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Along Came a Spider

A large female golden orb-web spider

A large female golden orb-web spider is often surrounded by many small males competing for proximity to her on the web. But size isn’t the only factor determining who wins each fight.

By Michael Kasumovic

The comparative size and weight of two animals determines the outcome of 80% of fights. Now a small spider has revealed the physiological factors that help explain the other 20% of contests.

Predicting winners and outcomes in sporting events is a billion dollar industry. There are complex algorithms that use winning and loss percentages, the location of the game, the players playing and even personal player feelings. Given all the variables, it’s quite impressive that anyone can predict anything at all!

Yet predicting contest outcomes in the animal kingdom is relatively simple. Think of male lions brawling over a pride, elephant seals fighting over a harem or butterflies aggressively flitting over a sunny spot. In each of these instances you could predict the winner by simply measuring the size, weight or weaponry of each of the competitors before the contest. In fact, using any one of these traits alone gives you about 80% accuracy.

But if you’re like me you realise that this is only a part of the story, and wonder why these traits aren’t perfect predictors. Which factors explain the other 20%?

One of our recent studies provides some insight into these questions. When controlling for weight and size, we discovered that a male’s internal physiology plays an important role in winning competitions. What this means is that what our parents have been telling us is true: it really is what’s inside that counts.

Why Size Isn’t Everything

Along with navigating a world full of predators, individuals must acquire various resources that are necessary to reproduce. Given that populations are usually large and resources scarce, it means that competitions in the animal world are common. One of the interesting outcomes from the frequent competitions is the evolution of ritualised contest behaviour.

Not only do contests involve a great deal of energy expenditure, they can also result in injury or even death. To minimise these costs, rivals inherently agree to a set of rules: start things off slowly and escalate only when necessary. What this means is that males begin a contest by displaying towards one another, which allows each individual to assess its likelihood of winning.

As you can imagine, external traits play an important role at this point. If you walked up to Mike Tyson in an alley, I don’t think it would be long before you turned and fled! This is no different in non-human animals. Smaller individuals quickly retreat if the size difference is large, which means that most contests don’t escalate further than displays, and size becomes a strong predictor of winning.

This changes when the size difference decreases. Individuals are less willing to give up to a similar-sized rival, and contests escalate to physical interactions. In escalated contests, external traits become poorer predictors of contest outcomes. In other words, when differences in size become less dramatic, other traits become more important. What those other traits are, however, are difficult to pinpoint.

One of the factors that may play a role is individual motivation. In non-human animals, motivation is determined by changes in social environment that result in differences in the value of a resource or the perception of the quality of an opponent.

My research shows that such factors can be quite important. For example, a previous win can increase the likelihood of winning again by 25%, and sole access to a mate leads to an individual fighting 12% harder. Despite their importance, in neither of these instances do social factors trump external traits. They simply moderate their importance.

Motivation, however, cannot explain contest outcomes in naïve individuals, suggesting that there still must be some unknown factor that contributes to the higher competitive ability of males. This is the question that I aimed to answer.

Along Came a Spider

Golden orb-web spiders are part of an Australian summer. Backyards, walkways and hiking trails are often covered in large golden-tinged webs, with large hand-sized females occupying the centre of the web. When looking closer, you can also see minute males about 5% of a female’s body size waiting on the web for an opportunity to mate.

The females attract many suitors, so the males end up competing for the closest position to her on the web. Proximity to a female is extremely important for male golden orb-web spiders, as the closest males reach females first when the opportunity arises – first in, best-dressed really does ensure more offspring for males.

To examine what other factors may play an important role in contest outcomes, Frank Seebacher and I collected 210 males and 12 females and brought them back to the laboratory, where we could better control the contests. For each trial, we released three females within an artificial 15.5m3 screened-in enclosure in a glass house and allowed them to build a web overnight. The next morning we released 12 individually marked, similarly-sized males and allowed them to find and settle on the webs.

Males generally found females within 15 minutes, and began jockeying for position almost immediately after. To ensure males were provided a long enough time to settle their differences but not long enough that any contest outcomes would affect their physiology, we returned and collected the closest and furthest males on each female’s web an hour after first introducing them.

Since males were all similarly sized, their external traits did not predict who won. But this provided us with a unique opportunity to examine the role of the physiological system in determining contest outcomes.

To do this, we temporarily sealed males within airtight containers and, using an oxygen meter, examined how quickly males consumed oxygen. We first examined how much oxygen individuals consumed while at rest. In invertebrates, this provides information into how much energy the physiological system requires to maintain the body in working order.

We next forced males to exercise, and examined how much oxygen they consumed to estimate how active individuals can become. Although it is wonderful to think of little spiders running on treadmills, the reality is a bit less exciting. Within each container we also placed a small magnetic bar. When the bar within the container is turned continuously from an outside source, it induces a male’s natural prey-catching behaviour and allows us to estimate how much oxygen they consume while they are physically active.

Such a measurement is important as it allows us a glimpse into how much energy individuals have when they are performing a specific behaviour. As you can imagine, a runner that has more access to energy while active will be able to run faster and further.

But the different types of energy required for burst speed and continuous behaviour differs from a metabolical perspective. Thus, along with measuring their metabolic rate, we also examined three different biochemical pathways through which metabolic energy is supplied to better understand where the energy is stored and how it is produced.

Over the course of the experiment we collected information on 36 size-matched spiders, and compared the winners with the losers and found some interesting results.

Incy Wincy Spider

The winners and losers did not differ in their resting metabolic rate. This means that the energy associated with repairing and maintaining all the internal processes isn’t related to how well an individual performs in a contest.

However, males that were physically closer to females, and therefore have a greater likelihood of mating, 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, thereby providing them with a competitive advantage.

We further found that this extra energy production was a result of an increased density of mitochondria – the energy powerhouses of the cell. Males that were more likely to win differed in their ability to produce energy at a cellular level.

The male spiders that were closest to females were thus 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, our results suggest that these differences may be determined during development.

Another interesting facet of male golden orb-web spiders is that they are developmentally plastic. This means that they alter how they look and perform in response to specific environments they experience while they are developing.

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 that along with this plasticity in development time, size and weight, males may also shape their underlying physiology. But we’ll have to complete more manipulative experiments to determine this conclusively.

Our experiment provided some of the strongest evidence that an animal’s internal physiology determines how well he will perform and the number of offspring he will leave in the next generation.

Results like ours help us to understand why we sometimes see much smaller males outcompete larger counterparts. In these cases, an individual’s internal physiology may allow it to escalate and compete harder, which outdoes the size benefit we all take for granted.

For now, I’m happy to ease the collective minds of all the little spiders out there and highlight that, with a little training, maybe we can all become better competitors.

Michael Kasumovic is a Lecturer and ARC DECRA Fellow at the University of New South Wales. Along with examining competitions in spiders and crickets, he’s interested in understanding how competitions (both real and virtual) affect how humans behave as well.