Changing Tune

Image credit: (copyright) Sergei Elkin

Noise is often a problem for many of us living in large cities. Yet, high noise levels may affect more than just a good nights sleep for some urban animals.

Recent research published in Behavioural Ecology has suggested that noise pollution is affecting components of bird song in the European Robin. In this species the male is very territorial and uses his song to warn away competing males, and serenade potential mates. 

Although it is sexually selected, bird song is a fairly flexible trait. This means that components of an individuals song can be altered in frequency, complexity or timing and so on. High levels of background noise in the environment can reduce the transmission of bird songs and make them harder to hear. 

Image credit: Roger Butterfield 

Through comparisons of individual bird songs with varying levels noise exposure, the researchers determined that males in the loudest areas had to change components of their songs in order for them to be heard. In particular, minimum frequency at which the males sang was increased by high levels background noise. However changing one component had a knock-on effect for other song components. Increasing the minimum frequency led to reductions in song length, rate and complexity of the song. 

As the elaborate nature of bird song is well documented for attracting mates, comprises in song quality caused by human noise pollution could reduce the reproductive success of males living in noisy areas. 

Find Out More:
BBC Earth News - How noise pollution is affecting many different species

Sources:

• MONTAGUE, M. J., DANEK-GONTARD, M. & KUNC, H. P., 2012. Phenotypic Plasticity affects the response of a sexually selected trait to anthropogenic noise. Behavioral Ecology. 342 - 348

Risky Business

Don't be fooled! It's not only males who advertise their genetic quality to mates, females can too (although it is less researched).

Once a female had build a nest, it was removed from the bird box and enlarged or reduced in size. Control nests were also taken out of the box but replaced without changing size. Aromatic plants (lavender) or control plants (grass) were then added at random. Nests were placed back in the box for egg laying.
Image created using Word Clip Art 2007

A paper published last month suggested that males are prepared to take greater risks for females with larger nests that are well-stocked with aromatic plants (to deter parasites). It is known that females without parasites or with better feeding resources build larger nests, so these features may advertise a healthy female.

Risk-taking behaviour was determined by males behavioural responses after being trapped and tagged inside the bird box. Using a little aluminium door propped open with a stick, both males and females could be trapped inside the nest box. The researchers insist that returning to the nest once this had occurred was "risky" behaviour, because it could happen again:

"... [It] may show the risk birds are willing to incur to feed their offspring".

Blue tit entering its nest box with a beak full of moss
Image credit: Frank Boxell

Reproductive success was improved by males who were willing to take greater risks to provision for their offspring. However, there was no difference between male risk taking in large and control nests. Perhaps there is a limit to male risk taking that is not explored. 

How "risky" this behaviour was and whether most blue-tits would act this way is questionable. 

Although this study was done in a woodland with little human disturbance, many blue tits are found throughout urban areas and should be used to humans. Human presence is probably not considered a great threat as we are not a natural predator of blue tits. Being trapped inside a bird-box or having a radio-tag are also unlikely natural situations. It would have been interesting to see how they respond to a real predation threat in the area. 

Females who build larger nests and add more plants gain higher investment from males in the form of 'risk-taking'. 

Find Out More:

VIDEO: Cigarettes in the Nest  - Urban birds have found a new way to deter parasites

Nest size changing in response to climate change - Really interesting read about what else may affect nest size of blue tits, by ScienceDaily 

Sources:

• TOMAS, G., MERINO, S., MARTINEZ-DE LA PUENTE, J., MORENO, J., MORALES, J., & RIVERO-DE AGUILAR, J. 2013. Nest size and aromatic plants in the nest as sexually selected female traits in blue tits. Behavioural Ecology [online] Available at: <http://beheco.oxfordjournals.org/content/early/2013/03/25/beheco.art015.abstract> [Accessed 28 March 2013]

For 'Choosy' Males, Big is Beautiful

It isn't easy being a male seahorse
Image credit: Nathan Rupert

For many males 'mate and run' seems like the best method of producing offspring; leaving females to bear the post-mating investment costs. That is, unless you're a male seahorse. 

