If the earth gravity fluctuated between +50% and -50% over the course of 200 years, what specific adaptations would be useful to account for the change?
If gravity varied only between 0.5 g and 1.5 g over hundreds of years, most animals could adapt through changes to bone density as they grow up. Higher gravity might shorten the lifespan of some animals at the extreme (giraffes?) due to elevated blood pressure, but the major phenotypic changes are to bones and muscles.
Thankfully, mouse centrifuges are a thing (unfortunately there aren't any videos) so there are some studies in the 'hypergravity' area. Kwano N, et al. (2016) found that mice raised at 3 g have higher bone density and muscle mass, but they noticed that the inner ear was needed for some effects to occur. In their graphs, they have a "Sham" control group that was subjected to 1 and 3 g, but the "VL" (vestibular lesion) group had their middle and inner ear effectively destroyed by a labyrinthectomy, which is a (drastic, last-resort) procedure to treat vertigo, or here to see what the absence of the organic 6-axis IMU does.
So, animals on this variable gravity world would be slightly more adaptable in the bone and muscle-mass area and have slightly better inner-ear function. Higher changes over less time would demand more adaptability. The muscle mass might also include stronger blood vessels (arteries are wrapped in smooth muscle) to tolerate higher blood pressure.
$\begingroup$ Thank you, Nick T; that is very helpful. How would birds handle flight? I imagine they wouldn't get by with hollow bones like earth species, but perhaps they would be able to fly only when gravity is near .5. Otherwise, they adapt to be more maneuverable on the ground. $\endgroup$ Jul 8, 2017 at 3:25
1$\begingroup$ I don't quite know how it would affect lift, but the density of air at 10,000 ft is about 2/3 what is at sea level, so might make flying 50% harder? Many birds could tolerate that, but again, some at the extremes (albatrosses?) would be much more drastically affected. Albatrosses are also extremely dependent on tradewinds to get around, and I'm not sure what higher/lower gravity would do to air currents. $\endgroup$– Nick TJul 8, 2017 at 3:37
2$\begingroup$ @AdamHalatek Higher gravity will increase air density, while drag forces will be greater, flight will in general become easier. Birds that dive off cliffs, for example, to get up to flying speed will have easier job of it. $\endgroup$ Jul 8, 2017 at 4:32
3$\begingroup$ Gravity shifting on the same planet wouldn't have much effect on birds being able to fly. Air density changes would generally offset the changes in gravity. Things might be a little easier or a little more difficult, but if it flies, it should remain capable. Same goes for most aircraft. $\endgroup$– AndonJul 8, 2017 at 4:36
2$\begingroup$ Metamorphosis in any literal sense enters the realm of magic. Just keeping wings folded in high gravity would be okay. But your timespan (200 years) is over ten times the lifespan of most earth animals (birds, dogs, cats) and triple that of humans; so likely no 0.5G features would evolve, only the harsher 1.5G; because 0.5G would not be a survival advantage or necessity for 2/3 of the time, but 1.5G would aid in survival 1/3 of the time, while big muscles and bone density likely more useful the other 2/3 of the time. I expect only 1.5G animals to evolve, and expand faster in 0.5G times. $\endgroup$– AmadeusJul 8, 2017 at 13:53