A few days ago I asked a question on a spherical desert creatures movement here: Movement for a spherical desert creature. I chose the Rocsphere which is @HDE 226868's answer. I was wondering what weaknesses/problems would a creature using this movement technique encounter?
Can it climb the typical sand dune inclination, which can be up to 34 degrees?
The steep (downwind) side of sand dunes consists of sand grains on the very edge of collapsing and depending on the type of sand is between 30 and 34 degrees. This is quite steep for a big ball to puff itself against. The other side is usually more gradual, but not necessarily by that much. Even if it could navigate the shallow side, it would not be able to get back without going around the long way, which can be anything from a few hundred meters to several hundred kilometers
Can it reach the top of the average dune before running out of "steam"?
With no way to anchor itself, the creature needs to ascend every dune in one go. This can be up to 200 meters high, but even 20 meters may be a challenge. The Rocsphere appears to have a CO2 filled bladder for propulsion. That will not contain enough mass (even with some sand mixed in) to propel the creature that far up, so it likely needs to inhale normal air to keep going, at a lesser efficiency.
Those would be the biggest risks to a creature that cannot dig its feet into the sand and slog its way up step by step. Note that all earth desert animals with a rolling mode employ it going downhill and have an alternate means of getting back up.
Rocky, uneven terrain would seem even worse, but if the Rocsphere could produce enough of a burst to lift itself a few feet in the air, it could probably navigate hop by hop, gathering its energy after each hop.
All this could be averted if the creature lives exclusively on perfectly flat, compacted ground like dried lake beds, but your typical desert environment with rocky outcrops and/or sand dunes will prove quite challenging, especially if there are legged predators around who can easily overtake the Rocsphere as soon as they hit an incline.
This will have to be basically opinion, since we're considering a what-if scenario about a creature of imagination (so far), but I'll give you my best guesses.
Assuming only the data for the Roksphere as given in HDE 226868's answer (barring future edits thereof), and your specific request regarding possible weaknesses/problems that it might encounter:
- a large central chamber (filled with carbon dioxide);
- six holes (two along each axis);
- muscular expulsion of CO2;
- single exhaust point on average ("the other five remain closed with flaps - which generates a torque")
- "like a furry rocket".
a single central CO2 bladder is a crippling point, should it be compromised;
greatly reduced mobility in the event of even a single clogged exhaust hole -- this could be remedied by providing redundant exhaust ports (though also providing extra routes of ingress for foreign bodies and pathogens), in addition to the organism's own methods of cleaning and repair;
if the exhaust ports aren't vectored, then the value of the exhaust is greatly reduced (this is noted in passing, since the geometry isn't explicitly addressed);
lactic acidosis (or an equivalent, depending upon a Roksphere's biochemistry) is a possibility that might be circumvented by fueling its propulsion via chemical mix, rather than via muscular exertion, though such would also come with a biochemical investment (for manufacturing and/or storing same) somewhat greater than that of ATP;
Rokspheres would likely require rather high exhaust speeds and/or low body density in order to achieve/maintain velocities necessary to climb dunes -- these might be achieved via chemical propulsion (see paragraph above), hollow organs (bones or otherwise), swim bladders;
the furriness could help (traction if cleat/root-like, laminar airflow if adjustable, grip if adding the parameter of any prehensile capability) or hinder (detritus increasing weight, surface area increasing drag).