Whales must exercise to stay fit, according to a new study that finds humans aren’t the only animals that benefit from working out.

Humans might, however, be the only animals that require exercise as adults, because just young whales were observed exercising.

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“Young humpback whale calves frequently engage in extended sequences of breaching, even at a very young age,” co-author Rachel Cartwright from California State University, Channel Islands, said in a press release.

“These high levels of exercise have always been something of a paradox,” she continued, “given the limitations on maternal energy resources during the breeding season.”

To investigate the matter, Cartwright and her team studied muscle tissue samples from 18 stranded baleen whales. They looked at muscular myoglobin stores in both young and adult baleen whales. Myoglobin is a red protein-containing compound that carries and stores oxygen in muscle cells. It is structurally similar to hemoglobin, the primary functional constituent of red blood cells.

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The scientists determined that whale calves only have about 20 percent of the muscle myoglobin stores as adult baleen whales. The levels elevate over the course of maturation.

Additionally, comparisons of myoglobin levels between and within muscles, along with differences in myoglobin accumulation rate in very young baleen whales, suggest that exercise influences the rate of development of myoglobin stores in young baleen whales.

The findings help explain why young whales frequently breach for seemingly no good reason, and why they engage in other bouts of energetically expensive exercise.

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Cartwright said, “This study provides a functional explanation for these high activity levels; this intense exercise drives development of oxygen stores in the muscle tissue, allowing young whales to build their breath-holding capacity and make sustained, extended dives.”

The study, published in the journal PLOS ONE, also indicates that young whales are particularly vulnerable to changes affecting their foraging habitat and marine resources.

It remains to be seen if energetic playtime enjoyed by other young animals is also considered to include exercise. If so, then working out could be very common throughout the entire animal kingdom.