Most people probably wouldn’t consider bustling towns and cities good places for nature to thrive. Yet a few species of birds have so successfully adapted to city living that they boast large and thriving urban populations. Now, research has suggested that the success of these city-dwelling species may lie in their behaviour.
Urban habitats are quite different to the natural environments in which birds evolved. Cities are noisy places, they are lit almost continually with artificial lights and they contain an abundance of food. Cities are also full of people. This means birds living there rarely get any peace and must cope with almost constant disturbance from both humans and their pets.
But birds living in cities are known to be much more tolerant of human disturbance than their rural compatriots. In a study of 44 European bird species, all but four allowed humans to approach them more closely in cities than in rural habitats. This suggests that city birds are bolder in the face of a potential threat.
It’s not only towards humans that urban birds seem to be bolder. We recently studied the territorial behaviour of male great tits (Parus major) in cities and rural habitats in the UK and found significant differences between them.
During the early spring, males of this species defend breeding territories. Once a male has a good territory he signals his ownership to other birds using song. In fact, together with attracting females, one of the main uses of song is territory defence.
When a rival male enters another’s territory and begins to sing, the territory holder usually responds aggressively to drive the intruder out. At first, this may mean simply singing back, but can quickly escalate to full-on physical conflict.
In our experiment, we used playbacks of great tit song to mimic an intruding male. This allowed us to measure how strongly urban and rural great tits respond to intruders. We found that city birds approached the intruder mimic 35 seconds faster and almost two metres closer than rural birds. This suggests that urban great tits are both bolder and more aggressive than rural great tits.
And it’s not just great tits which are known to be more aggressive in cities. Urban song sparrows (Melospiza melodia) have also been found to be more aggressive towards each other during the breeding season.
Why urban birds should be bolder and more aggressive than their rural cousins is not yet fully understood. One possibility is that in cities, where space is limited, only the most aggressive individuals are able to hold a territory.
In addition to researching levels of aggression, we also looked at how consistent males were in their aggressive displays. This consistency in the way animals behave over time is often referred to as “animal personality”.
We found rural great tits were very consistent in the way they behaved towards an intruder over two consecutive days, but urban birds showed lower levels of consistent behaviour. This could be due to the fact that urban males experience greater fluctuation in their environment on a day to day basis.
Urban bird territories are also packed closer together in cities, which means that the birds are likely to cross each others’ paths more often, increasing the chances that squabbles will break out.
The increased likelihood of aggressive encounters may also explain the lack of consistency of urban male behaviour. Animals remember fights they have had and their experience alters the way they behave the next time they encounter an opponent. Urban males are more likely to have had an aggressive encounter with males in their territory, which could then alter the way they behave towards our mimic of an intruder the following day.
As well as aggressive behaviour, being bold may also help birds survive in the city. Bolder birds have been shown to be more willing to explore new environments and to find new types of food to eat. As cities are vastly different to natural habitats, and contain many new types of food, boldness is likely to be a very useful trait for urban birds to have.
The big question which remains for ecologists is whether birds in cities are evolving to suit their new urban environment. A recent study has shown genetic differences between rural and urban great tit populations which suggests this may be the case.
Animals in urban environments face many unique challenges including finding and adapting to new sources of food, coping with almost continual human disturbance, and learning to avoid introduced predators (domestic cats I’m looking at you).
Another factor that all animals in urban habitats must learn to cope with is noise. Noise in cities comes primarily from cars and other road vehicles, but is also produced by factories, building sites, roadworks and numerous other human activities. In my own experiments I have measured noise levels in excess of 70 dB next to major city roads. For comparison, that’s as loud as a vacuum cleaner from 3 metres (10 feet) away! Even in quieter areas and at the quietest times of day urban noise rarely dips below 50 dB which is around the same level as in a bustling office.
For animals in urban environments there is almost no escape from noise. This presents a major problem for species which use sound to communicate as their calls and signals can easily be drowned out and lost in the din of the city. However, as is often the case in nature animals have found a solution to this problem and it is surprisingly simple.
In noisy environments many species, including all mammals and birds so far tested, unconsciously increase the amplitude of their vocal signals to ensure they are heard. This response to noise is known as the Lombard effect, named after its discoverer the French scientist and doctor Étienne Lombard (1869 – 1920).
