Liar, liar, pants on fire…

… or telling the truth is probably wiser!

Before you read my post, watch this video of Springbok leaping high into the air when escaping the predation of the fast cheetah. I promise you, you will be entertained.

Now, some of you might be thinking the same thing as I did when I first saw the video. Why on earth would these animals not expend their energy on running away as fast as possible instead of wasting their time and energy on ballerina displays, as impressive as they might be…

 

Well, funnily enough, impression is exactly what these Springbok’s are trying to do. Through this behaviour (called ‘stotting’ displays), they convey a message to their predator that there is no point in chasing them because they are far too fit and strong. The process of signalling quality to a receiver (in this case a predator) is a concept known as the honest signalling theory, or, the handicap principle (Nur & Hasson 1984).

Honest Signalling Theory/Handicap Principle

 

Whether you are exploring the reef, hiking through valleys or strolling through a forest, you are constantly being bombarded with signals, most of them intended for non-human receivers. These could be in the form of calls, colours, smells, just to name a few. Several of these signals are costly to produce and send, and I have discussed some of these in my past blogs.

 

Check out the other examples below:

 

Baby Northern Cardinals begging loudly for food whilst being fed by their parents:

 

 

Performing these loud cries might be useful in signalling to their parents that they are hungry, however it would also undoubtedly attract predators to the nest.

 

Big Bull Elk rubbing the velvet off their huge antlers 

 

 

These horns are impressive, attractive to females and useful in battle, but incredibly energetically expensive to create and carry (some of them weigh up to 18kg!).

 

So why don’t these animals send a more subtle message to the receiver instead of partaking in these elaborate displays? These signals obviously work well in convincing the receiver as they have evolved through natural selection, but how?

 

The Handicap principle proposed by Amotz Zahavi in the 1970’s suggests that if an organism can signal something costly, it means that it has a higher fitness than other individuals as it can afford the signal in the first place. That is, it is strong and healthy enough to deal with the cost and pass the test of survival, when compared to another individual who has an inferior fitness and therefore cannot afford the costly traits. Through this honest signalling organisms are effectively conveying their ‘quality’ to other individuals (Zahavi & Zahavi 1977).

 

This can be to attract females, such as with the peacock or large rack of antlers. Peacock’s with dull feathers that drag on the ground and collect mites signal to females the lower quality and fitness of that male, and she will find someone else with beautiful feathers (making the grooming worth it in the end!). Likewise, small antlers signify to females that the male cannot “afford” to grow bigger ones and carry them and is therefore of lesser quality than the spunky male with the oversized bone/skin/horns projecting from his head (Nur & Hasson 1984, Grafen 1990, Johnstone 1995)

 

Which one would you choose?

 

 

It can also be to deter predators, in the case of Gazelles and Springbok. Jumping high signals to the predator that they are fit and strong, and that they should probably focus more on chasing the one who keeps tangling his hooves and who can’t even jump 2 feet of the ground (how shameful!). The hard to catch prey benefit by distinguishing themselves from less healthy individuals and escape predation, and the predators benefit by understanding this signal and not wasting their time on prey who are more likely to escape. Other prey do not benefit y being distinguished, however the cost of the leaping signal is so high that they cannot pretend to be a harder-to-catch individual, meaning that is an honest signal! (Nur & Hasson 1984, Yachi 1995)

 

In the case of baby birds, they are informing their parents about how hungry they are. These birds might all pretend that they are hungry and all make noise. However, this takes energy and could also possibly attract predators to the nest. This means nestlings end up honestly conveying their hunger levels as the ones who are truly hungry continue making noise (need for food outweighs the risk of predation), whereas the satisfied ones will stay silent due to the cost of predation (Rodríguez-Gironés et al 1996).

 

Signals have to be honest, or the receiver would eventually evolve to ignore them, leading to the signallers evolving not to send them in the first place as they would be completely useless. And in order for the signals to be honest, they must also be costly. Each signaller makes a choice of the size of their handicap they want to produce, taking into consideration both the cost and the benefit (this is often not a conscious decision, but one that is encoded in the genes and acted upon through natural selection). Stronger individuals can assume greater handicaps at lower costs and hence will grow ‘bigger and better’ ornaments, whereas the same handicap would hinder a weak individual. Consequentially, handicap size is a reliable signal of strength to the receiver. 

