Some Saturday mornings, I’ll get so engrossed in what I’m doing that I totally lose track of time. I’ll get into a good book, a big LEGO set, or streaming a whole season of my favorite show, and suddenly, hours have gone by. If it wasn’t for the rumbling in my stomach, I might completely miss lunchtime. 🕛
How do our digestive systems know when it’s time to eat? The hormone ghrelin is known as the "hunger hormone." It’s our bodies' natural reminder to eat our next meal. An empty stomach produces ghrelin and pumps it into the bloodstream. The chemical triggers the brain to tell us that it is time to eat. This cycle keeps us eating on a healthy rotation throughout the day.
Almost every vertebrate uses ghrelin the same way. Most...but not all.
Bears that hibernate in the winter can’t eat on a regular daily cycle. They eat heavily for months, then sleep through the months when food is less available. Normally, ghrelin levels drop after an animal eats. Scientists discovered that during the summer and early autumn, grizzly bear ghrelin levels stay high, constantly sending the “eat” signal. Once the seasons change and the bears hibernate, the ghrelin levels drop to nearly nothing. Without the modified ghrelin signals, the bears would never be ready for their long sleep.
Boa constrictors and chameleons are ambush hunters. 🐍 Rather than actively hunting for food, they stay still and wait for prey to come within their reach. This style of hunting means they might have to wait a long time between meals - sometimes weeks or months! Their bodies survive by lowering their heart rates and making other energy-conserving changes in the times when it's hard to catch prey.
Are they just ignoring their ghrelin signals all this time? No! These reptiles have modified genes that simply don’t produce ghrelin. Their digestive system is on its own clock!
The vertebrates with the strangest relationship with ghrelin has to be migratory birds. Before they start their journeys, migrating birds pack on loads of fat to sustain them through the long flights. How do they know when to stop eating and start flying? Ghrelin! Ghrelin levels increase when a bird's fat level reaches a safe amount for their trip. The ghrelin in the birds' bodies doesn't tell them to start eating; it tells them their bodies are prepped and ready to travel!
Check out how little golden-mantled ground squirrels change mass all year. Their weights vary before, during, and after hibernation cycles, as you can see in the graph below:
Here are some questions I'd think about with this graph:
💡What is the independent variable in this graph? What is the dependent variable?
💡If you observed a ground squirrel for a week during the month of June, what do you predict the animal would be doing? Use data from the graph to justify your claim.
💡Let's say you were studying the ghrelin in these ground squirrels. Based on the graph and what you know about hibernators, during which times of the year would you expect there to be high levels of ghrelin in a squirrel's body? Why do you say so?