had quite a few pairs over the years and noticed that (with the older models at least with the grey centre) they get very sticky/filthy and almost begin to melt when the temps climb such as in spain in summer.
I did contact ergon but they were pretty unhelpful and sometimes on other queries have had no reply at all.
perhaps it's just me ?

noticed mine have been a bit stickier recently – they are the older grey ones.

Yup – same here with older grey compound ones.

Now got new slimline ones with the black grips on all 3 bikes. Seem much better in the heat (rode the Transalp last year with them with no "melting" problems despite temps in the high 30s).

Mine have gone sticky too, but I suspect its the use of bike cleaner such as "muck off" that has effected the rubber, rather than the weather.

mine too but i don't use any cleaners as such just water really.