Although it may be less common in nature, for seahorses post-mating care is performed exclusively by the male. Despite this, evidence suggests that males have kept the 'traditional role' in sexual selection by competing for 'choosy' females. Now new research into pot-bellied seahorses (Hippocampus abdominalis) has now suggested that role reversal may be occurring in areas where female numbers are high. 

Observations of wild seahorses have noted a sexual dimorphism, with females being larger on average than the males. Past studies have shown that female size is related to reproductive out-put. Big females produce the largest and the most eggs, and subsequently the largest offspring. In the pot-bellied seahorse, male size does not seem to influence reproductive out-put, and even small males can hold (and successfully care for) large clutches of eggs in their extended pouch.

Who to choose?
The focal seahorse (male or female) could see both a larger and small individual of the opposite sex (chambers 1 & 2). Their choice of partner preference was based on courting behaviours in the preference zone (grey hatched areas). Black walls are opaque, grey walls are clear.
Image credit: Modified from Mattle and Wilson, 2009

By examining seahorse behaviour in a test tank (see above image), Mattle & Wilson determined that males were not only more active in courting females but they liked larger ladies. Females on the other hand, were fairly inactive and female preference was not linked to male size. A well defined set of courting behaviours was used to decide if seahorse interest was orientated towards mating. 

Unfortunately, not all seahorses felt in the mood for mating and the final analyses included a small and skewed number of both sexes. In particular, the smallest females displayed no courting behaviours. The authors suggest that future studies use larger samples of captive bred seahorses and allow more direct interactions. 

Find Out More:

Male Seahorse like Big Mates - ScienceDaily summary of the article

Free Access Article - Click Here - Read the original article online

Video: Male Seahorse Releasing Young - How does a male "give birth"? Good quality video from The Deep, Yorkshire Aquarium.

Source:

• MATTLE, B. & WILSON, A. B. 2009. Body size preference in the pot-bellied seahorse Hippocampus abdomonalis: choosy males and indiscriminate females. Behavioural Ecology and Sociobiology, 63, 1403-1410.

Video Link: Why Sex?

Worth watching! 

Want to know more about why animals have sex to reproduce? 6 part Evolution documentary covers different species reproductive behaviour and why sex is so important. 

Length: just under an hour

Evolution: Why Sex?

No Costs Attached

Growing up to two-thirds of their body length, the horn of the male rhinoceros beetle (Trypoxylus dichotomus) looks like a heavy burden to bear. The large and elaborate horn is a sexually selected trait used in male-male competition to gain control of the best feeding territories and access to females. 

Rhinoceros beetle in flight. The sharp angle helps reduce the aerodynamic costs to locomotion.
Image credit: Tetsuya Shimizu

But according to recent research the cost of possessing a large horn is less than first predicted. Experimental evidence has found that males with larger horns do not suffer from reduced locomotor abilities, impaired immune systems or reduced resource allocation for other body functions. 

The rhinoceros beetles horn can almost more than 
half the length of its body
Image credit: Wikipedia (some rights reserved)

The males' horn is surprisingly lightweight; causing little difference to total body mass and drag when flying. In all males, both the weight and drag of the horn increased the force required to fly by less than 3 %. Males were also able to fly as fast as females thanks to their extreme angle of flight (Flight video). 

One suggestion for the lack of locomotor costs is that horns were costly in the evolutionary past of the species but compensatory traits have since evolved to reduce these costs. Some evidence has indicated that males with long horns for their body size had larger wings than males with relatively short horns. Relative to body size, all males had larger wings and flight muscles than females. 

