Coevolution occurs when two or more species interact so that the evolution of both is affected by the selective pressure each exerts on the other. Mutualism is when the interaction of two species contributes essential benefits to the other. Mutualism and coevolution are intertwined.
Whitebark pine (Pinus albicaulis) and Clark’s nutcracker (Nucifraga columbiana) are examples of coevolution and mutualism. Clark’s nutcrackers cut whitebark pine cones open to get the seeds. As they acquire the seeds, the nutcrackers stash the seeds in a special pouch under their tongues and carry the seeds away. The seeds are cached in the soil and provide sustenance for the nutcrackers throughout the winter. In the spring any seeds which the nutcracker did not retrieve and eat germinate and become a new generation of pines.
In 2015 Ronald Lanner wrote a paper summarizing the evidence for coevolution between whitebark pines and Clark’s nutcrackers. There is also evidence of coevolution between nutcrackers and limber pines and some species of pinyon pines. The seeds, cones and tree shape of these pines, whose seeds are distributed by nutcrackers, are different from other pine species that rely on wind for dispersal of their seeds:
*seed morphology – the seeds are large, flightless and highly caloric,
*cones – do not open, have breakaway scales that clasp the seeds and are sessile (no stalks),
*branches – limbs are vertical and forking to increase area for cone production and “display” of the cones,
*seed dormancy – germination is postponed so the nutcracker can eat the seeds over the winter and into the spring. Since the nutcracker has a stashed food supply it can begin to breed earlier in the spring and feed its brood cached seeds.
The Clark’s nutcracker has its own characteristics that help in the utilization of whitebark pine seeds:
*well-developed memory: the nutcracker stores tens of thousands of seeds yet remembers the location of most of them,
*a sublingual pouch that is used to collect and transport quantities of seeds,
*a powerful bill that can cut pine cones and break through to their seeds.
Taza Schaming recently showed that there is also mutualism between Clark’s nutcrackers and whitebark pines. Following periods of low whitebark pine cone production Clark’s nutcrackers do not breed because there is not enough food available. Once the pinecone crop rebounds, the Clark’s nutcrackers begin to again breed.
I find this mutualism and coevolution most interesting.
The whitebark pines were growing at Cloudcap on the rim at Crater Lake National Park in Oregon. The Clark’s nutcrackers were photographed near the lodge at Crater Lake.