34 species of fruit-feeding butterflies were included to capture as much diversity in terms of phylogeny and putative defensive tactics, as possible. In Lowercamp and Mikana, we focused on three butterfly species: Euphaedra medon (L.), E. alacris Hecq and Charaxes fulvescens Aurivillius in order to obtain large sample sizes for selected species. We focused on medium to large bodied species that are less likely damaged by handling. All included species hold their wings closed over their back while at rest and are thus expected to show symmetrical wing damage if they were attacked while at rest, although the Adoliadini and Cymothoe species hold their wings open during sun basking and can open their wings during feeding (FM pers. sobs.).

 

Scoring damage

Focal species were carefully removed from baited traps by hand. To avoid pseudo-replication, butterflies were marked with a unique number before release. Most individuals were captured only once (the proportion of captures that were recaptured is given in Table B3 and the frequency of recaptures in Table B4 of Appendix B). We visually estimated the proportion of wing surface missing on each wing as well as the percentage of scale loss of all wings taken together. Any entire number could be noted, albeit obviously a difference of 1% would not be interpretable. We compared estimates of wing surface loss with detailed drawings of the wing surfaces of 538 of the included specimens and corrected systematic biases accordingly (e.g. overestimation of minor damage, underestimation of severe damage: Online Appendix B). We also counted the number of tears (ripped wings without surface loss) in the wings (Fig. 1). To gauge the realized repeatability of estimates of butterfly wing damage in this study, we took data from individual butterflies that were captured and recaptured at most one day apart (estimates often made by different observers), and determined the correlation between the two estimates of wing damage. Since the butterflies could have incurred new wing damage during this one day, it is likely that we slightly underestimate repeatability. Across 1100 instances of individuals that were captured on two consecutive days, the correlation coefficient of wing surface loss was 0.74 on average, wing tears 0.53 and scale loss 0.98. We note that stronger correlation, i.e. reproducibility, did not correspond to stronger statistical signal in the later tests of our hypotheses (Tab. 1). We calculated the degree to which wing surface loss was biased toward forewings as the damage to forewings minus that in hindwings, divided by the total wing surface loss; such that this variable had positive values when wing surface loss was biased towards forewings, and negative values when biased towards hindwings. For each pair of wings, we scored whether any of the surface missing was symmetrical (i.e. the surface loss on left and right wings represented a mirror image of each other). Even when some of the wing surface loss had a symmetrical shape across wing pairs, the extent of wing surface loss of wings in a wing-pair often differed between the two wings, because there was additional non-symmetrical wing surface loss.

We attempted to avoid damage due to handling by focusing on species of large body size (forewing length over 2.8 cm.), and by working with local field assistants with several years of experience in handling butterflies. Fingerprints on butterfly wings are readily recognizable and were ignored when scoring butterflies. We noted if a specimen was damaged during handling, and excluded any subsequent recaptures of these individuals from the analyses.

 

Wing length

Forewing measurements were made using callipers at the study site for the 34 species, represented by 12,271 live individuals that were not included in the study of damage (separate data set).

 

Flight speed 

Flight speed was measured in a 3 m long tunnel. A house at the field station was darkened except for one exterior door that was left open, and the doorframe was covered with white mesh, providing a light target to butterflies. Butterflies were taken from baited traps in the morning during a four-month period, provided water and mashed banana and used during the afternoon between 13:00 and 16:00 of the same day for flight speed measurements. Therefore, the ambient temperature was roughly the same for all trials, ranging between 20.5 and 25 ̊C. Butterflies were individually released 1 m from the floor and 4 m from the open door, oriented towards the open door. Butterfly flight away from a human experimenter is likely escape behaviour, thus we presume that butterflies were displaying escape flight tactics and were ostensibly maximizing their speed. The time they took to reach the mesh covering the open door was recorded, and flights that were not straight towards the target door were excluded from analyses.

Funding

National Institute on Aging, Award: PO1 AG022500-01

National Institute on Aging, Award: PO1 AG608761-10

Bixby International Travel Grant

US National Science Foundation, Award: 1309425

Estonian Science Foundation, Award: 9215

Estonian Science Foundation, Award: IUT20-33

European Regional Development Fund, Award: FIBIR

Région Bretagne, Award: SAD

Région Bretagne, Award: ACOMB

Centre National de la Recherche Scientifique, Award: ATIP

Centre National de la Recherche Scientifique, Award: ATIP

Department of Science and Technology, Government of India, Award: DST/INSPIRE/04/2013/000476