CrowdScience BBC World Service

How do we know where a sound is coming from?
Another chance to hear this ear-opening episode, exploring a question from CrowdScience listener Chiletso. One day, he heard his son bounce a ball and instantly knew the direction it was travelling. How?
Anand Jagatia sets out to discover what makes left, right, up and down sound so different.
First, he gets blindfolded, so Alan Archer-Boyd, former auditory scientist and lead engineer at BBC R&D, can put his sound localisation skills to the test. It turns out that having two ears and pinnae, those flappy bits of cartilage on the side of your head, help a lot.
Professor Eric Knudsen shares how the barn owl’s asymmetrical ears allow it to hunt mice, even in complete darkness.
And Anand uncovers how far he can push his own spatial hearing. Blind activist and researcher Thomas Tajo teaches him how to echolocate like a bat, and Dr Lore Thaler explains what is going on in the brain of experienced echolocators.
This programme was originally broadcast in March 2023.
Presented by Anand Jagatia
Produced by Florian Bohr for the BBC World Service
Image: Boy with hands at his ears
Credit: Silke Woweries/Getty Images

Today there are over eight billion people on Earth. That’s an awe-inspiring figure… but how does it compare to the vast numbers who came before us? Listener Alpha wants to know how many people have ever existed, so CrowdScience sets out to do a historical headcount.
The Population Reference Bureau in the USA estimated this number back in the 90s, and have been updating their calculations ever since. Demographer Toshiko Kaneda explains how their model works, the assumptions it makes – and the huge uncertainties around the number it comes out with.
We first need a date for when ‘humans’ first began, so Caroline travels to the Natural History Museum in London to meet human evolution expert Chris Stringer, and marvel at his collection of replica fossil skulls. Chris demonstrates how to distinguish our species, Homo sapiens, from other species like Neanderthals. When did these species first appear - and which of them count as human?
And once you know where to start the clock, how do you estimate the numbers of people alive at different points in history? For a population demographer like Walter Scheidel, it helps that some ancient civilisations kept detailed censuses, a few of which have survived to the present day. Caroline and Walter pour over one of these census fragments, and learn how to combine them with other archaeological clues to get some very rough numbers.
And finally: what does the future of our population look like? Poonam Muttreja from the Population Foundation of India discusses developments in the world’s most populous country, as well as the big demographic trends ahead for humanity.
Presenter: Caroline Steel
Producer: Phil Sansom
Additional Recording: Umaru Fofana
Editor: Cathy Edwards
Production Co-ordinator: Connor Morgans
Studio Manager: Sue Maillot
Featuring:
Toshiko Kaneda, Technical Director of Demographic Research, Population Reference Bureau
Chris Stringer, Research Leader in Human Evolution, Natural History Museum London
Walter Scheidel, Professor of Classics and History, Stanford University
Poonam Muttreja, Executive Director, Population Foundation of India

We spend a lot of our time thinking about climate change, but listener Paul has a question that isn’t usually part of the conversation. He wants to know whether a hotter atmosphere will affect how often volcanoes erupt, or make them more explosive when they do.
CrowdScience travels to New Zealand to search for answers, exploring volcanic craters and discovering traditional Maori knowledge about volcanoes.
Contributors:
Geoff Kilgour, Volcanologist, Geological and Nuclear Sciences Taupo, New Zealand
Heather Handley, Volcanologist, University of Twente, The Netherlands
Pouroto Ngaropo, Historian and Matauranga Māori expert, Rotorua, New Zealand
Presenter: Caroline Steel
Producer: Emily Bird
Editor: Cathy Edwards
Production Co-Ordinator: Connor Morgans
Sound Engineer: Steve Greenwood
(Photo: Icelandic volcano. Credit: KRISTINN MAGNUSSON/Getty Images)

Many of us have habits that calm us down in times of stress. Things we find deeply comforting, like sucking our thumb or biting our nails. We might not even be aware we’re doing them, but they play a fundamental role in helping us regulate our emotions.
Our question this week comes from CrowdScience listener and nail-biter, Ash. He wants to know where these habits come from. And since his pet dog is also a nail-biter: do we share these traits with other animals?
Recently, a video of a mouse cleaning up a man’s shed took the internet by storm. Was this a house-proud mouse, or was it the animal's way of making sense of a frenetic environment? An emerging field of scientists focusing on animal behaviour and emotions help us shed some light on such questions.
Along the way we meet a dog training specialist, learn what a sniffari is, go for playtime with a thumb-sucking otter, and visit an OCD clinic. We’ll also be getting tips on how to give your pets the best home environment, and meet an animal enrichment officer in South Africa, who knows how to spot the signs of an unhelpful habit developing.
Contributors:
Karolina Westlund, Ethologist, Stockholm University and ILLIS
Ben Terry, CBT Therapist, Priory Hospital North London
Karin Pienaar, Animal Behaviourist, COAPE International
Candice Ward, Animal Behaviourist, Johannesburg Zoo
Jaak Panksepp clip: The science of emotions: Jaak Panksepp at TEDxRainier
Producer: Robbie Wojciechowski
Presenter: Alex Lathbridge
Editor: Cathy Edwards
Production co-ordinator: Connor Morgans
Additional recording by Elna Schutz
(Photo: Portrait of border collie puppy biting a curtain. Credit: Rawlstock/Getty Images)

Crowd Science listener Maik wants to know what the BMI is and what his BMI score says about his body. He trains dogs for a living and wonders if, like different breeds of dog, we simply have different body types?

Marnie Chesterton comes up with some answers, talking to doctors about how the BMI is used and misused in clinical practice, and looks at some alternative methods for measuring our body composition. She also sits down with philosopher Kate Manne to discuss the realities of living in a fat-phobic world.

We hear from Tonga in the South Pacific, where high BMI scores have labelled the country highly obese. But this is not necessarily how Tongans see themselves.

And Marnie finds out if the BMI will continue to be used across the world as an important health marker or whether it is destined for the scrap heap of medical history.

Contributors:
Professor Kate Manne
Dr Francesco Rubino
Dr Naveed Sattar
Professor Brendon Noble
Technician Leah Siegel
Fononga Pulu
Sela Latailakepa

Presenter: Marnie Chesterton
Producer: Richard Walker
Editor: Cathy Edwards
Production co-ordinator: Connor Morgans
Studio manager: Emma Harth

CrowdScience listener Gregory wants to know what affects the way we see the colours of the world. He was looking at a blue summer sky with a friend and they got to wondering whether they both see the same colour blue. So what does influence our vision of the colours that surround us? Could eye colour have anything to do with it? And can we ever really know if your blue sky is the same as mine?
Caroline Steel comes up with some answers, talking to colour scientists about their research into the multiple factors that enable us to see in multi colours, from the intricate biology of our eye to the changing environment around us.
She also investigates her own colour vision and solves a personal mystery, discovering why the world has always looked a slightly different colour from each eye.
Contributors:
Professor Jay Neitz, Department of Opthalmology, University of Washington, US
Professor Hannah Smithson, Department of Experimental Psychology, University of Oxford
Dr Juan Perea García, researcher, Department of Cognitive Psychology, University of Leiden
Dr Lauren Welbourne, researcher, Department of Psychology, University of York
Dr Adam Bibbey, lecturer in sport, Department of Sport, Oxford Brookes University
Presenter: Caroline Steel
Producer: Jo Glanville
Editor: Cathy Edwards
Production co-ordinator: Connor Morgans
Studio manager: Jackie Margerum
(Photo: LWA)