Chien Lee / Nymphalid Butterfly (Cupha erymanthis) caterpillar standing guard over a clutch of parasitic wasp pupae that secreted hormones into the caterpillar to guard them after they had c the caterpillars insi...

Jabruson / African Forest elephant (Loxodonta africana cyclotis) bull in musth following female elephant scent, Dzanga-Ndoki National Park, Central African Republic

Christophe Courteau / Sable Antelope (Hippotragus niger) bull showing Flehmen response to female hormones, Okavango Delta, Botswana

Christophe Courteau / West African giraffe male smelling female in oestrus {Giraffa camelopardis peralta}.

Christophe Courteau / West African giraffe male smelling female in oestrus {Giraffa camelopardis peralta}.

Charlie Summers / African elephant (Loxodonta africana), close up of face showing patch of secreted hormone, a sign that the animal is in musth, Ngorongoro Crater, Tanzania, East Africa

Charlie Summers / African elephant (Loxodonta africana) close up of face showing a patch of secreted hormone, a sign that the animal is in musth, Ngorongoro Crater, Tanzania

Toby Sinclair / Musth gland on tusker Indian elephant {Elephas maximus} Gangaramaya Temple, Sri Lanka

Nigel Cattlin / Close up showing the effect of Gibberellic acid (GA, gibberellin), a growth hormone, on the internode length of nodes 2 - 3 in Barley (Hordeum vulgare) crop plants.