 

References:

Grafen, A. 1990, "Biological signals as handicaps", Journal of theoretical biology, vol. 144, no. 4, pp. 517-546.

http://www.shutterstock.com/pic-108779864/stock-photo--three-young-rocky-mountain-elk-bulls-with-spike-antlers.html (26th April 2014)

 

http://www.shutterstock.com/pic-108779921/stock-photo-rocky-mountain-elk-in-habitat-with-beautiful-autumn-leaves.html?src=pp-same_artist-108779864-4 (26th April 2014)

 

http://www.youtube.com/watch?v=1tWLDhJ6mjQ (26th April 2014)

 

http://www.youtube.com/watch?v=nPWHdJM8bJ8 (26th April 2014)

 

https://www.youtube.com/watch?v=2Sl250079Kk (26th April 2014)

 

JOHNSTONE, R.A. 1995, "SEXUAL SELECTION, HONEST ADVERTISEMENT AND THE HANDICAP PRINCIPLE: REVIEWING THE EVIDENCE", Biological Reviews, vol. 70, no. 1, pp. 1-65.

Nur, N. & Hasson, O. 1984, "Phenotypic plasticity and the handicap principle", Journal of Theoretical Biology, vol. 110, no. 2, pp. 275-297.

Rodríguez-Gironés, M.A., Cotton, P.A. & Kacelnik, A. 1996, "The Evolution of Begging: Signaling and Sibling Competition", Proceedings of the National Academy of Sciences of the United States of America, vol. 93, no. 25, pp. 14637-14641.

Shigeo Yachi 1995, "How can Honest Signalling Evolve? The Role of Handicap Principle", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 262, no. 1365, pp. 283-288.

Zahavi, A. & Zahavi, A. 1977, "The cost of honesty. Further Remarks on the Handicap Principle", Journal of Theoretical Biology, vol. 67, no. 3, pp. 603-605.

 

 

 

 

Sneaky exploitation in nature

In my last post I talked about females selecting certain traits in males. In these cases, the males developed a trait and the females in turn selected for it.

Well there’s actually a very controversial theory being debated in the literature that says females evolve their preferences in mates first, and then males evolve their mating display afterwards. How is this possible?

Females that are picky might have pre-existing sensory biases

It is hypothesised that this begins with a trait evolving in a non-mating context, and then is in turn exploited by the other sex to receive more mating opportunities (Ryan, M.J. 1998).

For example the female water mite Neumania papillator adopt a posture called the net stance (shown below) in order to catch copepods until it senses the vibrations in the water caused by its prey, and then proceeds to clutch at it (Proctor 1991). The male water mite has evolved to take advantage of this by finding a female (through scent) and creating vibrations in the water, causing the female to instinctively clutch onto him, which is then followed by the male releasing his spermatophores. This is a perfect example of males evolving a courtship technique that exploits the predation technique of females.

  

Sourced from Proctor 1991

Another example is female three spined sticklebacks (Gasterosteus aculeatus), where females have a preference for males with a red throat and jaw. Some have proposed that this has evolved on a runaway basis (Fisherian hypothesis), however others have tested the possibility of it originating in the context of foraging (Smith et al 2004). Red fruits fall into the streams where guppies live. As these fruits are a rare treat, guppies have adapted an ability to find them quickly, and over time developed an affinity for the colour red. Males have then taken advantage of this preference and evolved red colouration to attract mates (Smith et al 2004). Isn’t that cool!?

Red throats in male Three Spined Sticklebacks
Author unknown

If you want to check more of this out, sexual selection through pre-existing sensory biases is also evident in auklets, wolf spiders and manakins.

References 

• Smith, C. Barber, I. Wootton, R.J. & Chittka, L. 2004, ‘A receiver bias in the origin of three-spined stickleback mate choice,’ The Royal Society, no. 271, pp. 949–955
• Proctor, H.C. 1991, ‘Courtship in the water mite Neumania papillator: males capitalize on female adaptations for predation,’ Animal Behaviour, No. 42, pp. 589-598