Surprisingly however, despite numerousstudiesoftheLombardeffect in many different species very little is known about how quickly the Lombard effect acts after sudden increases in noise. For city dwellers the ability to respond to changes in noise levels quickly may be vital as things such as passing vehicles or sporadic building or road works can cause noise levels can fluctuate wildly over the course of a day. Even in wilderness habitats noise levels can change quickly due to wind, rain or the even the calls of other animals.
Last year my colleagues and I set out to fill this gap in our knowledge by testing how fast a songbird could exhibit the Lombard effect when noise levels suddenly changed. As a test species we used wild-type canaries. This species was perfect for our study as canaries sing a lot with little encouragement and produce really long and complex songs. These songs are composed of long strings of repeated bursts of sound known as song elements which can be grouped into different element types.
Here you can listen to the song of one of the canaries from our experiment while the image shows a visualisation of song and song elements.
To test how quickly canaries could respond to noise we created software which would detect when a canary began to sing, then play a 20 second burst of 75 dB white noise after a random delay of between 5-10 seconds. In this way the first half a canary’s song was sung during quiet conditions while the other half was overlapped by noise which caused the bird to respond by singing louder.
In this recording you can hear how song sounds when overlapped halfway through by white noise.
Using recordings we were able to work out the strength of the Lombard effect in our canaries by comparing how loud they sang before and after noise began.
We found that in comparison to song elements sung during quiet conditions, song elements sung during the 20 second white noise playbacks were 5.3 dB louder on average. That may not sound like much, but is actually an impressive 84 % increase in song amplitude.
To work out how fast this increase in song amplitude occured we used a statistical technique known as broken-line regression. Using this method we were able to work out exactly how quickly the amplitude of song increased after noise began, and it was extremely fast. Our canaries were able to respond to noise extremely quickly. After just 0.32 seconds of noise exposure their song showed a statistically significant increase in amplitude.
Our study shows that canaries are able to rapidly increase the amplitude of their song in response to sudden increases in background noise levels. This means that, despite fluctuating and unpredictable changes in noise levels, canaries can ensure that their songs ares till heard. This ability is likely to be particularly useful in urban areas where noise from anthropogenic sources is loud, fluctuating and unpredictable. Given that the Lombard effect is known in all other birds so far tested it is highly likely that other species also possess this ability.
This study is published in the Journal of Experimental Biology and is available here.
If you don’t have access to the journal but would like a copy of the article let me know and I will send you a copy.
As I was coming home from work yesterday there was an unmistakable feeling of spring in the air. After what feels like a very long and harsh winter (it dropped to an incredible -16°c for a few days) the sunshine and warmth is certainly welcome.
It’s not just me that’s enjoying the sunshine either. At this time of year the breeding season for great tits (Parus major) really gets going and when the sun shines the males sing as they attempt to attract females to their territories.
For some people the first snowdrops mark the start of spring, for others the melting snow or the lengthening days. For me, great tit song signals that spring is on its way and is always a welcome sound.
Great tit song is one of the most distinctive sounds in nature. From February until early June these little birds can be heard singing in fields, parks, woodlands and even in the middle of cities from Ireland all the way to China.
The most common great tit song contains two repeating notes, one high and one low which are said to sound a bit like “tea-cher, tea-tea-cher”. Here is an example I recorded in Derby (UK) a few years ago with a spectrogram below so you can see how it looks.
Remarkably however, this is just one of over 70 different calls and songs great tits are known to produce with individual birds having repertoires of up to eight song types. In fact, the great tit’s musical repertoire is so vast that if you hear a bird song you don’t recognise there is a good chance it’s a great tit!
Here a few different variants on the “classic” great tit call I have recorded in the past with spectrograms so you can see the difference.
Recorded in Starnberg (Germany)
Recorded in Bath (UK)
Recorded in Durham (UK)
Recorded in Leamington Spa (UK)
In the past I travelled all over the UK recording these birds as part of my research. I have recorded so many great tit songs that I have an almost Pavlovian response whenever I hear one as I feel the need to grab my microphone and start recording. This year however, all I need to do when I hear their song is look forward to longer days and better weather.
