Masquerading predators deceive prey by aggressively mimicking bird droppings in a crab spider

Abstract

In aggressive mimicry, a predator accesses prey by mimicking the appearance and/or behavior of a harmless or beneficial model in order to avoid being correctly identified by its prey. The crab spider genus Phrynarachne is often cited as a textbook example of masquerading as bird droppings in order to avoid predation. However, Phrynarachne spiders may also aggressively mimic bird droppings in order to deceive potential prey. To date, there is no experimental evidence to support aggressive mimicry in masquerading crab spiders, therefore, we performed a field survey, a manipulative field experiment, and visual modeling to test this hypothesis using Phrynarachne ceylonica. We compared prey-attraction rates among bird droppings, spiders, and control empty leaves in the field. We found that although all prey combined and agromyzid dipterans in particular were attracted to bird droppings at a higher rate than to spiders, other dipterans and hymenopterans were attracted to bird droppings at a similar rate as spiders. Both spiders and bird droppings attracted insects at a significantly higher rate than did control leaves. As predicted, prey were attracted to experimentally blackened or whitened spiders significantly less frequently than to unmanipulated spiders. Finally, visual modeling suggested that spiders and bird droppings can be detected by dipterans and hymenopterans against background leaves, but they are indistinguishable from each other. Taken together, our results suggest that insects lured by spiders may misidentify them as bird droppings, and bird dropping masquerading may serve as aggressive mimicry in addition to predator avoidance in P. ceylonica.

From crypsis to masquerade: Ontogeny changes the colour defences of a crab spider hiding as bird droppings

“…To defend themselves as they grow, masquerading as bird dropping may be a better strategy for defending against predators because chicks hesitated much longer and were less likely to attack individuals resembling bird droppings in comparison to dark individuals when they were large, even though bird dropping-like individuals were more readily detected. Together, pressure from predators (this study) and from prey-luring (Yu et al, 2021) acts simultaneously to select for masquerade as bird droppings in P. ceylonica spiders when they increase in size.…”

“…), Napo County, Baise City, Guangxi Province, China. Maintenance procedures and cage design were used as in our earlier studies (Li et al., 2008; Yu et al., 2021), so we only provide essential details on the present study. Spiders were reared individually in transparent cylindrical cages (diameter × height: 6.5 × 8 cm) under controlled environmental conditions (relative humidity: 80 − 85%; temperature: 25 ± 1°C; light regime: 12 hr:12 hr; lights on at 08:00 hr).…”

“…We measured spectral reflectance of two types of patches: dark patches (on the carapace and abdomen) and the leg I femur (with the naked eye, at the 3rd instar this appears dark, while from the 4th instar to sub‐adult females, this appears white), for each of 10 individuals in each life stage. We used an Ocean Optic USB4000 spectrometer with a DH‐2000 deuterium and tungsten halogen light source (Ocean Optics Inc.) in a dark room following a similar protocol described in previous studies (Yu et al., 2021). We randomly selected five spots on each colour patch or body part of each individual and recorded the reflectance reading (300–700 nm) at each spot of the spider perpendicular to and 2 mm above the sample.…”

“…We randomly selected five spots on each colour patch or body part of each individual and recorded the reflectance reading (300–700 nm) at each spot of the spider perpendicular to and 2 mm above the sample. We used previously published data on the spectral reflectance of 10 adult P. ceylonica females and of 10 bird droppings produced by the light‐vented bulbuls Pycnonotus sinensis that were commonly found and also observed on the same substrate at the study site (Yu et al., 2021). We also measured the spectral reflectance of 10 adult males.…”

“…We also measured the spectral reflectance of 10 adult males. For incorporation into physiological visual models (see below), we used previously published spectral reflectance data of 10 leaves of elephant ear taro Alocasia macrorrhiza (five readings for each leaf, thus 50 readings in total), a plant where P. ceylonica spiders are commonly found (Yu et al., 2021). We averaged these 50 readings and used the mean as the background reflectance spectrum.…”