Morley Read / Close-up of Agua rica leaf frog (Phyllomedusa ecuatoriana) eggs after 11 days development, showing beating heart and blood flowing over gills, Ecuador. Endangered. (non-ex)

Morley Read / Developing embryo of the Ecuadorian monkey frog (Phyllomedusa ecuatoriana), showing heart beating and blood flowing, Morona Santiago Province, Ecuador, (non-ex)

Danny Green / Japanese crane (Grus japonensis) displaying, wings in heart shape, Hokkaido, Japan, February

Morley Read / Close-up of Agua rica leaf frog (Phyllomedusa ecuatoriana) eggs after 11 days development, showing beating heart and blood flowing over gills, Ecuador. Endangered. (non-ex)

Morley Read / Close-up of Agua rica leaf frog (Phyllomedusa ecuatoriana) eggs after 11 days development, showing beating heart and blood flowing over gills, Ecuador. Endangered. (non-ex)

Morley Read / Close-up of Agua rica leaf frog (Phyllomedusa ecuatoriana) eggs after 10 days development, showing beating heart and blood flowing over gills, Ecuador. Endangered. (non-ex)

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two adults brooding begging chicks aged five weeks. Atka Bay, Antarctica. September.

Matthew Maran / Meat offal, including sheep hearts, for sale at butchers counter. North London, England

Frederique Olivier / Panning shot from an Emperor penguin (Aptenodytes forsteri) to a chick looking around and calling, Adelie Land, Antarctica, January.

Frederique Olivier / Emperor penguin (Aptenodytes forsteri) chick calling, with a heart shaped patch on its chest, Adelie Land, Antarctica, January.

Frederique Olivier / Emperor penguin (Aptenodytes forsteri) chick with a heart shaped patch on its chest approaching an adult, calling, Adelie Land, Antarctica, January.

Frederique Olivier / Emperor penguin (Aptenodytes forsteri) chick walking, with a heart shaped patch on its chest, Adelie Land, Antarctica, January.