Of squirrels and conifers coevolving

A review submitted by Jordan Anderson for the Evolution class.

The feeding preferences of pine squirrels (Tamiasciurus) on conifers lead to two separate patterns of evolution; one of divergence and one of convergence. Squirrels first forage indiscriminately, and then switch to discriminate feeding when supplies run short. This causes selective pressure on the species that is least easily used and results in divergence (Smith, 1970). This selection also produces a divergence between the two tree species in regards to defense mechanisms, such as early shedding of seeds. An example of convergence results, when part of the prey is temporarily inaccessible to the squirrels, thus resulting in energy withheld. Squirrels tend to feed the most on individual trees that are different from the rest in that they are producing more cones, or more seeds per cones (Smith 1970).

The study conducted by Smith took place in the Cascade Mountains of southwestern British Columbia, which is significant, because the Cascade Mountains create a rain shadow with the forests to the east being very dry and prone to burning. In this area, lodgepole pines are ubiquitous and their cones are serotinous, they remain closed for years after maturation to enable them to reseed areas after frequent burns (Smith, 1970). Douglas-firs are also common in this area and will alternate between years of crop failures and years of crop masts; large production. The squirrel population in this area is very stable, they first feed on Douglas-firs but switch to lodgepole pines during crop failures (Smith, 1970). The lodgepole pines serve as a nearly constant food source. On the west side of the Cascade Mountains, the situation is quite reversed. Lodgepole pines are rare, are nonserotinous, and may have cone crop failures (Mowat 1960). This results in a fluctuating squirrel population exerting less selection pressure on lodgepole pines, as they do not exploit as much of the cones. The squirrels in the east exert a selection pressure for harder lodgepole pine cones, and less seeds per cone in the Douglas-firs. Thus, the lodgepole pines exert a selection pressure for squirrels with stronger jaws, those that can eat the stronger cones (Smith, 1970). This change in squirrels is evident as the squirrels in the east have stronger jaw muscles than those in the west.

Another study, explored other information of populations of red squirrels Tamiasciurus, and found that they are territorial. An individual will have its specific area that it maintains year-round. The individual will forage in autumn to assemble a cache of food items. The cache will be located near the center of the individuals’ territory (Larsen et al, 1997). During periods of shortage, squirrels depend on cached food (Vander Wall, 1990). The squirrels will leave their midden to forage on their territory only if the environment happens to be milder on that particular day (Larsen et al, 1997). The squirrels are then foraging the cones that still remain on the trees. Unfortunately, winter foraging does have some drawbacks. It may be more energetically costly to forage than to feed on the food already stored in their midden. It also increases the risk of predation (Larsen et al, 1997).

A study conducted by Lindsay inadvertently tested Smith’s hypothesis about squirrel size relative to cone morphology or cone anatomy. More specifically, Lindsay tested whether squirrel size was influenced by pressure in maintaining efficiency within foraging for a cone cache. He observed that changes in cone morphology selected for squirrel size (Lindsay, 1986). Small squirrels were found in areas dominated by spruce, hemlock, and redwoods (all of which have small cones with little energy per cone). Conversely, large squirrels were found in forests that had larger cones with a greater energy per cone (Lindsay, 1986). His study showed that discriminatory feeding tactics based on the individuals’ size could minimize loss of energy from the food source as they became more efficient in handling time. Efficiency, was calculated by measuring the amount of energy in a particular food while taking into consideration the energy used to process the food. This relationship is noted by Palmer, who expressed the total energy as “net energy” with the energy lost in processing known as “handling time.” Thus, minimizing handling time is a highly efficient tactic for feeding discrimination and efficiency (Palmer, 1981). This is particularly applicable to squirrels, because they need to obtain lots of energy to store up for winter; they need to be efficient in their foraging techniques in order to survive.

Lindsay noted more evidence in support of Smith’s research by observing T. douglasii in association with small cones. This makes sense as douglasii has weak jaw muscles and can manipulate the smaller cones. T. hudsonicus was found in association with large, thick cones, suitable for hudsonicus as it has strong jaw muscles (Larsen, 1986).

I found Smith’s experiment to be exceptionally thorough. This is evident by many authors who have cited Smith’s work within their own studies. However, he was unable to discern if any animal was exerting a selection pressure to maintain thick seed coats (as a defense mechanism by the conifers). I have not yet found an article exploring this important topic. More research needs to be conducted to discover if insects or birds are inhibited by the thick seed coats of ponderosa pine, western white pine, and Douglas-fir (Smith, 1970). If animals are selectively choosing seeds with a thinner coat, then they are creating a selective pressure for the conifers to produce a thicker seed coat in order to escape predation.

References:

K. W. Larsen, C. D. Becker, S. Boutin, M. Blower (1997). Effects of Hoard Manipulations on Life History and Reproductive Success of Female Red Squirrels (Tamiasciurus hudsonicus). Journal of Mammalogy, 78 (1), 192-203

S. L. Lindsay (1986). Geographic size variation in Tamiasciurus douglasii: Significance in relation to conifer cone morphology Journal of Mammalogy, 67 (2), 317-325

E. L. Mowat (1960). No serotinous cones on central Oregon lodgepole pine Journal of Forestry, 58, 118-119

A. Richard Palmer (1981). Predator Errors, Foraging in Unpredictable Environments and Risk: The Consequences of Prey Variation in Handling Time Versus Net Energy The American Naturalist, 118 (6), 908-915 DOI: 10.1086/283883

Christopher C. Smith (1970). The Coevolution of Pine Squirrels (Tamiasciurus) and Conifers Ecological Monographs, 40 (3), 349-371 DOI: 10.2307/1942287

Wall, V. 1990. Food hoarding in animals. University of Chicago Press.