In 1800 only 3% of the world’s population lived in urban areas, yet as the industrial revolution picked up pace in the early 18th and 19th centuries the number of people moving from the countryside to work in the newly industrialised cities soared. By 1950 29% of the world’s population were living in cities and by 1985 this had grown to 42% while in 2025 it is estimated to that it will be over 60%1. That is just 10 years from now. In highly developed nations such as those in Western Europe and North America the 50% threshold has already been surpassed which means that if you live in a western nation and do not live in a city you are in a minority.
It’s not just where people choose to live that is rapidly changing either, but how many of us there are. The human population is currently expanding more rapidly than ever before and has grown from just a few hundred million people less than a thousand years ago to over 7 billion today, and this number is still rising2. As the human population grows cities are expanding quickly to meet the need for additional housing while the surrounding countryside is farmed and developed ever more intensely to provide us with the food, water and other resources we need to live.
Naturally, many people are concerned about how we are going to continue to feed and clothe ourselves as populations continue to expand, but there is also a growing concern among many about how such huge numbers of people are affecting the environment and the animals that inhabit it.
For animals which depend on natural habitats such as forests, meadows, or wetlands to survive, the growth of urban areas often spells bad news as these habitats are removed and paved over to make way for new suburbs, factories and roads. In the UK numerous species have declined for just this reason. For example, the bittern, a close relative of herons, was once widespread in reedbeds and wetlands across the UK but is now confined to a tiny area of the south-east after its habitats were drained to make way for agriculture and urban developments.
While many species cannot survive in urbanised areas, others are able to tolerate moderate levels of urbanisation and may continue living within cities despite drastic changes to their habitats. Life in the city is not without its challenges however, even for the most adaptable and resilient of species. Cities typically contain different threats to rural areas such as an abundance of cats which are responsible for killing huge numbers of birds and small mammals4, and high levels of chemical5, light6 and noise pollution7 which all have negative impacts.
This all sounds bad, and it really is, but while many species suffer badly from the effects of increasing urbanisation and habitat loss, there are a few species that have been able to adjust remarkably well to life in urban areas. One group that has been particularly well-studied in this regard are the songbirds and over the past 15 years or so, biologists have discovered some fascinating behavioural adaptations which have allowed some species to become successful city dwellers.
One of the most notable features of cities across the world is that they are incredibly noisy places. With heavy traffic, building sites, aircraft flying overhead and all manner of other sounds and distractions it’s a miracle anyone can hear anything at all. For songbirds however, all this noise is more than just a distraction, it can seriously affect their chances of finding mates and successfully reproducing and for males it is likely to affect how well they can defend their territories against rivals.
While we may find bird song pleasant to listen to (or annoying depending on how early in the morning it is), for songbirds it has a serious purpose. Males sing during the breeding season to attract females8 and to signal to other males that their territory is occupied and should not be entered9.
City noise can overlap and interfere with these signals making communication among birds difficult and unreliable. The background noise of a city is typically continuous low rumble concentrated at around 2kHz in frequency. Unfortunately for many birds this overlaps neatly with the frequency of their songs and this can make it difficult for other birds to hear them as they do not stand out from the irrelevant background noise.
Clearly this is a problem for birds which rely on song to communicate, yet research has revealed that birds have ways of overcoming this problem and one of them is to increase the frequency at which they sing so that their songs literally rise above the background noise and can be clearly heard.
Evidence that birds sing at higher frequencies in noisy cities than they do in quieter rural sites has now been found in numerous species including great tits (Parus major)7, blackbirds (Turdus merula)10, European robins (Erithacus rubecula)11 and song sparrows (Melospiza melodia)12.
In great tits the difference in song frequency between urban and rural sites has been measured at 478Hz13 and tests have shown that this is enough to substantially improve the distance over which song can travel in urban environments before it degrades and becomes inaudible14.
Birds may also face challenging noisy conditions in natural environments too such as where running water or wind creates high levels of low-frequency noise and these naturally noisy sites have allowed scientists to confirm that it really is the noise in cities and not some other factor which is causing city birds to sing at high frequencies. Biologists Henrik Brumm and Hans Slabbekoorn recorded the songs of white-throated dippers (Cinclus cinclus) living around noisy fast flowing streams in Scotland and found that they call at frequencies well above that of the background noise and higher than usual for this species suggesting that dippers in this area have adapted their calls to suit their noisy habitat15.
