That Horn's Not Cheap

  He may be good at fighting off rivals but this male dung beetle pays a high price for a large horn
Image credit: (copyright) Alexander Wild

The major male dung beetle protects the entrance to his burrow with his large thoracic horn. Underground are his females. This male knows he can out-compete other males for access to females because his large horn makes him a good fighter. But unfortunately for him, the females he guards are less than faithful. These females are secretly mating with 'sneaky' minor males who lack the cumbersome horn used in male-competition. 

Various studies on the dung beetle, Onthophagus nigriventris, have suggested that having the biggest horn is not always as profitable or desirable as you might think. In this species sexual dimorphism among males is highly evident. Major males possess a large pair of thoracic horns (the long horn is the primary horn and can reach up to 40% of total body length); whilst minor males have only a rudimentary horn. The difference in morphology represents not only a difference in mating strategy between these two types of males, but also reveals a trade-off between obtaining matings and fertilisation success.

Thanks to female promiscuity, males not only have to compete to obtain matings but their sperm must compete with that of other males to obtain fertilisation of eggs. 

So what costs are involved with the thoracic male horn?

Horns vs. testes

Several years back, a study by Simmons & Emlen revealed that thanks to their large horn, Onthophagus nigriventris, males experience a trade-off between the sizes of primary and secondary sexual traits. Through experimental manipulation of larvae, the researchers were able to compare the morphologies of horned and hornless males.  

They discovered that hornless males grew into larger adults than the horned males, and possessed disproportionately larger testes whilst the horned males were very small in this department. Testes are a primary sexual structure and required for the male to produce sperm. The smaller the testes the fewer sperm a male can produce at any one time. Of course, the males with the most and the best quality (motile) sperm are likely to win the fertilisation race. 

The difference in morphological trait sizes (horns vs. testes) between major and minor males was found to be due to differences in resource allocation during the larval development. Intrigued by their finding, the researchers wanted to know if this relationship existed in any other Onthophagus species. However, horn size in these other species did not seem to limit testes size. 

Simmons & Emlen suggest that in some cases, males of different species may have developed strategies to reduce costs to fertilisation such as horn developing in different places. Horns developed from the head, instead of the thorax, are further from the testes in larval development. Compromises in resource allocation may be instead made with eyes, antenna or wings.

Smooth Mover

Sexual dimorphism between minor (left) and major males. The huge horn on the major male can make negotiating tunnels tricky
Image credit: (copyright) Alex Wild

With such a large morphological structure, it is hard to image that this would not cause a problem for locomotor capabilities. Onthophagus nigriventris, is a tunnel building species. They build their networks of tunnels in soft soil or sand, and use them for mating, protection and as a place for larvae to develop. 

Tunnels are used for a variety of activities.
Males use them to defend their females,
fight other males and mate. Females store
their eggs inside dung balls hidden
inside these tunnels
Image credit: Jim Heath

Through an experimental study, Madewell & Moczek investigated whether body size or horn size would affect underground manoeuvrability. Male and female beetles of two difference size groups (large: 6.2-7 mm; small: 5.2-6 mm) were allowed to run along a plastic tunnel towards a light. When the light was moved to the other end of the tunnel, the beetles tried to turn around. 

It was found that horn size rather than body size constrained manoeuvrability in large males. 

The natural range of tunnel sizes vary from 6-18 mm in width. The tunnel used in this experiment was 13 mm width so that it would allow >95% of males to turn around eventually. 

I'm not sure how useful a measure of cost this is. Whilst it has shown that male horn size rather than body size constrains manoeuvrability; large males are likely to dig bigger tunnels than small males, so they can fit through easily. 

The manoeuvrability of males (A) and females (B) was compared in terms of body size and horn size. Small horned males, large and small females all took around the same time to turn around. Large horned males took significantly longer to turn around. 
Image credit: Madewell & Moczek, 2006

Unfortunately, we don't know whether horn possession or lack of horns will produce different results for reproductive success because neither study tested this. 

Find Out More:
Original article by Simmons & Emlen & Original article by Madewell & Moczek - Free access online articles
Horniest males have tiniest testicles - New Scientist blog summary

Sources:

• SIMMONS, L. W. & EMLEN, D. J. 2006. Evolutionary trade-off between weapons and testes. Proceedings of the National Academy of Science of the United States of America, 103, 16346-16351.
• MADEWELL. R. & MOCZEK. A. P. 2006. Horn possession reduced maneuverability in the horn-polyphenic beetle, Onthophagus nigriventris. The Journal of Insect Science, 6, 1-10.

Hitching a Ride on a Giant Fly

Even for the tiniest of species, sexual selection provides advantages to the 'best equipped' males. For the pseudoscorpion, Semeiochernes armiger, new research has shown that male-male competition for dispersal sites, rather than female-choice, is a strong determinant of large sexual dimorphism. 

Tiny pseudoscorpion hitching a ride. Males attempt to mate with females whilst on the fly
Image credit: Jana Christophoryová

Living in the forests of Panama these tiny, wingless creatures are faced with a challenge when it comes to dispersal. Luckily for them, transportation is available in the form of the the giant timber fly. 

The female timber fly lays her eggs on the rotting wood of the Ficus tree (where S. armigers are also found). Her larvae then burrow their way into the wood where they remain for  5 - 12 months before re-emerging as adults and flying away. In order for S. armigers to disperse, they must hang onto the body, legs and wings of the fly as it emerges from its pupal case. Using another animal for dispersal is known as phoresy. 

Sexual dimorphism: female (left); male (centre) with large chela peg (indicated by arrow) and triangular palps; small male (right) with no chela peg and small triangular palps
Image credit: Zeh & Zeh, 2013

Labelled pseudoscorpion (not the same 
species) notice the palps and the chelae; 
these are regions affected by sexual 
selection. Overall body size was 

not different between males and females. 

Image credit: reprinted from the Canadian Journal of 
Arthropod Identification with permission of the 
photographer Christopher Buddle

Observations discovered that each of these fly "bore holes" was guarded by a single S. armigers male. Body measurements showed that males with the largest pedipalps, especially the chelae; were the most likely to control a bore hole. The pedipalps are required for feeding in both males and females, but males also use them for competition. Males with smaller pedipalps were unlikely to control bore holes, and have reduced mating opportunities. Sexual selection selects for males with bigger pedipalps as they will be the ones mating. 

Females wishing to disperse gather near the bore holes of soon-to-emerge flies. The male who controls the bore hole will mate with females whilst on the fly or when they reach their destination. As many as 22 pseudoscorpions were seen climbing aboard their fly host!  

Q:What benefit does transporting pseudoscorpions have for the fly? 

Sources:

• ZEH, J. A. & ZEH, D. W. 2013. On the threshold of dispersal: hitchhiking on a giant fly favours exaggerated male traits in a male-dimorphic pseudoscorpion. Biological Journal of the Linnean Society, 108, 509-520

Video Link: Living in Paradise

Fantastic way to spend an hour! 

When it comes to impressing the ladies, few species could out-compete the extravagant Birds of Paradise! A great documentary by the BBC and narrated by David Attenborough. Beautiful  footage and really informative. 

BBC Natural World - Birds of Paradise

Photo Credit: Tim Laman

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?