Behaviour: Mammals category. Giant Gathering by Tony Wu, USA. Dozens of sperm whales mingled noisily off Sri Lanka’s northeast coast, stacked as far down as Tony could see. This was a congregation of dozens of social units, like a gathering of the clans. Aggregations like this could be a critical part of the whales’ rich social lives but are rarely reported. Some two thirds of the population was wiped out before commercial whaling was banned in 1986. This kind of major gathering could be “a sign that populations are recovering”, says Tony. (Photo by Tony Wu/Wildlife Photographer of the Year 2017)

In this 2017 photo provided by Simon Pierce, Jonathan Green checks on a fin-mounted satellite tag on a whale shark in the Galapagos Islands area of Ecuador. Despite typically being bigger than a double-decker bus, the elusive whale shark has only tiny, almost useless teeth. It's also one of the least understood animals in the ocean. (Photo by Simonjpierce.com via AP Photo)

Female of pygmy hippopotamus called Malaya from Netherlands in zoo in Dvur Kralove nad Labem, Czech Republic, February 22, 2016. (Photo by David Tanecek/CTK via ZUMA Press)

Fans dressed up in costume await the cast members on the red carpet at the world premiere of the film "The Avengers: Endgame" in Los Angeles, California, April 22, 2019. (Photo by Mario Anzuoni/Reuters)

Dressed in ancient Chinese clothes, children at a local kindergarten dance and make traditional perfumed medicine bag to celebrate the upcoming Dragon Boat Festival, Ganyu district, Lianyungang city, east China's Jiangsu province, 19 June 2020. (Photo by Imaginechina via AP Images)

People cling on to a crowded train as it leaves a railway station during the ongoing Coronavirus Disease (COVID-19) outbreak in Ghaziabad, India, September 21, 2021. (Photo by Anushree Fadnavis/Reuters)

Burning candles illuminate graves as a Bangladeshi man observes a contemplative moment on All Souls' Day at the Dhaka Christian Cemetery at Wari in Dhaka, Bangladesh, 02 November 2021. People on All Saints Day and All Souls Day pay respect to deceased relatives and loved ones by placing flowers on their tombs or maintaining their graves and lighting candles. (Photo by Monirul Alam/EPA/EFE)