A woman walks past a stall at a market in Banjul, Gambia December 15, 2016. (Photo by Afolabi Sotunde/Reuters)

Six month- old baby elephant “Clear Sky” is kept afloat with the help of a harness during a hydrotherapy session at a local veterinary clinic in Chonburi Province on January 5, 2017. After losing part of her left foot in a snare in Thailand, baby elephant 'Clear Sky' is now learning to walk again – in water. (Photo by Roberto Schmidt/AFP Photo)

A tea garden worker plucks tea leaves inside Aideobarie Tea Estate in Jorhat in Assam, India, April 21, 2015. Unrest is brewing among Assam's so-called Tea Tribes as changing weather patterns upset the economics of the industry. Scientists say climate change is to blame for uneven rainfall that is cutting yields and lifting costs for tea firms. (Photo by Ahmad Masood/Reuters)

A man poses for a photo with a life size cutout of Pope Francis at Bicentennial Park where the pontiff celebrated Mass for an estimated half-million people, in Quito, Ecuador, Tuesday, July 7, 2015. Francis told an estimated half-million people gathered for the Mass that in a world divide by wars, violence and individualism, Catholics should be “builders of unity”, bringing together hopes and ideals of their people. (Photo by Ana Buitron/AP Photo)

Residents make their way along a fence as they try to cross a flooded street after heavy rainfall hit Wuhan, Hubei province, China, July 23, 2015. Severe downpours hit the city on Thursday, flooding streets and shutting down public transportations, local media reported. (Photo by Reuters/China Daily)

A woman, wearing a traditional Sevillana dress, performs as she asks for alms in downtown Malaga, Spain, April 28, 2016. (Photo by Jon Nazca/Reuters)

Yuanlei Liao, the singer of hardcore band called Thank Ray wears a read scarf after performing on the stage of Mao Live House during the club's last public concert night in central Beijing, China April 23, 2016. Mao Live House, a prominent live rock music venue in Beijing, shut its doors on the weekend, the latest closure to hit China's rock music scene. Owner Li Chi said the club, popular among fans of punk, metal and alternative rock since it opened nine years ago, was forced to close due to tighter rules on live performances. (Photo by Damir Sagolj/Reuters)

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)