Visual complexity of egg patterns predicts egg rejection according to Weber's law

Abstract

Visual complexity is ubiquitous in nature. Drivers of complexity include selection in coevolutionary arms races between antagonists. However, the causes and consequences of biological complexity and its perception are largely understudied, partly because complexity is difficult to quantify. Here, we address this by studying egg pattern complexity and its perception in hosts (tawny-flanked prinia Prinia subflava ), which visually recognize and reject mimetic eggs of their virulent brood parasite (cuckoo finch Anomalospiza imberbis ). Using field data and an optimization algorithm, we compute a complexity metric which predicts rejection of experimentally placed conspecific eggs in prinia nests. Real cuckoo finch eggs exhibit significantly lower pattern complexity than prinia eggs, suggesting that high complexity benefits hosts because it distinguishes host eggs from parasitic eggs. We show that prinias perceive complexity differences according to Weber's law of proportional processing (i.e. relative, rather than absolute, differences between stimuli are processed in discrimination, such that two eggs with simple patterns are more easily discriminable than two with complex patterns). This may influence coevolutionary trajectories of hosts and parasites. The new methods presented for quantifying complexity and its perception can help us to understand selection pressures driving the evolution of complexity and its consequences for species interactions.

Combined measures of mimetic fidelity explain imperfect mimicry in a brood parasite–host system

“…pattern connectivity. This result was surprising since another measure of number of markings (the number of SIFT features, extracted using NaturePatternMatch [ 16 ]), shows the opposite relationship: prinia eggs have more SIFT features than cuckoo finch eggs [ 13 ]. This apparent discrepancy may also be due to scribbles: one scribble often corresponds to multiple SIFT features, due to large changes in orientation of some SIFT features.…”

“…We extracted measurements of colour and pattern traits from a representative egg (prinia n = 215; cuckoo finch n = 123) from clutches laid during 2018–2020 (alongside cuckoo finch eggs laid in 2012–2016), as described in [ 13 ]. Colour measures were extracted as in [ 7 ]; briefly, reflectance spectra were obtained using an Ocean Optics USB2000 Spectrophotometer, with a PX-2 pulsed xenon light source and an R400-7-UV/VIS reflectance probe, standardized with a Spectralon 99% White Standard (Labsphere) and a black felt cloth.…”

“…Measures of principal marking size, marking size variation, pattern contrast, pattern coverage, and the extent to which pattern is dispersed between blunt and narrow poles of the egg were extracted using granularity analysis in MATLAB [ 8 ] and adaptive thresholding in the MICA toolbox in ImageJ [ 15 ], as described in [ 13 ].…”

“…To determine whether the presence of scribbles affected host responses, we conducted experiments ( n = 44) largely as in [ 13 ]. Briefly, one egg from a completed host clutch was replaced with an ‘experimental' egg, with the nest observed for 4 days to determine whether the replacement egg was rejected [ 8 , 13 ]. Hosts were given close colour matches to human vision (cf.…”

“…Unlike [ 13 ], real parasitic eggs (taken from other prinia nests) rather than conspecific eggs were used as experimental eggs. The test treatment ( n = 17) was five scribbles painted on a real cuckoo finch egg; at least a part of one scribble could be seen by viewing any angle of the egg.…”