Whether it’s two lions fighting over a pride or two butterflies fighting over a sunny spot, decades of nature shows have led the average watcher to conclude that bigger, stronger males win competitions.
But despite David Attenborough’s confident and assuring voice, research demonstrates it’s common to find smaller males out-competing larger males. In fact, the larger and more similar in size the combatants, the less of an accurate predictor size is.
But if it size isn’t the reliable predictor we always assumed, what other traits are important? My colleague Frank Seebacher and I – in research that was published today in Biology Letters – found that when male spiders were similar in size, a male’s success depended more on his physiological capacity than his size.
Along came a spider …
Just like the underlying differences in physiology between sprinters, marathon runners and the average individual, it seems that a spider’s internal physiology plays an important role in winning a mate.
Golden orb web spiders are part of an Australian summer. When walking or hiking along paths, it is common to find groups of females' webs in aggregations and males jockeying for the closest position.
Proximity to a female on her web is extremely important for these males as the closest males have the first opportunity to mate – first in, best-dressed really does ensure more offspring for males.
Although competition for position is intense, a male’s size doesn’t always declare him a winner. So, we collected 210 males and 12 females and decided to examine what they did. We released three females within an artificial 2.5m3 screened-in enclosure in a glass house at UNSW and released 12 individually marked, similarly sized males.
After an hour, we returned and collected the closest and furthest males on each female’s web and examined how physiologically capable they were.
By sealing males in airtight containers and using an oxygen meter, we could examine how quickly males were consuming oxygen when at rest. We then compared this to a male’s active metabolic rate by forcing males to exercise.
We did this by using a spinning magnetic bar to activate their natural prey-catching behaviour. Males that consumed more oxygen while exercising indicated a superior metabolic capacity.
Along with measuring their metabolic rate, we examined three different biochemical pathways to examine whether differences in metabolic rates could be attributed to differential energy production.
After collecting information on 36 spiders, we compared the winners against the losers and found some interesting results.
It’s what’s inside that counts
The winners and losers did not differ in their resting metabolic rate. But males that were physically closer to females had a significantly higher active metabolic rate. What this means is that winning males were able to ramp up their performance to a higher level when in competition.
We further found that this extra energy production was a result of an increased density of mitochondria – the energy powerhouses of the cell.
Our results demonstrated a strong link between an individual spider’s internal physiology and his potential fitness: a difficult connection to make. The male spiders that were closest to females were more like sprinters than marathon runners as they had the physiological capacity for burst activity that provides them with a competitive, and therefore fitness, advantage.
But unlike sprinters that need to train to improve their physiology, these differences may be determined during development. Male golden orb web spiders are developmentally plastic, which means they alter how they look in response to specific environmental triggers.
When developing around a high density of available females, males mature smaller and more quickly which allows them to locate and mate with females earlier. In contrast, when males are developing around more rival males, they take longer to mature larger to have the capacity to out-compete males.
Our new results suggest sexual selection has also shaped plasticity in underlying physiological traits. But we’ll have to complete more manipulative experiments to determine this conclusively.
For now, we’re happy to know we’ve eased the collective minds of all the little spiders (and maybe humans) out there.
Felix MacNeill
Environmental Manager
Yay - as a professional short person, let me thank you from the bottom of my heart (which is probably at a slightly lower altitude to the bottom of yours!).
I have always treasured an old Roman hunting adage (I think from Senecca): Saepe teneture aper a cane non magno - 'Often the boar is held by one of the smaller dogs.' It's nice to see some scientific validation.
Chris Saunders
retired
Certainly makes sense to me. If you remember your teenage years, upon entering a party and all the young men are leaning on walls and lounging on the sofa and yet some sharpen up and jump to the fray. Also explains why a depressive becomes such a burden around one’s neck. Can we go so far to suggest that the ability to pick up speed like a sprinter gets first choice and then the ability to adapt to being a ‘stayer’ promotes longevity?
Paul Rogers
logged in via Twitter
Increased density of mitochondria in animals (including humans), usually implies better endurance performance than sprint performance.
But maybe arachnid physiology in relation to fitness and speed is different.
Did you measure their muscle fibre type :-)
Dale Bloom
Analyst
"Did you measure their muscle fibre type"
Are you suggesting the spiders be dissected?
I would think the 222 golden orb spiders were collected from the wild, and then many, or all would have died.
Paul Rogers
Manager
Dale, you need to learn about smilies my good man (pejorative).
I don't even kill the redbacks that infest the lid of my compost bin; not to mention gently transporting the huntsmen outside because they scare guests . . .
BTW, you're a vegan (non-pejorative) are you?
Dale Bloom
Analyst
I think it would be difficult to measure the muscle fibre type of the male spiders without dissecting them.
I myself have been taught not to capture wild animals, and I don’t.
I also have been taught not to torment animals, or put them under stress, and I don’t.
In fact, at this time of year, I go around the house a pick up beetles and put them on the grass outside.
But I wonder how many evolutionary biologists in universities have been trained not to capture wild animals, or torment them, or put them under stress.
Dale Bloom
Analyst
Personally I try not to denigrate males or make up stories about males, and I haven’t been trained to do this anyway.
I also don’t go along with carrying out absurd experiments on voiceless animals, or carrying out cruelty on voiceless animals, but to prove the theory made up by this university evolution researcher and lecturer about the male gender, then the following could be done.
Collect 210 male evolution researchers and 12 female evolution researchers, and then release three females within…
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