Marie Read / Wood Ducks (Aix sponsa), females behave aggressively toward a male (second from right) that is trying to join them on a floating log, autumn, New York, USA, October.

Marie Read / Canada geese (Branta canadensis) two during territorial fight in early spring, Ithaca. New York, USA

Marie Read / American avocets (Recurvirostra americana) two during aggressive (territorial) encounter, Bear River Migratory Bird Refuge, Utah, USA, May.

Paul Hobson / Parrot snake (Leptophis ahaetulla) in aggressive pose with mouth open. Costa Rica.

Ingo Arndt / Blue Crab (Discoplax hirtipes) with claws raised in defensive / aggressive posture, endemic to Christmas Island, Indian Ocean, Australian Territory

Ingo Arndt / Blue Crab (Discoplax hirtipes) with claws raised in defensive / aggressive posture, endemic to Christmas Island, Indian Ocean, Australian Territory

Krijn Trimbos / Great crested grebe (Podiceps cristatus) with head low to the water in aggressive display, The Netherlands. March.

Doug Perrine / Female Hawaiian monk seal (Monachus schauinslandi) defending her pup from two males, which are aggressive with each other, Molokai Island, Hawaii, USA.

Alex Hyde / Baboon Spider (Pterinochilus murinus) in aggressive posture, photographed on a white background. Captive, originating from Africa.

Shattil & Rozinski,Shattil & Rozinski / Black bear (Ursus americanus) cub climbing a fence, Minnesota, USA, May.

Denis-Huot,Denis-Huot / White-backed Vulture (Gyps Africanus) feeding, Masai-Mara Game Reserve, Kenya

Andy Rouse / African Lioness (Panthera leo) portrait, with aggressive expression because of cubs nearby Maasai Mara, Kenya, Africa