Breaking up the Edge to Hide in Plain Sight

A guest post by Rich Webster

Whether hiding from Lions on the Serengeti or dodging bullets on the battlefield, avoiding unwanted eyes should be high up on your priorities!

In nature, animals have evolved different strategies to achieve concealment. Resembling the appearance of ones surroundings (background matching) can reduce the chance of being detected. Surprisingly background matching is not a golden bullet solution to camouflage. Even species with faithful background matching can suffer from having visible edges, due to a small mismatch between where the animal starts and the background ends. For instance, predators can use these visible edges as cue to recognise camouflage prey from there characteristic shape.

Alternatively, an animal can trick the eye-of-the-beholder into seeing but failing-to-recognise its prey by masking distinctive features using disruptive coloration. A 100-year old theory speculated that background matching camouflage can benefit from a complimentary camouflage strategy of disrupting outlines visibility (Fig. 1).

Fig. 1. Do edge patches contribute to camouflage through background matching or disruptive coloration? (a) White-tailed Ptarmigan Lagopus leucura, (b) Marai giraffe Giraffa camelopardalis tippelskirchi, (c) Canadian Disruptive Pattern (CADPAT), the Worlds first digital camouflage, (d) Emu chicks Dromaius noaehollandiae, (e) tree frog Hyla versicolor. Photographs by Michael Webster (a-d) and Micheal Runtz (e).

Disruptive edge markings intersect animals’ outline, breakup its edge, making boundary and overall shape less recognisable. Whilst many animals, such as zebras, tigers and cuttlefish, have been proposed to have disruptive camouflage—not to mention a host of military uniforms and equipment—to date there is no evidence show that disruptive patterns are harder to recognise, a key prediction of this concept.

Here at Carleton University, Ottawa, Canada, we set out to test experimentally if disruptive camouflage truly misleads humans when searching for animals. Using humans hunting on computer screens (Fig. 2a) we looked at the survivorship of artificial moth targets with varying numbers of edge patches. Further, we used eye tracking technology to measure recognisability (Fig. 2b). Targets that were looked at for longer were assumed to be harder to recognise.

Fig. 2. a) Photograph of a human subject hunting on a computer screen for an moth hidden (see grey arrow) on the tree image, b) a close up of the eye-tracking apparatus and the screen with an overlay showing where the subject was looking during their search.

We predicted that if number of edge patches improves camouflage by disruptive coloration, then targets with more edge intersecting patches should have a high survivorship due to impaired recognition.

Indeed, we found that targets with more edge intersecting patches taking longer to be found. This cannot be explained due to background matching alone because this was even the case for edge markings that were dissimilar from the background. Crucially, targets with more edge patches took longer to be recognised and were overlooked more often.

In our recent Biology Letters publication, we offer new evidence to support the hypothesis that disruptive coloration can achieve camouflage by masking animals’ outline visibility, which makes animals less recognisable.

Further, these novel methods provide a means to test if seemingly ‘disruptive’ markings, such as those of zebras, tigers or even soldiers uniforms function to disrupt recognition (Fig. 3).

Fig. 3. Tiger stripes have long been speculated to break up the cats like shape. The next step that scientist studying camouflage will take is to apply their new methods of assessing camouflage, to start studying animals in the wild.

This research will enable us to better understand—and  design—camouflage patterns, as well as appreciate the beauty of animals colorations, that we sometimes don’t always see.

