What are these two up to?

What are these two up to?

Observation - What are these two up to?. Description: Observed these two on the banks of the Bridgewater canal, just outside Preston on the Hill. It's looks obvious that this isn't mating, so what are they doing. At first glance it looked like the one underneath has bloated itself up whilst the other o

Observed these two on the banks of the Bridgewater canal, just outside Preston on the Hill.

It's looks obvious that this isn't mating, so what are they doing. At first glance it looked like the one underneath has bloated itself up whilst the other one is throttling it.