Researchers at the University of Derby and colleagues at the University of Cambridge believe they have found which species has the largest testicles in relation to body weight on the planet – and why!
Yet the research team also discovered that large testes did not necessarily relate to a larger amount of sperm produced – which goes against traditional thinking in the science world.
Biologists at the University of Derby, which led the research project, say that the Tuberous Bushcricket (Platycleis affinis) produces testes which are 14% of the male body mass, according to research published today in Biology Letters, a Royal Society Journal.
It compares to a species of fruit fly (Drosophila bifurca), whose testes to body weight ratio has been recorded as 10.6%.
Lead researcher Dr Karim Vahed, Reader in Behavioural Ecology at the University of Derby, said: "We couldn't believe the size of these organs, they seemed to fill the entire abdomen.
"We are also interested in the reason why they are so large. An almost universal evolutionary rule appears to be that such variation in relative testes size is linked to female mating behaviour; testes tend to be larger in species where females are more promiscuous, as has been demonstrated in various species in fish, birds, insects and mammals.
"But at least two hypotheses could account for this pattern – sperm competition on the one hand and male mating rate on the other.
"Yet our study appears to be the first study to show that, in the case of the Tuberous Bushcricket, bigger testes don't necessarily produce more sperm per ejaculate."
As part of the Derby-led study, Dr Vahed, Darren Parker (a recent Derby Biology graduate now studying a PhD at St Andrews University) and Dr James Gilbert at the University of Cambridge, compared relative testes size across 21 species of bushcricket. As with other studies, they found testes were proportionately larger in species where females mated with more males – female Tuberous bushcrickets mate with up to 23 different males in their two-month adult life.
However, the surprise was that the Tuberous bushcricket did not produce more sperm: in fact they produced less voluminous ejaculates.
Dr Gilbert said: "Traditionally it has been pretty safe to assume that when females are promiscuous, males use monstrously-sized testicles to deliver huge numbers of sperm to swamp the competition - even in primates. Our study shows that we have to rethink this assumption. It looks as though the testes may be that big simply to allow males to mate repeatedly without their sperm reserves being exhausted."
Dr Vahed said: "This strongly suggests that extra large testes in bushcrickets allow males to transfer relatively small ejaculates to a greater number of females. Males don't put all their eggs (or rather sperm!) in one basket."
Traditionally, the assumption is that larger testes produce more sperm per ejaculate and thereby provide males with an advantage in sperm competition (when males are vying for the fertilisation of the female's eggs).
Sperm competition is most intense when the female of the species mates with many males; the male that has produced the most sperm is often assumed to be at an advantage, hence the development of larger testes in such species.
But more promiscuous females also increase the number of mating opportunities available for the male. It is therefore possible that larger testes have evolved in more polyandrous species because they allow an increased rate of ejaculate production, enabling the male to engage in a greater number of successive matings, as indicated by this latest study.
The researchers suggest these findings offer food for thought about the links between endowment, promiscuity and reproduction within insects in the biological world.
For more information contact Deputy Head of Corporate Relations Simon Redfern on 01332 591942 or 07748 920038 or email s.redfern@derby.ac.uk
– Read our latest news at www.derby.ac.uk/connected
Notes to editors:
The University of Derby is a thriving institution inspired by a dedication to quality and opportunity. It is an innovator in flexible modular study and e-learning solutions tailoring programmes to students' needs.
The University is home to a diverse community of more than 20,000 students from the UK and overseas. Subjects offered to students include a wide range of disciplines in arts, design and technology; business, computing and law; and education, health and sciences.
The University has won and been shortlisted for a series of national awards in recent years including HEIST Best Website (Gold Award, 2008); HEIST Marketing Department of the Year (Bronze Award, 2008); Outstanding Finance Team, Times Higher Education (Winner, 2009), Universities Human Resources Excellence Award for Staff Engagement (Winner, 2009); CIPR Best Broadcast category (National finalist, 2009); Times Higher Education Best Community Project (National finalist, 2009).
The University Quarter in Derby incorporates sites at Kedleston Road, Markeaton Street and Britannia Mill to the west of Derby city centre.
The University of Derby Students' Union, a number of research centres and academic schools, including The Derbyshire Business School, are all based at the Kedleston Road site. Facilities include our £1.5m Clinical Skills Suite and a pioneering computer games development suite.
The UK's first dedicated site for arts, design and technology at Markeaton Street was opened by Sir Richard Branson, President of Virgin Atlantic, in autumn 2007. Britannia Mill, a restored Victorian mill, houses the University's School of Social Care and Therapeutic Practice, and some of our nursing students are trained in Chesterfield.
We also have a multiple award winning £23m campus in the Peak District spa town of Buxton. The Devonshire Campus is the result of five years of careful restoration. Teaching there commenced in September 2005 and the Devonshire Campus was opened officially by Their Royal Highnesses Prince Charles and Camilla, Duchess of Cornwall, in February 2006. The building's centrepiece is a magnificent Dome, larger than St Paul's in London – its facilities are among the finest in Europe and the Buxton Faculty has further education and higher education provisions.
In 2008, University of Derby Corporate, the business-to-business arm of the University, was launched.
Derby achieved University status in 1992. Professor John Coyne has been the University's Vice-Chancellor since the summer of 2004.
For media information please contact the University's Press Office Administration Assistant Anne Wake on 01332 591187, Press and PR Officer Sean Kirby on 01332 591891 or 07876 476103, Press and PR Officer Joanna Colburn on 01332 593004 or 07909 533242; Deputy Head of Corporate Relations, Simon Redfern on 01332 591942 or 07748 920038, or Head of Corporate Relations Peter Gallimore on 01332 591041.
If the story relates to a graduate story, you can also contact Laura Ratcliffe, Alumni Relations Officer, on telephone: 01332 591368, or email: l.ratcliffe@derby.ac.uk
Journal
Biology Letters