Listen to Rich Webster discuss this research in a 3 min YouTube video:

 

Richard Websterrichard.j.webster@gmail.com Skype username: Richard.j.webster

Dr. Chris HassallC.Hassall@leeds.ac.uk www.christopherhassall.com

Related articles

• Camouflage on the edge
• Kings of Camouflage: Cuttlefish (theboldcorsicanflame.wordpress.com)
• The Art of Blending In (huntercourse.com)
• What octopus camouflage tells us about the nature of reality and successful people (douglasernstblog.com)
• On the function of zebra stripes. (blogs.discovermagazine.com)

Middle age females are most interested in mating: Lessons from a cricket

How age influences phonotaxis in virgin female Jamaican field crickets (Gryllus assimilis)

A new article in PeerJ written by Karen Pacheco, Jeff Dawson, Mike Jutting, and Sue Bertram

Click here for a copy of our PeerJ article

Click here for the Bertram Lab website

Female mate choice can heavily influence the evolution of male sexual signals. While mate choice was once thought to be a consistent and stable behaviour[1], research has shown mate selection is often very dynamic and variable throughout a female’s life. There are a number of environmental and internal factors that can affect a female’s mate choice (Figure 1). For instance, a female suddenly in danger of encountering a predator may alter her final mate decision[2]. A female’s past sexual experience[3] and her age[4] [5] [6] can also play a role in determining her mate choice. We currently have a vague understanding of how and why various intrinsic factors, such as female age, can cause variation in mate preferences and what the overall effect is on the evolutionary direction of male sexual traits.

Figure 1. Network of some of the intrinsic and extrinsic factors that could influence the final mate choice expressed before copulation; the interaction of these mate preference cues can drive sexual selection[7].

In one of our studies, which was recently published in PeerJ, we examined how one of these factors, female age, influenced female mate preferences in the Jamaican field cricket, Gryllus assimilis. Research on other species including cockroaches (Nauphoeta cinerea)⁶ and guppies (Poecilia reticulate)⁵ has revealed that as females age, their health and likelihood of future reproduction progressively declines. Because of this, older females may be less choosy in selecting mates compared to younger females who have higher reproductive potential.

Figure 2. A) A cricket tethered on top of the sphere (part of trackball). An optical sensor picks up the movements made by the trackball, which is rotated by the cricket walking. This information is collected on a computer system[10].

Figure 2. B) Female cricket tethered on top of our trackball via spring attached to magnet.

In crickets and other acoustic insects, females find and assess the attractiveness of potential mates using acoustic signals produced by the males of their species (termed ‘phonotaxis’)[8] [9]. We tested a wide range of female ages and assessed female phonotactic response to a male acoustic mate attraction signal using a specially built apparatus called a trackball. The trackball enables us to suspend a female over rotatable sphere and measure her movement associated with her attempt to move towards a speaker playing a male’s acoustic signal (Figure 2). Using the trackball and associated computer software, we measured how fast a female moved towards the speaker (velocity), how far she moved (total path length), and whether she generally seemed to be attempting to move towards the speaker (net vector score).

We found that female age influenced phonotaxis response: older females (13 days post adult moult) moved further and at higher velocities than females in other age groups in an attempt to reach the speaker (Figure 3).

Figure 3. How age affects female phonotaxis measures. Letters above each age reveal significant differences.

We also found that older females (10-13 days post adult moult) tended to more accurately orient themselves and move towards the active speaker than younger females (1-7 days post adult moult). The polar plots below (Figure 4) show the average direction and velocity (red arrows) towards the active speaker (0°) for females across all 10 age groups.

Figure 4. Polar plots indicating speed and direction moved for each age group tested. Inner circular gridlines indicate velocity magnitude (cm/s) and tic marks around the outside of the circle represent velocity angle. Red arrows show the mean velocity for all 20 females in each age group. The large, multi-colored plot shows the differences in average velocity across age groups.

