Changing colors is as energetically demanding for octopuses as jogging for 23 minutes is for humans.
You might not realize this, but quickly changing colors, as octopuses do, is energetically costly. This, according to a recently published study by two biologists who measured oxygen consumption rates in live octopuses whilst they changed colors.
Rapid color change is an adaptation that has evolved multiple times in animals. It is used for dynamic camouflage, communication, thermoregulation, or ultraviolet light protection. Color changes can occur quickly, as with chameleons, tree frogs or octopuses, or slowly, such as with snow hares or many types of birds. However, our understanding of the evolution of rapid color changes is hindered by a lack of information about the energetic costs associated with this ability.
Amongst all animals, the speed of color changes and of overall diversity of color patterns seen in cephalopods is unmatched. This ability is a kind of superpower. Cephalopods, particularly many species of octopuses, possess specialized skin cells called chromatophores. These are tiny flexible sacs of pigments that are connected to 15 to 25 radial muscle fibers, resembling the spokes of a wheel attached to the hub. When the muscles are relaxed, the pigment sacs shrink to nearly invisible specks and the octopus appears to be white, but when the muscles contract, the sacs expand, spreading pigment granules across a small area of the skin and revealing the color.
Not only are these color changes rapid, but in octopuses, these changes are remarkably precise. Each chromatophore is like a tiny pixel on a computer screen and shallow water octopuses, like the ruby octopus in this study, have an astonishing 230 chromatophores per square millimeter of skin -- far exceeding the 180 pixels per square millimeter on a 4K 13-inch laptop monitor. By precisely controlling each chromatophore using their nervous system, octopuses can produce intricate camouflage patterns or elaborate visual displays.
Do octopuses pay a metabolic price for their colorful superpower?
"Though octopuses make color change look effortless, it isn't for them," said Kirt Onthank, a professor of biology at Walla Walla University and director of the Rosario Beach Marine Laboratory. He noted that the high energetic costs associated with the chromatophore system would put pressure on octopuses to minimize these costs, and this may explain the use of dens or nocturnal lifestyles seen in some octopus species and reductions in chromatophore systems amongst deep-sea species.
To better understand the metabolic costs of rapid color change, Ms Sonner, a Master's student in biology, and Professor Onthank looked to octopuses for answers. They captured 17 wild ruby octopuses (also known as East Pacific red octopus, Octopus rubescens) and measured their oxygen consumption before, during and after they changed colors to calculate how much energy they used during this process.
Ms Sonner and Professor Onthank also measured chromatophore metabolic demands by collecting small skin samples and placing them under a flashing blue light, which activates the chromatophores, causing color changes. By measuring the metabolic demands of a skin sample, Ms Sonner and Professor Onthank were able to separate the energetic impacts of induced color change from the stress of manipulating an animal in a lab environment.
Ms Sonner and Professor Onthank found that the average octopus uses 219 micromoles of oxygen per hour when fully changing color, equivalent to the energy the octopus uses for all its other bodily functions whilst at rest, including digestion, respiration, circulation and organ function. To give you an idea of how metabolically demanding this process is for octopuses, Professor Onthank estimated that if humans had color-changing octopus skin, we would burn an extra 390 calories per day, roughly the same as completing a 23-minute run.
"Our results show that the octopus chromatophore system has an exceptionally high metabolic demand," reported Ms Sonner and Professor Onthank in their study.
"Due to the involvement of the nervous and muscular systems, it is likely that cephalopod color change is one of the most energetically expensive forms of color change, so our estimate likely represents the upper bound of the cost of color change in the animal kingdom."
Ms Sonner and Professor Onthank also suggested that the high energetic cost of changing color may explain common octopus behaviors, particularly hiding in dens and only venturing out at night.
"Octopuses outside of dens employ high degrees of crypsis, and consequently, a high proportion of chromatophores are active the majority of the time," wrote Ms Sonner and Professor Onthank in their study.
"However, octopuses in dens would be hidden from predators and not actively hunting prey, and therefore unlikely to be using their chromatophore system extensively. This reduction in energetic demand may be the reason that many octopus species spend the majority of their time in dens."
Sofie C. Sonner and Kirt L. Onthank (2024). High energetic cost of color change in octopuses, Proceedings of the National Academy of Sciences 121(48):e2408386121 | doi:10.1073/pnas.2408386121
Questions emailed to senior author, Professor Onthank, about this study went unanswered.