Eric Baccega / Chimpanzee (Pan troglodytes) female aged 37 years, portrait. Beauval Zoo Parc, France. Captive.

Karine Aigner / Scientist Nathan Cooper blowing feathers out of the way while measuring the size of the pectoral muscle of a Kirkland's Warbler (Setophaga kirtlandii) during research. Cat Island, Bahamas. April 2...

Karine Aigner / Scientist Nathan Cooper measuring the size of the pectoral muscle of a Kirkland's Warbler (Setophaga kirtlandii) during research. Cat Island, Bahamas. April 2017.

Floris van Breugel / High speed exposure of a Fruit fly (Drosophila melanogaster) taken through a microscope. In this image flight muscles express a genetically engineered protein called GCaMP, which fluoresces green in t...

Eric Baccega / Naro San Bushman making a bow string by rolling a strip of sinew from the muscle of an antelope, Kalahari, Ghanzi region, Botswana, Africa. Dry season, October 2014.

David Tipling / Xray of shot Turtle Dove (Streptopelia turtur) showing gun shot lodged in muscle, during BirdLife Malta Springwatch Camp, Malta, April 2013

Christian Ziegler / Leaf-nosed Bat (Phyllostomidae) flight muscles and blood vessels in the wing membrane, Smithsonian Tropical Research Station, Barro Colorado Island, Panama

Philippe Clement / Pullet carpet shell (Venerupis senegalensis) with labels showing anterior and posterior adductor muscle scars, pallial line and pallial sinus, Belgium

Philippe Clement / Pullet carpet shell (Venerupis senegalensis) showing anterior and posterior adductor muscle scars, pallial line and pallial sinus, Belgium

Nature Production / Japanese five lined skink {Eumeces japonicus} cross section of its tail that it has broken off by itself, Japan

Jane Burton / Domestic Cat {Felis catus} Polydactyl tortoiseshell showing muscle wastage and in poor condition, 'Tortie-toes'

Jane Burton / Domestic Cat {Felis catus} Polydactyl tortoiseshell showing muscle wastage and in poor condition, 'Tortie-toes'