Our guest post this week comes from Haley O’Brien, a PhD student at Ohio University, and Dr. Jason Bourke from the North Carolina Museum of Natural Sciences. If you would like to contribute a guest post, please get in touch on Twitter or Facebook.
Both giraffes and sauropod dinosaurs are renowned for their signature long necks. As scientists these super-long necks intrigue us because they seem to create a host of physiological problems that nature has had to solve. For instance, have you ever noticed that when you stand up too fast you can sometimes feel dizzy?
This is because our circulatory system, which must fight against the constant pull of gravity, gets strained by our brain’s sudden change in height. If you’re a giraffe, this height problem is increased by 2 meters (6.5 feet), and this distance would be even greater for many sauropod dinosaurs. For a long time, it was thought large arterial meshwork at the base of the giraffe brain, called the carotid rete, helped giraffes solve two major problems associated with their long necks: preventing them from fainting when they raised their heads, and keeping their brains from suffering catastrophic pressure increases caused by blood rushing to the brain when lowering their heads to drink. The proposed use of the carotid rete as a pressure-absorbing capacitor for giraffes led other researchers to suggest that sauropods may have employed similar structures to help protect their brains, too.
Despite its popularity, this hypothesis has been difficult to test in living animals. Recently, we used new digital 3-D modeling techniques to simulate what happens to blood as it flows through the carotid rete. Using a method called computational fluid dynamics, we digitally “pumped” blood through the rete under normal, head-raising, and head-lowering scenarios. We found that this arterial structure had essentially no impact on blood flow. The image below shows blood pressure change throughout the rete during the highest pressure phase. Warm colors indicate high pressure, and cooler colors indicate lower pressure.
Notice how almost all of the arterial meshwork is the same shade of green? (moderately low pressure) We found that pressure dropped by a measly 1.5 mmHg (around 0.002 atmospheres) before entering the brain. Given that typical blood pressure for a giraffe when drinking is about 330 mmHg (0.45 atm), a change of 0.5% doesn’t mean much for protecting the animal’s brain. We found this result surprising since it goes against the long-standing conventional wisdom that giraffes use their carotid rete to mitigate blood flow to their brains.
So then how do giraffes and sauropods keep themselves from fainting every time they raise their head to feed, or avoid brain bleeds when bending down to drink water?
Unfortunately, we’re still not sure. However, by testing this hypothesis, we’ve opened the door for others to test alternative processes, such as diverting blood into the jugular or vertebral veins or shunting blood away from the brain into arteries that don’t enter the braincase. Digital modeling helped us see what doesn’t work; now we can start looking for what does.