BORING EXPLANATION ALERTafter doing a bit of zooming in on the twitter versions (higher quality? also note the absence of the subJack one on there) on my laptop, I'm choosing to believe there is a five o'clock snail shadow (delusion).
feel like @mimimithis will know about mini nips? I dunno if the name for mens' ones is the same as the one for women.
I get proper hard nips when I'm working out too lol (soz everyone), dunno why.
I don't believe it has a special name, since it's not a pathology or anatomical variant. The tightening and hardening of nipples has to do with the sympathetic nervous system being more active during exercise - sport is a deliberate fight (or flight) reaction, which has the body produce more noradrenaline, which in turn lets muscles work harder and more readily. The side effect of that is that noradrenaline leads to hardening nipples, ergo those mini bullet nips we see here, despite the about 100°C (vast exaggeration, but the sun has been brutal here, I don't want to know what it feels like in Greece) he must be training in.