Find Out More:

Rhinoceros beetles' horns are not costly during flight  - BBC Nature summary

Sources:

• MCCULLOUGH, E. L., WEINGARDEN, P. R. & EMLEN, D. J. 2012. Costs of elaborate weapons in a rhinoceros beetle: how difficult is it to fly with a big horn? Behavioral Ecology, 23, 1042-1048.
• MCCULLOUGH, E. L. & TOBALSKE, B. W. 2013. Elaborate horns in a giant rhinoceros beetle incur negligible aerodynamic costs. Proc. Royal Soc. B, 280, 1-5

Disappearing Traits: What's the Cause?

We know sexually selected traits are important for the reproductive success of many species, so why are so many males losing them? An analysis of studies across different species has revealed possible causes for reductions in female preference for male traits and subsequent male losses.

Possible causes of male trait loss:

• Environmental
• Predation risk - the trait causes males to become highly visible to predators or reduces locomotor performance when trying to escape (e.g. less agile)
• Signal transmission - traits that are too easy or hard to transmit (e.g. environment too dark to see bright colours)
• Nutrients - limited availability so many males are unable to produce required traits (e.g. specific pigments for colouration)
• Random
• Genetic Drift - may occur in small populations if the pressure of sexual selection is weak or genetic drift is strong

What factors might change female preferences?

Social factors are influenced by environmental and random effects.

• High predation pressure on 'choosy' females
• Signal transmission (see above)
• Geographical overlap - Females may change their preferences when the territory of their species overlaps with a closely related species with similar sexually selected traits (e.g. colouration). This may help prevent interbreeding and hybrids
• Selection for different traits - Female choice for male parental care could reduce the need for males to present sexual traits to attract multiple females

The female preference in both the Pied Flycatcher (top) and Collared Flycatcher (bottom) is for black and white colouration. When these species are found in the same area, female preference in the Pied Flycatcher  changes to a dull colouration. 
Image credits: (top) Piotr Jonczyk; (bottom) Garth Peacock

But remember the effect of factors needs to be considered carefully! 

Factors influencing female choice and male sexual traits are likely to work in combination. Even when present, these factors may not always result in the loss of a male trait or changes to female preference. Sexual traits may not disappear simply because environmental pressures have increased.

Increased predation, decreased nutrients or signal transmission may instead highlight the most resourceful and genetically fit males (Good Genes Model) and increase the number of matings that these 'top quality' males receive. Thus, keeping these characteristics in the species and maintaining female preference. Although research shows that female preference is lost before the male trait is, researchers are yet to explain this!

To read about how male robins respond to difficult signal transmission see 'Changing Tune'. 

Source:

• WIENS, J. J. 2001. Widespread loss of sexually selected traits: how the peacock lost its spots. Trends in Ecology & Evolution, 16, 517-523

Small Neighbours Make You Look Good...

Male fiddler crabs will wave their enlarged claw to attract a female to inspect their burrow
Image credit: Tanya Detto

... at least if you're a fiddler crab.

When shopping around for the best mate, female fiddler crabs (Uca mjoebergi) like to know what's on offer. According to the researchers of a recent paper in Behavioural Ecology, females make decisions based on comparisons of competing males. 

Densely packed along the shorelines, the burrows of male fiddler crabs are fiercely defended by their occupant. Males are easily distinguished by their large claw used in male competitions for the best territories and for attracting the females to their burrow. 

Observations of male behaviour have noted that some larger males like to keep smaller neighbours nearby. In some cases they even 'lend a claw' to help small males when threatened by competitors. 

To test the idea that females make comparisons within close groups of males, the researchers created and painted robotic crab arms to tempt the females. These arms were placed in the sand at equal distances to the female (see image [2]). 

Females are known to be impressed by big claws and by the eager males who start their waving displays first. To make sure female preference was swayed only by claw size, all robotic arms waved in synchrony at the same rate. 

