Nature
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For snakes at supper, more is more. They’re nature’s binge eaters, capable of swallowing and digesting things so large it really appears to defy physics – but their time in between meals is equally mind-boggling in scale, lasting for months or even years at a time. It seems like it shouldn’t be possible, even to experts – but it is. And now, researchers think they know how.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.“The data […] was staring us all in the face – myself included,” Todd Castoe, a genomicist at the University of Texas at Arlington who was not involved with the study, told Science this week.
“I give [the researchers] a lot of credit for looking more deeply into [it],” he said.
So, what’s the story? It all comes down to ghrelin – a hormone primarily produced in the stomach which signals your brain when it’s time to eat. Often known as the “hunger hormone”, ghrelin was once hoped to be the key to treating and preventing diseases like diabetes, anorexia, and obesity – but in the end, it didn’t pan out. The hormone’s role in our metabolism was simply too complex, and it was found that just blocking it had no effect on appetite or weight in mice.
It all added up to an understanding of ghrelin’s mechanism and function that was incomplete at best – and so, in the new study, the team decided to focus on reptiles. After analyzing the genomes of some 112 species, including snakes, chameleons, turtles, agamas, and others, the team found that some species – 32 types of snake, four of chameleons, and two toadhead agamas – have all but completely lost the genes that produce the hormone.
“We were getting fragments, just small pieces of the sequence,” in some snake genomes, Rui Resende Pinto, an evolutionary biologist at the University of Porto and coauthor of the new study, told Science. In others, the genes were entirely absent; in still others, it was present, but so warped as to be useless for encoding the hormone.
This range of presentations implies something rather intriguing: that each order of animals that lost the ghrelin gene did so in independent events. In fact, it’s possible that it happened multiple times in snakes alone, Pinto said.
Ghrelin isn’t the only substance missing in these extreme eater species. So too is an enzyme essential for its activity, MBOAT4 (membrane bound O-acyltransferase domain containing 4). Combined, it’s hard not to link these missing genes to the affected species’ hunting and feeding habits: “[the] loss is a likely consequence of one of the many extreme physiological and behavioral adaptations that reptiles, particularly snakes, have evolved to survive in environments in which prey availability is uncertain and the need for energy budgeting is a constant,” the researchers write.
Put simply: in mammals, ghrelin tells a fasting body to burn fat for energy while the animal searches out food for refueling. But species like snakes, some chameleons, and agamas, do something different: they “employ a mode of foraging behaviour called ‘sit-and-wait’,” the team write, “which consists of waiting in the same area for prey to come within this area.”
To cope with this unreliable hunting method, species that use it “often employ strategies that limit metabolic energy consumption at rest,” they explain. Losing the genes associated with ghrelin production would do exactly that – in fact, interestingly, it seems to completely reverse the metabolic rates associated with fasting and digestion compared to mammals – allowing the studied species to survive these epic spans without food.
Overall, it’s a finding that Castoe called “pretty remarkable” – and potentially a first step towards a better understanding of how ghrelin works in other animals too.
“You never know where things are going to go,” Castoe said.
The study is published in the journal Open Biology.