Overall, we found age influenced female G. assimilis phonotaxis, where middle aged adult females’ (10-13 days old) displayed the strongest phonotaxis compared to younger or older females. The high speed and long distance travelled observed in young females (1-7 day) may signify higher energy reserves⁹, but their random orientation suggests little interest in the standardized acoustic mate attraction signal. We provide two possible explanations for the young females’ inattention towards the standard mate attraction signal: stronger preferences and lack of maturity. Very young females may have greater requirements in mate selection and prefer more extreme male traits[11]. Alternatively, very young females may be too young to respond to the call because they are not reproductively mature yet. Future research should present young females with multiple male signals to determine if indeed they are reproductively immature or only phonolocating toward signals with extreme trait values. Conversely, the older females oriented toward the speaker, suggesting interest, but walked at reduced speeds and covered shorter distances, possibly indicating senescence[12]. Overall, our findings are consistent with most studies on how age influences mating preference: diminished choosiness with increasing age[13] [14]. Our study suggests 10- and 13-day females are most responsive when quantifying the preference landscape for G. assimilis sexual signals.

Karen Pacheco is about to complete her MSc degree in Biology at Carleton University

Karen Pacheco, Jeff Dawson, Mike Jutting, and Sue Bertram (2013). How age influences phonotaxis in virgin female Jamaican field crickets (Gryllus assimilis) PeerJ : 10.7717/peerj.130

[1] Andersson, M. 1994. Sexual Selection. Princeton: Princeton University Press.

[2] Magnhagen, C. 1991. Predation risk as a cost of reproduction. Trends in Ecology & Evolution, 6, 183-186.

[3] Dugatkin, L.A. 1992. Sexual selection and imitation: females copy the mate choice of others. The American Naturalist, 139, 1384-1389.

[4] Jouventin, P., LeQuette, B., & Dobson, F.S. 1999. Age-related mate choice in the wandering albatross. Animal Behaviour, 57, 1099-1106.

[5] Kodric-Brown, A., & Nicoletto, P.F. 2001. Age and experience affect female choice in the guppy (Poecilia reticulata). The American Naturalist, 157, 316-23.

[6] Moore, P.J., & Moore, A.J. 2001. Reproductive aging and mating: The ticking of the biological clock in female cockroaches. Proceedings of the National Academy of Sciences, 98, 9171-9176.

[7] Widemo, F., & Sæther, S. A. 1999. Beauty is in the eye of the beholder: causes and consequences of variation in mating preferences. Trends in Ecology and Evolution, 14, 26-31.

[8] Solymar, B., & Cade, W.H. 1990. Age of first mating in field crickets Gryllus integer (Orthoptern: Gryllidae). The Florida Entomologist, 73, 193-195.

[9] Prosser, M.R. 1994. Effect of Age On Female Choice in the Field Cricket, Gryllus integer. Unpublished Masters thesis, Brock University, St. Catharines, Ontario.

[10] Hedwig, B. & Poulet, J.F. 2004. Complex auditory behaviour emerges from simple reactive steering. Nature, 430, 781-785.

[11] Ryan, M.J., Keddy-Hector, A. 1992. Directional patterns of female mate choice and the role of sensory biases. The American Naturalist, 139, 4-35.

[12] Prosser, M.R., Murray, A.M., & Cade, W.H. 1997. The influence of female age on phonotaxis during single and multiple song presentations in the field cricket, Gryllus integer (Orthoptera: Gryllidae). Journal of Insect Behavior, 10, 437-449.

[13] Roff, D.A. 1992. The evolution of life histories: theory and analysis. London: Chapman & Hall.

[14] Mautz, B.S., & Sakaluk, S.K. 2008. The effect of age and previous mating experience on pre- and post-copulatory mate choice in female house crickets (Acheta domesticus L). Journal of Insect Behavior, 21, 203-212.

Deceptive Males: Not All Sexual Signals Are Honest

Calling, Courtship, and Condition in the Fall Field Cricket, Gryllus pennsylvanicus

A new article in PLOS ONE written by

Sarah Harrison, Ian Thomson, Caitlin Grant, and Susan Bertram

Click here for a copy of our PLOS ONE article

Click here for a press release by Carleton University

Click here for the Bertram Lab website

Figure 1. Acoustic and physiological sexual signals

Males often produce signals to attract females to mate. These sexual signals come in many forms: acoustic (e.g. calls of songbirds and crickets), visual (e.g. deer antlers, bright colourful bird plumage, colourful tropical fish scales; Figure 1), and chemical (e.g. pheromones). These signals are thought to provide females with information about male quality and potential mating benefits to help them choose between different males.