As expected in most cases, 65-70% of females chose the largest males in the group. Interestingly though, females chose the focal 'medium sized' male more often when he had small males either side of him: 

"In the small neighbors treatment, 22.5% of females chose the focal male. In the large neighbors treatment only 5% of females chose the focal male"

As the largest male is not always chosen the researchers suggest that females may struggle to assess all 5 males at once; or that it is easier to perceive the difference between small and medium claw sizes, than medium and large. Hmm.. to me this seems unlikely (for this experiment at least), as the size difference between small and medium claws was 5.9 mm, and between medium and large was 6 mm.

Image [2] Male robotic arms were arranged in an arc and 3 different sizes were used in both scenarios (small, medium & large). The focus was on the focal 'medium sized' male and whether females chose to mate with him more often when he had direct large neighbours or small.
Image courtesy of Callander et al. 2012

Note-worthy: Female choice is not only based on male claw size. The quality of the burrow is very important for reproductive success and a smaller male with a fantastic burrow may have as good a mating chance as a large male who's burrow isn't up to scratch. 

In this experiment the robotic crab claws were not all the same colour. They were painted to within "the natural colour variation" of male fiddler crab claws. Other research has shown that females can use this colour to make sure she mates with a male of the correct species. Whilst she shows a distinct preference for yellow claws, the colour intensity plays no role.

Find Out More:

VIDEOS: Robotic crab arms -  BBC Nature has a couple of good videos showing the robotic crab arms in action plus another summary of the study

Source:

• CALLANDER, S., HAYES, C. L., JENNIONS, M. D. & BLACKWELL, P. R. Y. 2012. Experimental evidence that immediate neighbors affect male attractiveness. Behavioural Ecology,  730-733. 
• DETTO, T. 2007. The fiddler crab: Uca mjpebergi uses colour vision in mate choice. Proceedings of the Royal Society B-Biological Sciences, 274, 2785-2790.

Video Link: The Art of Seduction

Bowerbirds are unique for their sexual selection displays. Males spend 9-10 months of the year building these beautiful bowers and collecting objects to impress a female. Watch this great documentary narrated by David Attenborough as he explores the forests of New Guinea and Australia in search of these fantastic birds. 

Length: under an hour

Split into 4 parts:

Part 1        Part 2          Part 3           Part 4

Image credit: (copyright) Barry Hatton

Why the long neck?

Why did giraffes evolve such long necks?
 Image credit: Bernard Dupont

The incredibly long neck of the giraffe is a well known characteristic. Hypotheses for the giraffes long neck started flying about in the 1800s, with scientists Lamarck and Darwin both producing competing hypotheses. Whilst Lamarck's ideas about neck stretching have been thrown out the window, Darwin's "competing browsers" hypothesis has continued to be a strong contender. 

The "competing browsers" hypothesis proposes that giraffes necks evolved through natural selection to reduce competition with other foliage consumers in the environment. Naturally occurring (but small) differences in giraffe neck lengths, allowed those with the longest necks to reach leaves that were out of reach to other species (and shorter necked giraffe ancestors). Growing taller may have helped both sexes increase their browsing (until a certain height ~2.5-3 m) but it is often found that individuals still feed at shoulder height. 

In 1996 a new hypothesis emerged, that of the "Necks for Sex". This hypothesis suggested that the ridiculously long giraffe neck may owe its origins or continued maintenance to sexual selection. It is very obvious that males today use their necks for fighting rival males, both over territories and access to females. 

In some cases a trait that has arose for one purpose, has since been 'hijacked' for another purpose. This may be the case for the giraffes long neck. Evidence is suggestive of a variety of factors for the origin and maintenance of the giraffes neck but further study is needed. It is unlikely that the selection pressure on the origin of the neck is the same as the maintenance pressures today.