The effect of natural background noise on song frequency has also been shown in African little greenbuls (Andropadus virens) which sing at a higher frequency in areas where the rainforest is merging with open grasslands (known as ecotone forests) than they do deep within the rainforest itself16. Analysis of these two habitats revealed that the background noise in the rainforest is largely concentrated at higher frequencies while in ecotone forests there is more low-frequency noise. By singing at a lower frequency little greenbuls within the rainforest can ensure that their song does not overlap with the higher frequency background noise found in rainforests, while by singing at a lower frequency little greenbuls in ecotone forests avoid the lower frequency background noise in their habitat.
The evidence that birds change the frequency of their songs as an adaptation to noisy conditions may seem quite conclusive but not everyone agrees. An alternative explanation for the observed frequency shifts is that higher frequency song is actually just an unavoidable and possibly unimportant side-effect of singing more loudly, and it is higher volume, not frequency, which allows birds in noisy environments to overcome the background noise17.
In support of this argument Erwin Nemeth and Henrik Brumm of the Max Planck Institute for Ornithology in Germany found that the typical increases in song frequency found in great tits and blackbirds may be too low to substantially improve signal transmission whereas small increases in song amplitude were found to increase the distance over which a bird’s song could be detected much more effectively18.
But why should song frequency increase when birds sing more loudly? Nemeth and Brumm suggest two possibilities. Firstly, the increase in frequency observed in songs in noisy environments could be a side-effect of what is known as the Lombard effect (named after the French scientist Étienne Lombard) in which animals unconsciously increase the volume and frequency of their calls when the level of the background noise rises.
The Lombard effect is known to occur in humans (this is why it might feel like you have to shout to be heard at loud parties) and has also been shown in both lab and field studies of songbirds. Lab experiments on elegant crested tinamous (Eudromia elegans)19, and budgerigars (Melopsittacus undulates)20, have shown that these species both sing more loudly and at a higher frequency when background noise increases and the same result has been shown in the field in a study of Nightingales (Luscinia megarhynchos) in Berlin21.
A second reason why song frequency may increase when birds sing more loudly is that both the volume and frequency of bird songs depend on the same song producing organ which could limit how well birds can independently control frequency and volume. In birds this organ is the syrinx which is the bird equivalent of the mammalian larynx or voice box and is located at the base of the windpipe connected to the lungs. Birds produce song by forcing air at high pressure from the lungs through the syrinx causing membranes to vibrate creating sound. This sound can then be modified using numerous tiny muscles which alter the shape and tension of the sound producing membranes.
However, past studies of the avian vocal system have shown that without these tiny muscles altering the structure of the sound, both the frequency and amplitude of bird song unavoidably increase together. In other words, when birds sing louder they cannot help but also sing at a higher frequency22.
Of course, this may be totally irrelevant if birds are able to use muscles to independently control the frequency and volume of their songs but there is evidence to suggest that the frequency and volume of bird songs really are closely intertwined. One of the clearest examples of this comes from a study by Nemeth and his colleagues at the Max Planck institute who recorded blackbirds singing in sound-proof chambers and showed that volume and frequency really were strongly correlated17. When blackbirds sing more loudly they also sing at a higher frequency and this may be totally involuntary. Similar results have been found in other species including zebra finches (Taeniopygia guttata)23 and song doves (Streptopelia risoria)24 suggesting that this pattern may be widespread in birds as a whole.
It has become very clear over the past few years that urban noise is causing bird song to change however, opinion is still divided on whether it is the frequency or amplitude changes that are most important to improving song transmission in noisy environments. It is possible that both have important roles to play in helping birds to adapt to noisy urban areas and hopefully future research will provide an answer to this question.
