2000 Ford Ranger oil presure gauge low on cold start up

Just recently I noticed that when I start my vehicle in the morning after it has been sitting all night the oil presure gaguge will been showing low after pressing the accelator for a min the guage will rise slightly and after the vehicle warms up the guage showes to be in the proper place. For the rest of the day the guage will show proper pressure and the vehicle drives fine. The guage only show low upon a cold start untill the vehicle warms up for a min or two what could be the cause of this.