Daniel Heuclin / Human botfly (Dermatobia hominis) larvae in dogs skin, showing breathing tube, French Guiana.

Daniel Heuclin / Human botfly (Dermatobia hominis) larvae in dogs skin, showing breathing tube, French Guiana.

Daniel Heuclin / Human botfly (Dermatobia hominis) larvae, on dog fur, French Guiana.

Yukihiro Fukuda / Japanese squirrel (Sciurus lis) male trying to climb up a thin branch after an female in oestrus , Mount Yatsugatake, Nagano Prefecture, Japan, February. Endemic species.

Yukihiro Fukuda / Japanese squirrel (Sciurus lis) running after an female in oestrus in the snow, Mount Yatsugatake, Nagano Prefecture, Japan, February. Endemic species.

Yukihiro Fukuda / Japanese squirrel (Sciurus lis) running after an female in oestrus in the snow, Mount Yatsugatake, Nagano Prefecture, Japan. Endemic species.

Yukihiro Fukuda / Japanese squirrel (Sciurus lis) trying to climb up a thin branch after an female in oestrus , Mount Yatsugatake, Nagano Prefecture, Japan, February. Endemic species.

Anup Shah / Eland (Taurotragus oryx) male checking if female is in oestrus. Masai Mara National Reserve, Kenya, July

Anup Shah / Eland (Taurotragus oryx) male checking if female is in oestrus. Masai Mara National Reserve, Kenya, July

Anup Shah / Eland male (Taurotragus oryx) smelling females urine to establish if she is in oestrus. Masai Mara National Reserve, Kenya, July

Anup Shah / Celebes / Black crested macaque (Macaca nigra) female with swelling of sexual skin to indicate she is in estrus, Tangkoko National Park, Sulawesi, Indonesia.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) close-up of rears, one female in estrus and one male, Tangkoko National Park, Sulawesi, Indonesia.