Burying beetles don't learn from experience

Thursday, December 4, 2008

Male burying beetles' choice of mating tactics depends only on their intrinsic sense of how big they are, and not on experiences like fighting and mating, according to new research.

This means looking at a beetle's size, whether relative to others or absolute, is the best ways of predicting whether it will fight for a mate or try to pass on its genes through cunning and subterfuge.

Once a beetle has formed an idea of its own size and strength, no amount of losing fights or mating seems to change its mind.

'We thought these beetles' behaviour would be relatively flexible - it would seem to make sense to adjust what you are doing based on how well it works,' says Allen Moore, Professor of evolutionary genetics at Exeter University and one of the paper's authors. 'But we were surprised to find that when we tested all the possible environmental cues we could think of, none of them affected mating behaviour. They seem to be beetles of very little brain.'

Males of the species Nicrophorus vespilloides have two options in their efforts to find a partner. One is to find the corpse of a small vertebrate, defend it from other males, use it to attract a female and then bury it for consumption by larvae. The other is to release a pheromone to attract a female without taking possession of a carcass.

The first option often leads to violence - carcasses of the right kind are rare, so males compete fiercely for them. Bigger males are likely to benefit from this strategy, but smaller ones may lose out.

The second option is less demanding; if the male succeeds in attracting a female and mating with her, she will fly away in search of a suitable carcass. When she finds one, she may fertilise eggs using sperm that she has kept in specially-adapted storage organs.

Overall, the two methods have around the same effectiveness. Even when a female finds a carcass that is already guarded, around 15% of her offspring tend to come from previously-stored sperm. Indeed, females seem to prefer smaller males, despite their lower fighting ability. This is likely to be what maintains the diversity of sizes within the male population.

Size and fighting ability aren't the only factors at work; many males start the day looking for a carcass but switch to calling for a mate later on if they don't find one. And parentage seems to have an effect, with some males apparently inheriting a greater propensity to call for a mate.

A team of researchers at the Universities of Edinburgh, Exeter and Oxford set out to test how males decide which tactic to use. They asked if the beetles get their idea of their own size, and hence which tactic they should use, from experience.

The scientists tested whether larvae reared in different conditions became more likely to choose one tactic over another. The first difference they looked at was parenting - did larvae cared for by just one parent make different choices of tactics from those raised by two?

Alternatively, interacting with their peers while still at the larval stage could give individuals clues to their own size. Individuals from bigger or smaller broods would then be expected to have an exaggerated conception of their own largeness or smallness, since this self-image would have got more reinforcement early in life.

The researchers raised a large group of larvae from individuals caught in Cornwall. Once the larvae had hatched into adults, the scientists classified them as either big or small relative to its siblings, and then monitored which mating strategy each individual seemed to prefer.

Beetle combat

Another hypothesis was that individuals gain a sense of their own size by fighting others. The scientists tested this by taking a sample of individuals from both large and small groups and pitting them against each other in combats with stacked odds.

This involved dividing the sample into four categories - 'big winners', 'big losers', 'small winners' and 'small losers'. Winners were pitted against opponents smaller than them, while losers faced larger foes.

The hypothesis was that victory could lead 'small winners' to select mating tactics as if they were bigger than they in fact were, with 'big losers' subject to the reverse delusion.

A final possible explanation tested was that the beetles learn through experience of mating - individuals from each size class were given a variety of opportunities to mate, with some getting to mate lots and others not at all. The hypothesis was that successfully using a mating tactic might make males more likely to try it in future.

Statistical analysis of the results showed unexpected results. Larva conditions did have some effect on mating tactics, with individuals that were big relative to their siblings proving more likely to search for a carcass and less likely to call for a mate when they reached adulthood. Larvae that were big in an absolute sense showed the same trend even more strongly. Adult size also had an effect, with bigger adults calling less often.

But neither the number of parents nor the size of the brood of larvae had any effect. Success or failure in combat had no effect on future male behaviour. And experience of successfully mating using one strategy did nothing to change mating tactics in future.

The scientists think absolute rather than relative mass is the key factor, though the two potential causes are hard to disentangle. If this is the case, it raises the question of how the creatures can tell their own size without comparison to others.

The answer could be that the hormones and physiological pathways that determine an insect's size also form cues for its behaviour, so that big insects naturally act in the appropriate way without needing to confirm their size through experience.

Why might these insects have developed so as not to take new experiences on board? 'It could be that losing a fight is not a good predictor of whether you will lose more fights in the future - it's just the luck of the draw whether you happen to run into an opponent that is bigger than you,' suggests Moore.

Or it might just be that males get so few chances to reproduce that it is worth trying to exploit any that arise. Losing a fight usually just means running away; missing a trick may mean the beetle dies without mating.

Strategies for romance

'Burying beetles have very limited opportunities to mate, because doing so depends on finding a small, dead mammal, and they are not that common' Moore adds. 'The cost of making a mistake and not being able to mate is much higher than the cost of losing a fight.'

The researchers hope their work will shed light on more general questions of how males in various species choose between possible mating tactics. 'This may be more common than we realise,' says Moore. 'There is no reason to think burying beetles are different from other insects in this respect. We need to look at environmental cues in other species to see how much flexibility there is in their behaviour.'

Many other species possess more than one tactic for mating. Frequently, larger males choose a method that emphasises their strength and ability to fight off challengers, while smaller individuals take a more subtle approach, often managing to mate with females while bigger rivals are distracted.

In some species these differences are even mirrored in the animals' physiques - in many dung beetles, for example, big males develop horns to let them fight challengers for females and dung, while smaller males lack horns and try to sneak mating opportunities.

In these cases there seems to be a size threshold, above which one alternative always kicks in, meaning the male population is split into 'major' and 'minor' males.

But males of other species have sizes that are arranged along a continuum, and lack horns or any other physiological feature that makes it clear which tactic a male will use. In some cases which males choose which mating tactics seems to depend on a complex set of factors like the availability of resources, the distribution of sizes within the general population and how many other males are trying each tactic.

###

Natural Environment Research Council: http://www.nerc.ac.uk/

Friends