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