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The Mexican cave fish (code name: Astyanax mexicanus) isn’t just blind and eyeless, it also has a shrunken brain. Exactly how it ended up that way is a bit of an evolutionary mystery. In a recent article in the journal Science Advances, researchers suggest that the fish dropped the eyes and evolved a simplified brain in order to save energy. But is that really the best explanation?

In other words, do eyes and brain really require such disproportionate energy that a mere couple million years of cave lounging is enough to reach into the germline and reprogram them out? Would not a less contorted line of reasoning simply assume, like Darwin did, that they were somehow lost by disuse and nothing more? Unfortunately, the latter hints at Lamarckism, something that adherents of the strict blend of evolution by mutation and natural selection are programmed to abhor. Yet if we step over to the dark side for a moment, we might suppose that in order for luxuries like eyes and brain to persist, their physical imprint in the heritable germ must be continually refreshed somehow. Without that input — namely constant stimuli from the environment — these seemingly stable accessories are evidently as volatile as RAM.

Those in the know have a fancy word for the chalky etiolation and de-evolution that happens with too much cave living. Our own ancestral brethren, the newly discovered homo naledi, were no doubt quite familiar with its ills. I am speaking here of troglomorphism, the full morphological adaption to cave darkness. Creationists, we should probably mention, love the cave fish. They typically don’t know exactly why they love it, only that its forsaken visual system occasionally creates a bit of a problem for those more scientifically minded folk.

Now if raw unadulterated ‘energy’ perceived at the level of the organism, as opposed to more finely divided and nuanced stimuli is really the true troglomorphic driver, one might take the approach of breaking it down into three places it can act. In the first, there should be energy enough to build eyes and brain in the embryo. In the second, enough to maintain them in the adult, and in the third energy for them to actually turn on and be used.

While the latter two arguably have some overlap, clearly there is likely little in a dark cave to turns eyes on. The first demand, growing them, isn’t a big issue here either because as the author themselves report, the fishes develop eyes as an embryo. Only later on are they disassembled in the usual palimpsestic scratch that evolution uses throughout all development to repurpose obsolete organs to new ends.

It’s so much not that ‘energy’ is a bad explanation, but that it’s just an imprecise one. Energy equations, namely conservation relations, are probably the most useful tool available in physics. But in biology one is rarely so lucky. The authors attempt to measure relative energy use by recording oxygen consumption. In particular, they measured its use in blind and sighted fish, as well as in pieces of their eye and optic tectum, both in light and dark. The tectum is the part of the brain that was imagined quite famously not long ago in Zebrafish to create the first whole brain activity maps. It is the business end of the fish visual system. As seen below, the authors found that it is also the most dimorphic (varies in size) between cave forms of the fish, and their sighted surface cohorts.

The researchers used their data to build a model that predicted that the energetic cost of the whole brain for a 1-g surface fish was 15% of its resting metabolism. To give some idea of the consumption rates involved, the eyes used around 0.507mg O2 hour−1 g wet mass−1 in the light. In the dark, they actually were found to use a little more oxygen, although one might get away with attributing that to the anomalous inverse nature of photoreceptor responses in the retina. It would be nice to get a more intuitive feel for that consumption level, however, ii is in the nature of the business that the units generally need some massaging to be compared. For example, the hummingbird at hover, whose muscle allegedly can burn at ten times the rate of that of an elite athlete human, has been recorded at 68–85 ml O2/g/hr.

All is not lost in abandoning simple energy arguments. Genetics, and particularly epigenetics, has already offered a few places to start to look for rapid, environmentally-driven change. Previous research has highlighted a few genes that are directly involved in controlling things like eye and brain size. The de-evolution, or rather deconstruction of the eye in the embryo, also has been found to follow predictable programs or cell death or apoptosis.

Tedious transplantation experiments have revealed that it is the lens itself that is a key organizer of the entire eye. If the developing lens of a blind embryo is transplanted into a developing sighted fish, it can compromise the eye. On the other hand, transplanting a sighted fish lens into a blind fish eye can rescue many aspects of eye development, and possibly even brain.

Other rapidly changing structures which respond to the environment and reprogram parts of the cell, directly heritable or otherwise, include ubiquitous stress responsive proteins like HSP-90. These well-studied proteins readily alter their folding structure as things change around each cell in the organism, a much quicker process by comparison, than adjustments to the actual coding sequence. If the stress of an energy-depleted environment really calls for less brainpower, we might offer that a better way to achieve that end is to simply dial down processor speed. In other words, an untold number of additional ion channels — proteins fully visible to the hidden emissaries of evolution — regulate how fast neurons can fire. Neurons with a firing rate that idles high, and tops out much higher, burn an inordinate amount of energy.

How this neuron ‘firing’ energy compares to the energy used for growth and maintenance is an interesting point we’ve encountered before in the context of the ongoing extension and retraction of neurites in ambulating neurons. At an even more theoretical level, others have even estimated the energy needed by proliferating bacteria to replicate themselves, or for Eukaryotic cells to differentiatiate and migrate into a closing a wound. For those that just gotta know, they say that a single bacterium might need to roughly double its ongoing basal metabolic rate in order to become two, or something like that.

A final quick fix that energy-restricted troglodytes might try is simply reducing the amount of DNA they carry. Less DNA translates into slightly faster cell division and development cycles, but it also comes with an energetic bonus. Namely, you don’t need to burn as much ATP to replicate all those extra copies of genes and such. You also cut down on inadvertent expression of proteins you don’t really need. Jettisoning all your backup DNA may be a tough pill to swallow, but it might be a good thing to take a look to get a better idea of how the cave fish rolls.