Bull images - Joel Sartore

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A critically endangered Sumatran elephant bull (Elephas maximus sumatranus) at Taman Safari.

Picture of a bull elk with his antlers in velvet (Cervus canadensis) at the Oklahoma City Zoo.

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A bull elk with his antlers in velvet (Cervus canadensis) at the Oklahoma City Zoo.

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A bull elk with his antlers in velvet (Cervus canadensis) at the Oklahoma City Zoo.

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A bull elk with his antlers in velvet (Cervus canadensis) at the Oklahoma City Zoo.

A musk ox bull (Ovibos moschatus) charges at Joel Sartore, on Nunivak Island, Alaska. These animals descended from stock brought in from Greenland in 1935.

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A musk ox bull (Ovibos moschatus) charges at Joel Sartore, on Nunivak Island, Alaska. These animals descended from stock brought in from Greenland in 1935.

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Hungry Horse Dam, one of the biggest dams that blocks bull trout movements in Northern Montana. Note that biologists say this dam actually prevents non-native fish from moving upstream here, which today is a good thing. No fish ladder or passages were incorporated into this or any other dam in this region at the time they were built.

A bull trout (Salvelinus confluentus) in Bighorn Creek which is part of the Kootenay river system in British Columbia, Canada. (IUCN: Vulnerable; US: Federally threatened)

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A bull trout (Salvelinus confluentus) in Bighorn Creek which is part of the Kootenay river system in British Columbia, Canada. (IUCN: Vulnerable; US: Federally threatened)

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A bull trout (Salvelinus confluentus) in Bighorn Creek which is part of the Kootenay river system in British Columbia, Canada. (IUCN: Vulnerable; US: Federally threatened)

Bull trout (Salvelinus confluentus) swimming in the Bighorn Creek, in the Wigwam River drainage in British Columbia. This is one of the last, best places for spawning of the vulnerable (ICUN) and federally-threatened bull trout, and is part of the Kootenay River system, which sees an annual migration of bull trout from Lake Koocanusa, some fifty miles away. The fish prefer very cold water of 40 degrees or so in order to spawn, and the springs in this area provide that.

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Bull trout (Salvelinus confluentus) swimming in the Bighorn Creek, in the Wigwam River drainage in British Columbia. This is one of the last, best places for spawning of the vulnerable (ICUN) and federally-threatened bull trout, and is part of the Kootenay River system, which sees an annual migration of bull trout from Lake Koocanusa, some fifty miles away. The fish prefer very cold water of 40 degrees or so in order to spawn, and the springs in this area provide that.

Bull trout (Salvelinus confluentus) caught in fishing nets in Lake Pend Oreille, Idaho. (IUCN: Vulnerable; US: Federally threatened)

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Bull trout (Salvelinus confluentus) caught in fishing nets in Lake Pend Oreille, Idaho. (IUCN: Vulnerable; US: Federally threatened)

Biologists collect samples from a vulnerable (IUCN) and federally threatened bull trout (Salvelinus confluentus) Clark Fork River, Idaho.

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Biologists collect samples from a vulnerable (IUCN) and federally threatened bull trout (Salvelinus confluentus) Clark Fork River, Idaho.

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A fisheries technician for dam owner Avista, uses a radio antenna to track tagged bull trout in a stream that feeds into Noxon Reservoir. Biologists track a handful of tagged fish daily to try and learn about their migratory movements, which a series of dams on the nearby Clark Fork River have severely impeded.

A fisheries technician for Avista, checks a monitoring station for radio-tagged bull trout (Salvelinus confluentus) at the base of Noxon Rapids Dam, Montana. (US: Federally threatened; IUCN: Vulnerable)

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A fisheries technician for Avista, checks a monitoring station for radio-tagged bull trout (Salvelinus confluentus) at the base of Noxon Rapids Dam, Montana. (US: Federally threatened; IUCN: Vulnerable)