A female Xylocopa aruana-lieftincki, an Australian carpenter bee. Photo credit: Mario Vallejo-Marin.
A female Xylocopa aruana-lieftincki, an Australian carpenter bee. Photo credit: Mario Vallejo-Marin.
Bees have many different occupations. Some make perfume, while others chew leaves to build their nests, many are social, living in colonies, while others are solitary. But one thing unites all 20,000 bee species: they all buzz, beating their wings ∼200 times per second while rushing around. Bees can also range in size from minute stingless bees – no more than 2 mm in length – up to colossal leafcutters that are almost 4 cm long. But how do this enormous size range and lifestyle differences affect the characteristics of the defensive buzzes the bees produce when they want to frighten something off – from the pitch and duration, to the amplitude (swing range) of their body oscillations? ‘Bee vibrations have been studied in only a handful of species and we know very little about how they vary among species’, says Mario Vallejo-Marin from Uppsala University, Sweden, so he and a team of international collaborators from Australia, Mexico, Portugal, Sweden and the USA decided to set the record straight.
Initially, Vallejo-Marin visited Scotland's Orkney Islands and the Outer Hebrides with his wife, Gillian Vallejo, and their kids to collect bees, before joining colleagues in Australia and Mexico on separate bee-hunt expeditions. Across the field trips, the researchers collected 356 bees of 65 different bee types (taxa); back in each field lab, they pressed a tiny motion sensor against each bee’s body to encourage the insects to produce warning buzzes as they recorded the movement. ‘A big challenge was how to travel to remote places while collecting bee buzzes in freshly captured bees of all kinds. We had to improvise a lot’, he chuckles.
Eventually recording 15,000 warning buzzes, Vallejo-Marin describes how some only lasted 0.05 s, while a bee in Mexico maintained an astonishing 43 s long buzz. Some of the bees accelerated their bodies at just ∼0.2g, while others achieved extraordinary accelerations of ∼140g (1473 m s−2). But how did the strength and swing range (amplitude) of the body movements vary from species to species?
Looking at a graph of the width of the bees’ thoraxes (the wider the thorax, the more muscle is packed in), it was clear that the bees with the largest thoraxes produced the strongest buzzes with the greatest accelerations. However, the pitch of the buzz (determined by the frequency at which the bee vibrates its body) varied from 40 to 488 Hz regardless of the bees’ thorax size; so, thorax size does not affect the pitch of warning buzzes. In addition, when Vallejo-Marin took a closer look at the buzzes produced by bees of the same species, it was clear that individual bees could vary the strength of their buzzes and the strength varied from bee to bee. ‘The amplitude of the vibration seems to be under neural control’, says Vallejo-Marin, adding, ‘The bees can both turn the buzzes on and off and also modulate the amplitude of their vibrations’.
Knowing that some bees grip onto flowers and give them a good buzz to release pollen, Vallejo-Marin also looked at how the buzzes of buzz-pollinating bees and non-buzz-pollinating bees varied and admits that he was surprised that buzz-pollinating bees produce stronger buzzes with higher accelerations than bees that do not buzz pollen from flowers. ‘This might be a small step towards trying to solve the puzzle of why some bees, and not others, buzz pollinate’, he suggests.
So, the key to a big warning buzz is the amount of muscle that a bee can pack inside its thorax and Vallejo-Marin is curious to find out how much pollen bees liberate from flowers when giving them a good shake.