The study of how urban noise affects bird song is a very active area of research and there are many unresolved questions which are likely to be answered in the next few years. Most importantly we need to find out what the long-term impacts of urban noise are on bird populations. Although many species of birds do seem to be able to adapt to noise we do not know how the dramatic changes we are causing to their environments will affect them in the long-term. Furthermore, many species are not able to adapt to urban areas for numerous possible reasons. They may not possess the behavioural flexibility to cope with new environments or not they might not be physiologically capable of changes their songs or behaviour. That is why studies those discussed here matter, we are changing the planet in ways which have never been seen before and we know that many species are suffering as a result. The first step to protecting animals from these changes is to understand how they are affected and that is just what these studies aim to do.
1. Kegel, B (2014).Tiere in der Stadt: Eine Naturgeschichte. Köln: DuMont Buchverlag. (In German).
2. Keinan, A., & Clark, A. (2012). Recent Explosive Human Population Growth Has Resulted in an Excess of Rare Genetic Variants. Science, 336 (6082), 740-743 DOI: 10.1126/science.1217283
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A few months ago during the coffee break at an animal behaviour conference I was talking to a colleague about her research when she told me that to suggest that animals could feel fear or be afraid was anthropomorphism, the mistake of assigning of uniquely human characteristics to other animals. This view is not at all uncommon among practicing scientists and the term anthropomorphism is often extended to include a whole range of behavioural traits and emotions such as impatience, joy, expectation, boredom, anger, happiness or sadness, and yet there is good evidence that these emotions are not unique to humans. For example, dogs have been shown to exhibit jealousy, elephants have empathy, and Capuchin monkeys get visibly angry when treated unfairly as this video shows.
One of the main arguments against using anthropomorphic language to describe animal behaviours is that there is no way to know how an animal is really feeling, we can only describe what it looks like it’s feeling but not what’s actually happening inside it’s head. But the same is also true of humans, yet no one would question the use of anthropomorphic language to describe human behaviours.
If a person says they are excited we don’t actually know that what they feel as excitement is the same as what you or I feel as excitement, to them it may be a very different thing. All we can do is observe how that person acts and behaves and decide for ourselves if that matches up with our interpretation of what excitement is. The same is true of any emotion. If I say I am or happy or bored how could you tell that what I feel as happiness or boredom is the same as what you or anyone else feels as those things? At some point we have to use our subjective judgement to decide how a person is feeling. Do their actions match our expectations for a happy person? Then we can say they are happy. Do they behave as if they are sad? Then we can say they are sad. If this applies to humans then surely it can also apply to animals, at least in some cases.
I am not suggesting that we should abandon all caution and start using anthropomorphic terms carelessly. What I am suggesting is that so long as we clearly define our terms we should be able to use words like ‘afraid’ or ‘excited’ to describe animal behaviours when those terms well match what we see. If I want to describe anger in animals I should be able to use the word anger so long as I clearly state what I mean by the term and the behaviour that I am describing well matches what most of us would recognise as anger.
Of course, there are cases where using anthropomorphic terms really isn’t appropriate. If I read a paper that described ‘angry’ aphids or ‘jealous’ earthworms I would be dubious that those animals really could feel those emotions. In other cases I would have much less of a problem. Can chimps get angry? I think so. Are rats afraid of predators? It seems likely.
For many scientists, including people who I work with, the fear of using anthropomorphic language seems deeply ingrained and I think this affects how we view the behaviour of animals. Not all animals are mindless automatons that blindly follow their pre-programmed instincts. Many animals, especially among the vertebrates, have complex behaviours and emotions which are best described using the same terms we use for those things in human animals. I think it is time we started describing animals behaviours exactly as we see them. We must define our terms and we must be clear but so long as we are there should be little problem to this approach.
For those that still doubt that animals have can have thoughts and emotions like ours I recommend this TED talk.
In the closing paragraph of on the origin of species Darwin famously said that nature was a war in which individuals struggle against each other and the environment for survival. However, while survival may be important from an individuals point of view, from an evolutionary perspective mere survival is not enough. Reproduction is what matters and success or failure at producing offspring is what determines an individual’s evolutionary success. Of course, survival is important too, but only when it leads to reproduction.
In most species the reproductive success of females is limited by the rate at which they can produce offspring. When a female is pregnant or carrying eggs she has no choice but to wait until she has given birth or laid her eggs before she can reproduce again, and this can take a long time. Males have no such constraints to their reproductive success and can potentially mate with hundreds of females over their lifetime and raise an enormous number of offspring. The only thing stopping them is that there just aren’t enough females to go around. This shortage of females coupled with the need to reproduce leads to intense, and often aggressive, competition among males for limited mating opportunities.
