Whooping in Le Flore County, Oklahoma

We started a trip into the Horse thief Springs Trail from Cedar Lake on Saturday afternoon at 1730. We had a late start due to drive from Texas and waking up two teenagers on a Saturday. We were in a hurry to make the springs before dark, so we pressed pretty hard on the trail...we had 7 miles to cover quickly. We had planned on hiking The Old Military Road Trail several miles to the west, but audibled to this one due to available water sources. I made camp on the other side of the Ouachita National Recreation Trail where it intersects with the shorter HST...this was about 200 yards past where a small water flow crosses the ONRT. About 2100, my son's friend stated, "What was that?" My son looked at me and asked if I heard the noise. I wasn't paying attention because I was making our dinner. Another call immediately came...it was a series (3-4) of high-pitched whoops. They were very clear and distinct. My best guess would be within 1/2 mile to our north. At one point my son said, "It sounds like a monkey!" This continued for a while and eventually we heard the "whoops" start coming from our south. The ones from the south sounded like they were coming from past Hwy 1 because we could hear an occasional car pass and it sounded much closer. The calls seemed to go on into the night intermittently. I finally fell asleep around 0230 in the morning and was up around 0600. No other vocalizations were noted during the morning or hike out.