20 research outputs found
False holes as camouflage
Long noted by naturalists, leaf mimicry provides some of the most impressive examples of camouflage through masquerade. Many species of leaf-mimicking Lepidoptera also sport wing markings that closely resemble irregularly shaped holes caused by decay or insect damage. Despite proposals that such markings can either enhance resemblance to damaged leaves or act to disrupt surface appearance through false depth cues, to our knowledge, no attempt has been made to establish exactly how these markings function, or even whether they confer a survival benefit to prey. Here, in two field experiments using artificial butterfly-like targets, we show that false hole markings provide significant survival benefits against avian predation. Furthermore, in a computer-based visual search experiment, we demonstrate that detection of such targets by humans is impeded in a similar fashion. Equally contrasting light marks do not have the same effect; indeed, they lead to increased detection. We conclude that the mechanism is the disruption of the otherwise homogeneous wing surface (surface disruptive camouflage) and that, by resembling the holes sometimes found in real leaves, the disruptive benefits are not offset by conspicuousness costs.Funding provided by: Engineering and Physical Sciences Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000266Award Number: EP/M006905/
Iridescence impairs object recognition in bumblebees
AbstractIridescence is a taxonomically widespread and striking form of animal coloration, yet despite advances in understanding its mechanism, its function and adaptive value are poorly understood. We test a counterintuitive hypothesis about the function of iridescence: that it can act as camouflage through interference with object recognition. Using an established insect visual model (Bombus terrestris), we demonstrate that both diffraction grating and multilayer iridescence impair shape recognition (although not the more subtle form of diffraction grating seen in some flowers), supporting the idea that both strategies can be effective means of camouflage. We conclude that iridescence produces visual signals that can confuse potential predators, and this might explain the high frequency of iridescence in many animal taxa.</jats:p
Raw data - Kjernsmo et al 2018
Raw data from Experiment 1 and 2. Contains attack latency and side of prey attacked for each fish and trial. L = large spot, S = small spot, HC = High contrast and LC = Low contrast
Anti-predator adaptations in aquatic environments
Predation is an important selective force that has led to the
evolution of a variety of fascinating anti-predator adaptations, such
as many types of protective coloration and prey behaviours.
Because the evolution of life has begun in the aquatic environment
and many anti-predator adaptations are found already in relative
primitive taxa, it is likely that many of these adaptations evolved
initially in the aquatic environment. Yet, there has been
surprisingly little research on the mechanisms and function of antipredator
adaptations in aquatic systems. To understand the
function of anti-predator adaptations and natural selection
imposed on prey appearance and behaviour, I have investigated
how protective coloration can be used, either as such or together
with behavioural adaptations, to manipulate predator behaviour
and decrease predation risk. To this end I conducted a series of
behaviour ecological laboratory experiments in which I
manipulated the visual appearance of artificial backgrounds and
prey items.
In paper I of this thesis, I investigated background choice as an
anti-predator strategy, by observing the habitat choice of the least
killifish (Heterandria formosa) between pairs of artificial
backgrounds, both in the presence and absence of predation threat.
It has been suggested that prey could decrease their risk of being
detected by predators either by preferring backgrounds into which
they blend or by preferring visually complex backgrounds. The
least killifish preferred a background that matched their patterning
to a background that mismatched it, showing that they are able to
respond to cues of visual similarity between their colour pattern
and the surrounding environment. Interestingly however, in female
least killifish visual complexity of the background was a more
important cue for habitat safety and may override or act together
with background matching when searching for a safe habitat. It is
possible that in females, preference for visually complex
backgrounds is associated with lower opportunity costs than
preference for matching backgrounds would be. Generally, the
least killifish showed stronger preference while under predation
threat, indicating that their background choice behaviour is an antipredator
adaptation.
Many aquatic prey species have eyespots, which are colour
patterns that consist of roughly concentric rings and have received
their name because they for humans often resemble the vertebrate
eye. I investigated the anti-predator function of eyespots against
predation by fish in papers II, III and IV. Some eyespots have been
suggested to benefit prey by diverting the strikes of predators away
from vital parts of the prey body or towards a direction that
facilitates prey escape. Although proposed over a century ago, the
divertive effect of eyespots has proven to be difficult to show
experimentally. In papers II and III, I tested for divertive effect of
eyespots towards attacking fish by presenting artificial prey with
eyespots to laboratory reared three-spined sticklebacks
(Gasterosteus aculeatus). I found that eyespots strongly influenced
the behaviour of attacking sticklebacks and effectively drew their
strikes towards the eyespots. To further investigate this divertive
effect and whether the specific shape of eyespots is important for it,
I tested in paper III the response of fish also to other markings than
eyespots. I found that eyespots were generally more effective in
diverting the first strikes of attacking fish compared to other prey
markings. My findings suggest that the common occurrence of
eyespots in aquatic prey species can at least partly be explained by
the divertive effect of the eyespot shape, possibly together with the
relative simple developmental mechanisms underlying circular
colour patterns.
An eyebar is a stripe that runs through the eye, and this pattern
has been suggested to obscure the real eyes of the prey by visually
blending parts of the eyes and head of the prey and by creating
false edges. In paper III, I show that an eyebar effectively disrupts
an eyelike shape. This suggests that eyebars provide an effective
way to conceal the eyes and consequently obstruct detection and
recognition of prey. This experiment also demonstrates that
through concealment of the eyes, eyebars could be used to enhance
the divertive effect of eyespots, which can explain the common
occurrence of eyebars in many species of fish that have eyespots.
Larger eyespots have been shown to intimidate some terrestrial
predators, such as passerine birds, either because they resemble the
eyes of the predator’s own enemy or because highly salient features
may have an intimidating effect. In papers II and IV, I investigated
whether the occurrence of eyespots in some aquatic prey could be
explained by their intimidating effect predatory fish. In paper IV, I
also investigated the reason for the intimidating effect of eyelike
prey marks. In paper II, I found no clear intimidating effect of
eyespots, whereas in paper IV, using a different approach, I found
that sticklebacks hesitated to attack towards eyelike but not
towards non-eyelike marks. Importantly, paper IV therefore
presents the first rigorous evidence for the idea that eye mimicry,
and not merely conspicuousness, underlies the intimidating effect.
It also showed that the hesitation shown by fish towards eyelike
marks is partly an innate response that is reinforced by encounters
with predators.
Collectively, this thesis shows that prey colour pattern and the
visual appearance of the habitat influence the behaviour of fish.
The results demonstrate that protective coloration provides
numerous distinctive ways for aquatic prey to escape predation.
Thus, visual perception and behaviour of fish are important factors
shaping the appearance and behaviours of aquatic prey
Data from: Resemblance to the enemy's eyes underlies the intimidating effect of eyespots
Eyespots of some prey are known to deter predators, but the reason for this response has not yet been established, and thus the taxonomically widespread occurrence of this color pattern has remained an evolutionary conundrum. Two alternative hypotheses propose that (1) the eyelike appearance of the pattern falsely indicates the presence of the predator’s own enemy or (2) predators are hardwired to be cautious toward conspicuous prey. Earlier research has pertained mainly to eyespots in butterflies. Here we tested the hypothesis that eyespots resemble eyes by utilizing the lateral position of eyes in fishes. This allowed us to produce eyelike displays that did not have the round appearance of eyespots. Our study indicates that eye mimicry is an important factor evoking hesitation in predators. Moreover, we present direct evidence that this is because predators associate eyelike displays with the threat posed by their own enemies