Male red deer (Cervus elephus) fight for their chance to mate by using their huge antlers to batter their rivals into submission, while male northern elephant seals (Mirounga angustirostris) grow to enormous sizes allowing them to dominate harems of many females and guard them against the advances of smaller, weaker males. Not all species are so aggressive in their tactics. Males of many bird species such as peacocks (Pavo cristatus) and birds of paradise produce fantastic and colourful displays with which they attempt to attract females, as do a large number of insects and fish. In these species, rather than fighting with each other, males try to out-perform and out-class each other in the hope that females will choose them while their rivals are left unwanted on the sidelines. This may seem a more peaceful strategy but make no mistake, although these males don’t actively fight each other the competition between them is every bit as intense as among more aggressive species.
So fighting or displaying are two ways in which males can improve their reproductive chances, but what happens in species in which each female mates with lots of different males in quick succession? How is a male to improve his odds of being the true genetic father of the offspring? Well, as is often the case evolution has found a way and that way is called sperm competition (yes, really).
In species in which females mate promiscuously males compete not just for mating opportunities but also for direct access to eggs. In these cases competition between males happens after mating has occurred as the sperm of multiple males compete with each other within the females reproductive tract as they race towards the eggs. In species in which sperm competition is known to exist an incredible variety of different sperm adaptations have been found, all of which serve to improve the sperms chances of reaching the eggs first.
For individuals of many species adaptation to sperm competition simply means producing more sperm so as to swamp the sperm of their rivals and increase the odds that some of their sperm will make it to the eggs before anyone elses. For other species adaptation to sperm competition is more complex. For example, the wood mouse, Apodemus sylvaticus, has evolved sperm that have a hook-like structure on the head which allows them to intertwine with one another to form long sperm ‘trains’ which are much faster at swimming than individual sperm.
In a similar and recently discovered case, a team led by Morgan Pearcy of the Université libre de Bruxelles looked for evidence of sperm competition in the desert ant, Cataglyphis savignyi. The queen ants of this species mate with up to 14 males in rapid succession and store their sperm jointly in a special storage organ called the spermatheca. Only those sperm which make it to this storage organ have any chance of fertilising an egg and so competition for access to the spermatheca is intense. In response to this pressure C. savignyi males have evolved highly cooperative sperm that team up into bundles of 50-100 cells which can swim much faster than they could alone and so are better able to outcompete their rivals.
Some species have taken a more sinister approach to sperm competition and have evolved infertile “parasperm” which contain enzymes capable of breaking down the sperm of rivals. A similar and fantastically named kamikaze sperm hypothesis has even been proposed for humans in which some sperm are adapted to kill the sperm of rivals rather than fertilise eggs. The evidence for this hypothesis is equivocal at best but given the adaptations that have been discovered in other species it is not entirely unbelievable. In fact, given the adaptations that have been discovered so far, almost nothing is completely unbelievable.
