Gavin Hellier / Wall Street and the New York Stock Exchange signs, downtown Financial District, Manhattan, New York City, USA 2009

Marie Read / Pileated Woodpeckers (Dryocopus pileatus), pair during incubation exchange, female clinging to treetrunk as male comes out of nest hole entrance, spring, New York, USA, May.

David Pattyn / Great Crested grebes (Podiceps cristatus) performing courtship dance and exchanging weed as part of ritual, La Dombes area, France, June

LYNN M. STONE / Close up view of Red abalone shell (Haliotis rufescens) photo shows three apertures / holes that living abalone had used for water exchange and the shell reveals the mother of pearl, or nacre, surface...

Tom Gilks / Woman bank clerk in foreign exchange office, Senegambia, Gambia, 2007

Tom Gilks / Woman bank clerk in foreign exchange office, Senegambia, Gambia, 2007

Tom Vezo / Royal Tern (Thalasseus maximus) pair in food exchange part of courtship display, Rio Grande Valley, Texas

Piotr Naskrecki / Gossamer-winged Butterfly (Lycaenidae) caterpillar, tolerated by Weaver Ants in exchange for honeydew, Guinea, West Africa

Willem Kolvoort / Egg strings of Common european toad (Bufo bufo) strung along aquatic plants for good oxygen exchange, sand winning pit, Holland

Doug Perrine / Masked booby {Sula dactylatra melanops} pair exchange gift of coral in courtship display, Flinders reef, Coral Sea, Australia

Gerry Ellis / Mountain Gorilla (Gorilla gorilla beringei) protection, payment in exchange for snares has helped reduced poaching around Karisoke Research Center, Rwanda

Mark Moffett / Carpenter Ant (Camponotus sp) on tendril of host Villose Pitcher Plant (Nepenthes villosa) a symbiotic relationship where the ants help host digest insects in exchange for living space, Borneo