Figure 2. Bright plumage and plump body size in a crested partridge.

For instance, if bright colourful bird plumage is strongly dependent on nutrition, then feather colour might signal that a male is healthy and good at finding high quality food (Figure 2). Mating with males that have colourful plumage might benefit females directly if males share their food, or indirectly if they pass on good genes for health and foraging ability to their offspring. Since poor quality males should be incapable of cheating and producing sexual signals that rival those of high quality males, sexual signals are thought to honestly indicate mating benefits. But are all sexual signals honest, or do males sometimes cheat when trying to attract a mate?

Figure 3. Are you compensating for something?
Source: http://www.blogger.com/img/blank.gif

Males might be more likely to produce dishonest signals (cheat) if they have a bleak reproductive future. For instance, starving, diseased, old, or dying males might invest all of their remaining energy into attracting females at the risk of dying sooner (Figure 3). This alternative idea, known as the terminal investment hypothesis, suggests that poor quality males might sometimes be able to produce signals that rival the signals of high quality males. These poor quality males would not, however, be able to provide females with the mating benefits that they were dishonestly advertizing.

We questioned whether all sexual signals are honest using fall field crickets (Gryllus pennsylvanicus). These crickets are commonly found throughout much of the U.S. and southern Canada. Males produce two types of mating calls by rubbing their hind wings together: one they use to attract females from a distance, the other they use to court attracted females. Both mate attraction and courtship calls are made up of a series of sound pulses that they group into chirps (Figure 4).

Figure 4. Sonograms (top) and waveforms (bottom) of a G. pennsylvanicus long-distance mate attraction call (A & B) and a courtship call (C & D).

We break down these chirps into smaller pieces (call rate, chirp rate, pulse rate, tick rate, number of pulses per chirp etc) to study them. Males can invest more or less energy into calling by changing the temporal properties of these pieces. Higher condition males should have more energy to devote to signalling than low condition males (‘Condition’ is an individual’s ability to find food and turn it into useable energy). Females should benefit from using calls to find mates because high condition males should pass on good genes for condition to their offspring.

Figure 5. Size variation in G. pennsylvanicus

We recorded each male’s long-distance calls and courtship calls (click on the calls for examples), broke these calls down to their smaller temporal components, and then examined relationships between these temporal components and two different estimates of male condition: 1) body size (i.e. height; Figure 5) and 2) plumpness (i.e. fat).

We found evidence that males both honestly and dishonestly signal their condition. Male mate attraction calls honestly indicated body size, except that small males called with faster chirp rates than large males. Courtship calls dishonestly signalled male plumpness, as lean males produced calls with pulse and chirp rates similar to plump males, with males of intermediate plumpness having the lowest pulse and chirp rates. It costs more energy to call at faster pulse and chirp rates, so faster rates should be harder for low condition males. Our findings reveal that small and lean males cheat when calling to females: they call at equivalent or higher rates than large or plump males, suggesting not all sexual signals are honest.

Blog post “Deceptive Males: Not All Sexual Signals Are Honest” was written by Sarah Harrison, a PhD candidate in the Bertram Lab.

Sarah J. Harrison

Related articles

• Should Attractive Males Sneak: The Trade-Off between Current and Future Offspring (plosone.org)
• Females butterflies can smell if a male butterfly is inbred (phys.org)
• Ugly friends lift male’s mating chances (abc.net.au)
• Cheating monkeys try to hide their infidelity (futurity.org)

Harrison SJ, Thomson IR, Grant CM, & Bertram SM (2013). Calling, Courtship, and Condition in the Fall Field Cricket, Gryllus pennsylvanicus. PloS one, 8 (3) PMID: 23527313