Vanessa Couldridge of the Department of Biodiversity and Conservation Biology at the University of the Western Cape (UWC), South AfricaCredit: Morgan Morris
Few animals can escape the racket that their human neighbours make.
Creatures as diverse as frogs, bats and birds, beasts cordoned off in protected areas, and even dolphins venturing too close to human habitats will get an earful. This proximity has triggered sweeping changes in animal behaviour, impacting everything from their cognitive abilities (like city birds’ ability to recall their food sources) to their ability to keep track of predators. More worryingly, it can affect their reproductive habits.
Even noisy creatures can’t keep up, as Vanessa Couldridge of the Department of Biodiversity and Conservation Biology at the University of the Western Cape (UWC), South Africa has found.
Among Couldridge’s research interests is bioacoustics, a field that looks into how animals produce, disperse and receive sound, thus her interest in bladder grasshoppers.
Insects are by far the largest group of sound-producing creatures. But even in that vast company, bladder grasshoppers – most of whose 14 species can be found in South Africa – are no wallflower, says Couldridge. “What makes them unusual is that they’re very highly specialised for sound communication,” she explains. The males’ inflated abdomen, for instance, serves as an acoustic boombox, making the grasshopper among the loudest sound-producing insects in the world.
Both male and female bladder grasshoppers also boast six pairs of ears on their abdomens, while most other grasshoppers have to settle for a single pair. “That allows them to detect sounds and to localise sounds to a high degree of accuracy,” notes Couldridge.
“But it also makes them highly sensitive to other noises, with potential calamitous and far-reaching consequences.
“Because insects play such an important role in ecosystems – as food and pollinators, for instance – any reduction in their numbers can be expected to have a knock-on effect on biodiversity in general.”
Adapting the mating calls
In a recent study, Couldridge and a former doctoral student Rekha Sathyan sought to find how male bladder grasshoppers of the species Bullacris unicolor would adapt their ‘advertising’ or mating calls to their noisy environments. She compared two groups of the grasshoppers: one found in a small nature reserve conveniently abutting her university campus – but thus vulnerable to the noise of student and vehicle traffic, and an industrial area across the road – and the other on a measurably quieter nature reserve a few kilometres away.
While results, as expected, varied between the two sites, findings were significant and surprising enough to suggest that the insects are incredibly sensitive to human presence. For one, and contrary to other animal studies, the males on both sites lowered the frequency or pitch of their calls during noisy periods, reported the researchers.
What’s more, the more the traffic noise, the greater the intervals between calls. In addition, the grasshopper would also reduce the total number of calls the noisier things got. The grasshoppers on the noisier site also delayed their peak calling activities until the quieter hours in the evening. “This may help to conserve energy, as the production of advertisement calls is one of the most energetically expensive activities,” write the researchers.
One other finding of interest, that needs more exploring is the role weather conditions, particularly temperature, plays in calling. The warmer the conditions, the smaller the calling intervals while call pitch also increased with temperature.The challenge, say the researchers, is disentangling the effects of anthropogenic noise from environmental variables. But there is enough evidence to suggest that human noise plays a strong role in how and when, and how often, the grasshoppers go a-mating, and how successful they’re likely to be.
“This human destruction will lead to biodiversity loss and the extinction of species,” says Sathyan, now a postdoctoral research fellow with the South African National Biodiversity Institute (SANBI).“Human noise is increasingly encroaching on the habitats, behaviours, reproduction, and evolution and making a lasting imprint on this endemic species of insects of South Africa,” she adds.
