Compensating for something? Testicle size doesn’t matter for monogamous birds, which impress their partners with their colourful plumage and dance skills instead
- UK researchers analysed the testicles sizes of vertebrate animals over time
- They found many changes in endowments over the last 400 million years
- Bigger testes are often better as more sperm can increase reproductive success
- Yet monogamous birds appear to have repeatedly evolved smaller testicles
- Researchers think they traded in costly big testicles for other advantages
Testicle size does not matter for monogamous birds, who have traded in big testes for other traits like colourful plumage and dancing displays, a study has found.
The researchers analysed the largest ever dataset of animal testes masses to determine how testicle sizes have changed across evolutionary history.
They found many shifts in species’ testicle sizes over the last 400 million years, with many animals benefiting from bigger testes, which confer an advantage in mating.
However, the team found that birds were the only group to repeatedly evolve small testes over time.
Researchers suggest that the development of mating behaviour in these birds freed them up to ditch the cost of big testicles in favour of other traits.
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Testicle size does not matter for monogamous birds, who have traded in big testes in favour of other traits like colourful plumage and dancing displays, a study has found
IS BIGGER BETTER?
Bigger testicles produce more sperm.
This means that they can often give animals a competitive advantage when it comes to reproducing.
However, large testes are ‘expensive’ to grow and maintain.
Researchers found that birds have decided to trade down their testicle size to invest in other advantages.
These include colourful plumage, dancing displays, parenting and possibly increased intelligence.
Environmental biologist Joanna Baker of Reading University and colleagues conducted the research, which was devised to study evolutionary trade-offs.
‘All animals could benefit from having larger testes,’ Dr Baker told The Times.
‘Bigger testes produce more sperm and so give animals a competitive advantage over their relatives with smaller testes,’ she said.
The researchers found, however, that monogamous birds have undergone repeated and rapid decreases in testicle size over their evolutionary history.
Accounting for this, Dr Baker explains that testicles are very expensive to grow and maintain, meaning that they can be traded in for other advantages.
‘Around 90% of birds are socially monogamous — where producing more sperm doesn’t really matter — and so there has been lots of opportunity to reduce testes size in favour of other adaptations,’ she said.
‘Rapidly shrinking testes may have happened as a result of the fantastic diversity of behaviours and traits we observe in birds today.’
The include, she added ‘beautiful plumage colourations in birds of paradise, elaborate dancing displays in songbirds, through to dedicated parental care through extreme weather conditions in penguins.’
The researchers analysed the largest ever dataset of animal testes masses to determine how testicle sizes had changed across evolutionary history
The team suggest that smaller testes in monogamous birds evolved as they put energy into other traits — like colourful plumage, pictured, and dancing displays — instead
The researchers found that the birds with the smallest testicles were the Australian raven and the little raven.
Both birds are superficially rather dull in their appearance, however — a fact which initially seemed to contradict the main findings of the team’s analysis.
‘At first glance you might not think they are bright or colourful. But they are very iridescent. They are also notorious for their intelligence,’ Dr Baker told The Times.
‘Maybe,’ she added, ‘they trade testes size for intelligence.’
The researchers found that the birds with the smallest testicles were the Australian raven, pictured, and the little raven. Both birds are superficially rather dull in their appearance, however — a fact which initially seemed to contradict the main findings of the team’s analysis
Beyond just birds, Dr Baker and colleagues found that there was considerable variation in testicle sizes across all species with backbones, the vertebrates.
They noted enormous shifts in testes size — both getting bigger and smaller — across the last 400 million years of vertebrate evolution.
No other vertebrate group showed the same pattern of shrinkage as birds, however.
‘For example, we find massive testes size changes in a group of frogs called foam-nesting tree frogs,’ said Dr Baker.
‘These animals have an extremely unusual method of mating in which males gather around a female and fertilise eggs by whipping their sperm into a sort-of “foam” with their legs.’
‘Obviously, bigger testes that produce lots of sperm will give a male a competitive advantage in such foam nests: for example, the grey foam-nest tree frog has been known to mate with up to 12 males simultaneously.’
‘We find massive testes size changes in a group of frogs called foam-nesting tree frogs,’ said Dr Baker. ‘These animals have an extremely unusual method of mating in which males gather around a female and fertilise eggs by whipping their sperm into a sort-of “foam” with their legs’
‘We were surprised to find that no group of vertebrates showed any adaptations towards larger testes size during their evolutionary history,’ said paper author and evolutionary biologist Chris Venditti, also of the University of Reading.
‘Although all species could technically benefit from bigger testes, perhaps they are simply so expensive that maintaining size is enough.’
‘Although we thought that flight may have been an important factor in reducing testes size in birds, we don’t observe a similar pattern in bats – so clearly there is something else at play,’ he concluded.
The full findings of the study were published in the journal Ecology Letters.
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