Find Out More:
VIDEO: Giraffe Fight - Some amazing footage from the recent BBC series 'Africa' narrated by David Attenborough

Sources:

• SIMMONS, R. E & ALTWEGG, R. 2010. Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe. Journal of Zoology, 282, 6-12

The Great Illusionist

Sitting at the centre of his court, this Great bowerbird male is surrounded by his collection of stones, plastic, glass and children's toys
Image credit: (copyright) Tim Laman

A new perspective: research suggests the male alters the females perspective of nearby objects through careful manipulation of object placement. 
Image credit: (copyright) Tim Laman

Throughout the forests and grasslands of Australia and New Guinea, many beautiful structures take shape among the undergrowth. At first these floor-bound stick structures may look like the nest of a bird with little sense, but these structures have an entirely different purpose. These are bowers; the unique and elaborate courts of male bowerbirds. 

In most species of bowerbird, the male lacks distinct morphological features (e.g. bright colours) that could be used to attract a mate. Instead males have evolved a unique behaviour for building elaborate bowers and adorning them with an unusual and exotic choice of objects. Male bowerbirds can spend 9-10 months of the year working on their bowers in the hope of attracting female attention. When a female does appear the male works hard to impress her with his collection of objects, vocal repertoire and dance display. 

These spheres are entirely identical but it is 
easy for backgrounds to alter our visual 
perception of size.
(measure them if you don't believe me)
Image credit: 123opticalillusions.com

Now a new study has suggested that for one species at least, males have learnt to exploit their potential mates by altering the visual perspectives of their court. 

Great bowerbirds (Ptilonorhynchus nuchalis) build 'avenue' style bowers (see above images) from which the female observes the males display. Observations have discovered that male Great bowerbirds arrange their court objects in increasing size with distance from the bower. Kelley & Endler suggests that this positive gradient causes the female inside the bower to experience an altered perception of object size. 

They reported that object manipulation  increased the chance of mating, particularly for males who had a regular increase in object size length ways, but not widthways across their court.

According to these researchers the possible cause of this increased mating success was due to females attention being held for longer whilst the male displayed. They hypothesise that the positive gradient may make objects stand out more from the surrounding forest floor or make the male and his favourite objects appear larger when close to the bower. 

Borgia, Coyle and Keagy contended this view, pointing out that male success is based on a variety of traits such as vocal mimicry, object number and colour, as well as bower quality and male dance display. But watch out... not all these traits are used as sexual cues in all bowerbird species. Great bowerbirds do not use vocal mimicry and object number is not related to mating success. 

The different perspectives of a female Great bowerbird from inside the bower (left & right columns). Notice how the black focal object (right) appears larger with a positive slope (A, top right) than neutral or negative slopes. Gray walls are the side of the bower, grey circles are objects of different sizes. Centre column is a top view of the male court.
Image credit: Kelley & Endler, 2012a

An interesting study! It would be great to know if the gradient of the court altered the females perception of the males size and not just object size (big males may have access to better resources). On a final note, the use of the term "gesso" by Kelley and Endler to differentiate between certain bowerbird objects is not substantiated. There is no indication that some objects should be more important that others in a males display; though males will sometime select a favourite object to present to the female. 

Find Out More:

VIDEO: Great bowerbird wins a female
VIDEO: How to woo a female
The story of 'Donald' - MacGregor's bowerbird and his quest to attract a mate. Article from National Geographic

VIDEO: The Great Bowerbird - Male does his best to attract a female

Sources:

• BORGIA, G. COYLE, B. J. & KEAGY, J. 2012. Comment on "Illusions Promote Mating Success in Great Bowerbirds". Science, 337.
• ENDLER, J. A., MIELKE, P. W. & KELLEY, L. A. 2012. Response to Comment on "Illusions Promote Mating Success in Great Bowerbirds". Science, 337
• KELLEY, L. A. & ENDLER, J. A. 2012a. Illusions Promote Mating Success in Great Bowerbirds. Science, 335, 335-338.
• KELLEY, L. A. & ENDLER, J. A. 2012b. Male great bowerbirds create forced perspective illusions with consistently different individual quality. Proceedings of the National Academy of Sciences of the United States of America, 109, 20980-20985.