Snakes are renowned for their ability to survive extended periods without food – weeks, even months. Now, researchers are beginning to unravel the biological mechanisms behind this remarkable feat, and a surprising element appears to be missing from the equation: the “hunger hormone,” ghrelin. A study published on February 1 in Open Biology reveals that many snake species lack the gene that produces ghrelin, as well as the gene for an enzyme necessary to activate it.
Ghrelin, often dubbed the “hunger hormone,” plays a crucial role in mammals by signaling to the brain that it’s time to eat and prompting the body to utilize fat reserves during fasting. However, the relationship between ghrelin and appetite is complex, and its absence doesn’t necessarily equate to a lack of hunger, as demonstrated in studies involving mice. Researchers found that mice lacking ghrelin showed no change in appetite or food intake. In both mice and humans, levels of ghrelin and its activating enzyme actually increase after a meal, raising questions about its primary function.
The discovery stemmed from a comparative genomic analysis of 112 reptile species, including snakes, crocodiles, and chameleons. Researchers, led by evolutionary geneticist Rui Pinto of the Interdisciplinary Centre of Marine and Environmental Research in Porto, Portugal, found that 32 snake species were missing the genes for both ghrelin and its activating enzyme. Interestingly, this pattern wasn’t exclusive to snakes; some chameleon and lizard species, despite regularly consuming food, also exhibited this genetic absence.
Conversely, crocodiles – known for their ability to go over a year without eating – retained both genes. This observation suggests that the lack of ghrelin isn’t a universal prerequisite for prolonged fasting. “Snakes’ lack of ghrelin may have nothing to do with hunger,” Pinto explained.
The findings have prompted scientists to reconsider the role of ghrelin, suggesting its absence in snakes may be more closely linked to metabolic regulation than to appetite control. Ghrelin is also known to be involved in regulating fat storage and insulin response. It’s possible that snakes have evolved a metabolism so efficient that they simply don’t require ghrelin to manage these processes.
However, experts caution against overstating ghrelin’s metabolic importance. Tobias Wang, a zoophysiologist at Aarhus University in Denmark, notes that while ghrelin does have metabolic effects, there’s no strong evidence to suggest these effects are profound. He emphasizes that ghrelin is just one piece of a complex puzzle involving multiple hormones and pathways that regulate appetite and energy balance.
Todd Castoe, an evolutionary geneticist at the University of Texas at Arlington, who was not involved in the study, described the findings as “striking,” adding that many scientists had previously overlooked this “really cool pattern.”
The evolutionary implications of this discovery are significant. The repeated loss of ghrelin genes across different snake lineages suggests that it may have been a disadvantageous trait. Snakes, which often rely on ambush predation rather than actively hunting, expend less energy in acquiring food. A metabolism less driven by frequent hunger signals could be more advantageous in this lifestyle.
Rute Fonseca, an evolutionary geneticist at the University of Copenhagen and a study author, acknowledges that the current research represents only a partial understanding of ghrelin’s functions. Further investigation is needed to fully elucidate the hormone’s diverse roles and the consequences of its absence in different animal species.
Future research, as suggested by Wang, could involve deleting the ghrelin gene in crocodiles or introducing it into snakes to observe the effects. Such studies could potentially offer insights into human metabolic disorders, such as diabetes, and obesity. Castoe believes that this initial discovery will pave the way for further research, potentially revealing valuable information about metabolic processes in a variety of species, including our own. “I think there’s a lot more cool stories that we will see come out of this,” he said.
The study underscores the remarkable adaptability of reptiles and highlights the complex interplay between genetics, physiology, and behavior in shaping survival strategies. While the precise function of ghrelin remains a subject of ongoing research, its absence in snakes offers a fascinating glimpse into the unique metabolic adaptations that allow these creatures to thrive on infrequent meals.
