Bull Shark

0:00:02 > 0:00:03My name's Steve Backshall.

0:00:03 > 0:00:06Self-confessed shark-obsessed.

0:00:06 > 0:00:08This is Shark Bites.

0:00:10 > 0:00:13This time, we meet a shark considered the most

0:00:13 > 0:00:16menacing of all - the bull shark.

0:00:16 > 0:00:20These stocky and pugnacious predators probably have

0:00:20 > 0:00:23the worst reputation of all sharks,

0:00:23 > 0:00:26but there's more to these bruisers than meets the eye.

0:00:27 > 0:00:30They like to hunt in warm coastal waters,

0:00:30 > 0:00:35but the bull shark has another killer credential - they can

0:00:35 > 0:00:41live in fresh water and have been found 4,000km up the Amazon River.

0:00:42 > 0:00:46So how can a shark that evolved to live in salt water

0:00:46 > 0:00:48also travel into fresh water?

0:00:48 > 0:00:52Mexico's coast is the perfect place to see these feisty fish.

0:00:55 > 0:00:58This is the bull shark's ocean home,

0:00:58 > 0:01:01and it wasn't long before they came to check us out.

0:01:01 > 0:01:06And our first bull shark is coming in towards us right now.

0:01:06 > 0:01:07Look at this!

0:01:10 > 0:01:13In open clear water, these sharks are not going to attack.

0:01:14 > 0:01:17But that doesn't stop my heart from racing.

0:01:18 > 0:01:23There must be 10 or 15 of them around us at the moment,

0:01:23 > 0:01:26all circling us with real intent.

0:01:27 > 0:01:31Look at these three! Such dramatic animals!

0:01:33 > 0:01:35Whoa!

0:01:35 > 0:01:37There's no doubt - approaching dusk,

0:01:37 > 0:01:42these animals have a totally different attitude about them.

0:01:42 > 0:01:46It's clear that now they're ready to start hunting.

0:01:49 > 0:01:51But to satisfy their appetite,

0:01:51 > 0:01:55these hungry hunters also head inland to find more prey.

0:01:56 > 0:02:00And how they do that is down to some ingenious bull shark biology.

0:02:01 > 0:02:04One of the ways a bull shark will absorb water

0:02:04 > 0:02:07and vital salts is through its skin.

0:02:07 > 0:02:10But living in both freshwater and saltwater environments,

0:02:10 > 0:02:13the concentrations of salt are always changing,

0:02:13 > 0:02:16so the bull shark needs to adapt.

0:02:16 > 0:02:19Fortunately, they have very clever kidneys.

0:02:19 > 0:02:22When they're in the ocean, they need to get rid of excess salt but

0:02:22 > 0:02:24when they move into a freshwater environment,

0:02:24 > 0:02:28like a lake or a river, they do just the opposite.

0:02:28 > 0:02:31The kidneys hang on to the salts they need to survive

0:02:31 > 0:02:32and get rid of extra water.

0:02:34 > 0:02:38These sharks have the best of both worlds.

0:02:38 > 0:02:41Surviving in fresh water and salt water means

0:02:41 > 0:02:46they have access to even more prey - their very own bull shark buffet.

0:02:47 > 0:02:52The bull shark - seriously adaptable with a bullish attitude, it is

0:02:52 > 0:02:57a highly honed hunter, making it the most menacing shark of all.