Sperm competition by producing large quantities of sperm Moller, A. (1989). Ejaculate Quality, Testes Size and Sperm Production in Mammals Functional Ecology, 3 (1), 91-96 DOI: 10.2307/2389679
Sperm trains in the wood mouse Moore H, Dvoráková K, Jenkins N, & Breed W (2002). Exceptional sperm cooperation in the wood mouse. Nature, 418 (6894), 174-7 PMID: 12110888
Cooperative sperm in the desert ant Pearcy M, Delescaille N, Lybaert P, & Aron S (2014). Team swimming in ant spermatozoa. Biology letters, 10 (6) PMID: 24919705
Sperm competition in Australian frogs Byrne PG, Simmons LW, & Roberts JD (2003). Sperm competition and the evolution of gamete morphology in frogs. Proceedings of the Royal Society B: Biological Sciences, 270 (1528), 2079-86 PMID: 14561298
The two tailed sperm of the water beetle Mackie JB, & Walker MH (1974). A study of the conjugate sperm of the dytiscid water beetles Dytiscus marginalis and Colymbetes fuscus. Cell and tissue research, 148 (4), 505-19 PMID: 4836644
The world’s largest sperm in drosophila Bjork A, Dallai R, & Pitnick S (2007). Adaptive modulation of sperm production rate in Drosophila bifurca, a species with giant sperm. Biology letters, 3 (5), 517-9 PMID: 17594959
Killer ‘parasperm’ Buckland-Nicks, J. (1998). Prosobranch parasperm: Sterile germ cells that promote paternity? Micron, 29 (4), 267-280 DOI: 10.1016/S0968-4328(97)00064-4
Kamikaze sperm Baker, R., & Bellis, M. (1989). Elaboration of the Kamikaze Sperm Hypothesis: a reply to Harcourt Animal Behaviour, 37, 865-867 DOI: 10.1016/0003-3472(89)90074-2
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The power that natural selection has to sculpt both the appearance and the behaviours of creatures so that they intricately and precisely fit their respective environments is for me a source of endless fascination and wonder. Some of the most impressive examples of natural selection’s power lie among the mimics of the insect and spider world where a huge diversity of body forms are to be found, from insects which look uncannily like leaves or moss, to spiders that look just like ants. The benefits of these disguises vary from species to species. For many blending seamlessly into the background provides some protection against predators, while for others it allows them to creep up on their prey unnoticed or lure victims to their demise.
The south-east Asian orb-web spider known as Cyclosa ginnaga is a perfect example of how mimicry may be used to conceal an animal from its predators, in this case highly aggressive predatory wasps. Although by themselves individuals of this species are conspicuously silver in colour and not all that well disguised, they are able to spin white circular silk decorations which they stand on in the centre of their webs as a way of concealing themselves. That might not sound like a great way to hide but the size, shape and colour of the spider when viewed against the white background of its decoration look remarkably like a bird dropping which, of course, is of no interest to predators. This type of mimicry, in which animals mimic inanimate objects, is termed masquerading and the details of this particular case were recently published in a new paper by Min-Hui Liu and colleagues.
Liu and colleagues wanted to know if the decoration of C. ginnaga really did function as an anti-predator masquerade. To test this the researchers first used a technique called spectral reflectance imaging to examine how the spider and its decoration appears through the eyes of its predators. After all, what looks like a bird dropping to us may look completely different to a wasp. This method compared the way that light reflects from the body of the spider and its decoration to what is known about the sensitivity of insect eyes. The results were unequivocal, wasps cannot see the difference between bird droppings and the masquerade display of C. ginnaga.
The crucial test however, was to show that mimicking a bird dropping really does reduce the predation risk for the spiders and lead to real fitness benefits for individuals. To do this the researchers divided 39 wild caught spiders into three groups. To one group they coloured the bodies of the spiders black while leaving their decorations untouched, to another they coloured the decorations black while leaving the spiders themselves untouched, and to the final group they coloured both the bodies of the spider and their decorations black. They then observed the frequency of predator attacks on each group over 13 days and compared this to the predation rate on untouched spiders. From these three groups a huge increase in predation was observed on those spiders that had only their decorations blackened. This suggests strongly that having a white decoration really does help C. ginnaga to hide itself from predators.
As shown in the figure above, when both the spider and its decoration were blackened no increase in the frequency of predator attacks was observed. This is not so surprising as in this case the black spider is likely to be well camouflaged against its black background. What is surprising however, is that when the spiders body was blackened but the decoration was not there was also no increase in predator attacks. This seems strange since a black body on a white background might be expected to stand out very clearly to predators. It may be that the wasps recognise only silver spiders as their target species and so don’t see the black coloured spiders as potential prey. It could also be that when a black spider is on a white background it still looks like a bird dropping since bird droppings often have black bits in them. The authors don’t discuss this anomaly in their paper but it does cast doubt on the idea that the silver spider in combination with the white decoration together form a masquerade which reduces the risk of predation. Nonetheless it is clear from this study that the web decoration does substantially reduce predator attacks, why that is so remains an interesting question that is open for discussion.
Liu MH, Blamires SJ, Liao CP, & Tso IM (2014). Evidence of bird dropping masquerading by a spider to avoid predators. Scientific reports, 4 PMID: 24875182