Thursday, May 17, 2012

A Story of Bison, Wolves, and the Raging Lamar River

The theme of our (Karyn and I) visit to Yellowstone this year seemed to be Wolves and Bison. Each season of the year brings unique dynamics between the species, especially between predators and prey. As our visit coincided with bison calving season, bison were one of the major themes of the week (I will describe the other themes in a later post). In the Lamar Valley I expect the focus will shift to elk as the elk calving season kicks off in the next few weeks.

With two wolf packs occupying the Lamar Valley, we had ample opportunity to observe wolves both morning and evening. It was one of the richest watching experiences of all of our visits to the park in both quantity of observations and diversity of events.

To observe the Lamar Valley pack, the area near the confluence of Soda Butte Creek and the Lamar River provided the best opportunities. As the Lamar Canyon wolves den nearby, they have a fairly constant presence in the area. Climbing the hill north of the road provides spectacular views of the valley and aids in observing wolves hidden by sagebrush, tall grass, and the undulations of the landscape.

Confluence of Soda Butte Creek and Lamar River. Yellowstone National Park.

Bison. Hundreds of Bison occupy the Lamar Valley. For the most part, healthy adult Bison are out of reach of even a large wolf pack. Only one wolf pack in the park, the Mollie's Pack, regularly attack bison. Even during calving season, most bison have little to fear. In a past visit we have watched a single adult bison successfully defend her calf against a pack of six adult wolves. On this visit we watched numerous small groups of bison successfully defend a calf from a pack of 14 wolves! Unlike elk, bison exhibit group defense in the face of predators such as bears and wolves. However, we did observe one repeated threat that was unexpected. Even more unexpected was the magnitude of the threat. The threat: losing calves during river crossings.

Bison and calf crossing Lamar River, Yellowstone National Park.

Bison are herbivores which eat grass, small forbs, and occasionally shrubs. The landscape of the Lamar Valley is covered in sagebrush, grass, and forbs. From our viewpoint, it looked rather homogenous on both sides of the river. Indeed, the bison spend significant time on each side of the river. However, they often are observed crossing to the other side. As the saying goes, the grass is always greener... During these crossings the vast majority of the calves are successful. Even most one or two day old calves can swim the raging river. However, sometimes they don't make it. Those that don't are often swept downstream to be deposited on a river island or on a far away shore, left to slowly starve or more quickly succumb to predators. So why do they take the risk? I don't know but expect there is some evolutionary justification. It is possible that this early test of strength can determine a calf's future success. If the calf is not strong enough, maybe the mother is better off saving her resources for the next breeding season. It is harsh, but nature often is. As Jacob Dylan says, "the tyranny of nature's plan."

Our first morning in the Lamar Valley we were watching a pack of wolves on a carcass across the valley. We were also watching a stranded calf wandering around and around and around a small island. There was no adult anywhere in sight. We could only imagine that it was tired, hungry, and very confused. I wondered how long it would take for a predator to find it.

Stranded bison calf on island. Yellowstone National Park.

Little did I know that my question would soon be answered. As we descended the hill to our car so that we could move down valley, a friend pointed out the alpha male of the Lamar Canyon wolf pack. He was sitting next to the river watching the calf.

Alpha male, Lamar Canyon Wolf Pack. Yellowstone National Park.

Wow. Here we were just under 100 meters away... He rose and entered the water...

Alpha male swimming to island in Lamar River, Yellowstone National Park.
Alpha male approaches stranded Bison calf. Yellowstone National Park.

The calf appeared unaware of it's fate. From my observations deer and elk have an innate fear of predators, but apparently bison do not. I am sure this ties directly into the adult response which is for an elk to run and a bison to challenge. Anyway, the calf just watched as the wolf approached.

The final approach. Yellowstone National Park.
The takedown. Yellowstone National Park.

I would like to say that the kill was fast and painless, but it took a bit longer than I had expected. Regardless, the suffering of the calf was ended and the Lamar Canyon pups would be fed. Somewhere in the long chain of evolution, the available bison gene pool was just slightly altered for the better. The next generation will be slightly better swimmers...

The wolf ate some of the calf on the island before carrying, with difficulty, the carcass back across the river toward his den. What a prize a partially complete carcass would be for the four or five week old pups waiting there. This would augment their usual diet of breast milk and regurgitated meat.

Bison calf is carried back toward the den. Yellowstone National Park.

The outcome is not always the same. On another day we watched bison crossing the river. The first calf made it across successfully, but the second was washed far down stream away from it's mother. In addition, the calf appeared to panic and returned to the wrong side of the river. From our high observation point we could watch the whole drama play out. The mother appeared to have assumed that the calf made it across. She ran from calf to calf trying to find her offspring. She crossed back to the other side, searched, and crossed again continuing to search. The calf was moving too. The calf moved up the hill from bison to bison searching for its mother, but ended up moving farther and farther away. The mother crossed the river to search three more times! Time passed by and the distance between them was not getting any shorter. We were convinced the reunion would not happen. She crossed again and started heading in the wrong direction...then the right direction...then the wrong direction... It was agonizing to watch as we could see the whole hillside. They moved closer together, but a low ridge separated them. Finally the distance was short enough that their calls could be heard. Both animals ran at full speed to each other. The calf immediately started feeding. The whole drama lasted nearly 40 minutes. We didn't watch to see if they swam the river again.

Another day, another calf. We were now noticing stranded calves more and more often. Another was stranded on an island near the road. We could not bring ourselves to sit and watch. The day passed. It was there as we exited the park that evening. It was there as we re-entered the next morning. The photographers set up to catch it's fate. That evening it was still there with no signs of its mother. What happened next was unbelievable. We were not in attendance, but some other wolf watchers we know passed on the story. In the evening a yearling wolf from the Lamar Canyon pack noticed the calf. It crossed over and tried to attack the calf by the neck. This failed. It then grabbed it by the nose and pulled it to the ground. At this point the calf must have let out a wail as the mother, far uphill from the melee, came at full speed to its defense chasing the wolf away. With the wolf on the run, the calf immediately started nursing. Wow, lost for two days and then rediscovered at the final instant the animal could be saved. Amazing. The Lamar Canyon wolf pups would not be having bison this evening...

I can't stop thinking about all of these dynamics... Why do bison swim the river so often if the risk is so high? What could be the evolutionary benefit? It likely improves the swimming ability of the species, but does it benefit the individual female? It would be fascinating to study this more...if I wasn't up to my neck in bird research...

3 comments:

Jesse DeVoe said...

Wow, that is an incredible sighting!

Jesse DeVoe said...

Wow, that is an incredible sighting!

Adrian Langdon said...

Great photos and a good story as well. We've been over from Britain to Yellowstone a few times now and love to catch up on the latest details. Makes me want to get on a plane and spend more time in the Lamar valley!
Adrian.