After much searching on Google with no results, I decided to ask my question here.

If I'm not mistaken, swimmers' bone density is less than normal; at least for lifetime swimmers. Astronauts also lose bone mass from the lack of stress (from gravity) on their bones; it stands to reason that being in the water and thus being weightless, would lead to bone loss.

So my question is, how do marine mammals and penguins maintain their bone mass?

  • $\begingroup$ Wouldn't bones nearly completely wither away in a weightless environment, such as underwater? $\endgroup$
    – Rene
    Commented Jul 9, 2016 at 5:45
  • 1
    $\begingroup$ FYI, some of the densest mammalian bones belong to cetaceans. For instance, blainville's beaked whale has a mesorostral with 2.6g/cm3. Sperm whales have a bulla with 2.16g/cm3. For comparisson, human bone is around 1.7g/cm3 (source: Bones and Cartilage: Developmental and Evolutionary Skeletal Biology). Also, penguin bones are considerable denser than other birds. $\endgroup$
    – user24284
    Commented Jul 9, 2016 at 15:18
  • $\begingroup$ My comment was to OP. $\endgroup$
    – user24284
    Commented Jul 9, 2016 at 15:31
  • $\begingroup$ Thanks for the input. Question though, why do lifetime human swimmers lose bone mass, shouldn't the muscular activity required for swimming keep the bones strong? What about the hormonal component? $\endgroup$
    – Rene
    Commented Jul 10, 2016 at 0:09
  • 1
    $\begingroup$ Do you have a source for the bone loss in human swimmers? any vigorous activity puts stress on the bone, swimmers are not massless they still have to push hard against the water itself to swim. Also consider the alteration of an organism during its life is very different then evolutionary adaptation to an environment. $\endgroup$
    – John
    Commented Aug 10, 2017 at 4:26

1 Answer 1


I wouldn't refer to it as "bone loss" but more as bone strength. Lifetime swimmers spend a lot of their time in the water. As in water yes they are almost "weightless" and their bone strength will decrease because it has gotten used to the "low" gravity. Now in the case of penguins and other Marine mammals, they have adapted to the force that the water applied to them.

Here is a link to a picture of penguin's flipper.

  • $\begingroup$ This answer gives two reasonable hypotheses for a putative answer, however it lacks reference. Could you please provide some evidence for your suggestions? $\endgroup$ Commented Apr 12, 2017 at 15:54
  • $\begingroup$ How do you define bone strength? What do you mean by "adapted" as in "in the case of penguins and other Marine mammals, they have adapted to the force that the water applied to them"? It says nothing about the bone strength, however it is defined, and its evolutionary adaptation. What does the picture say about bone strength of a penguin's flipper? $\endgroup$
    – Hans
    Commented Oct 9, 2017 at 8:41

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