Mast seeding of Sasa borealis influences foraging behavior of field mice: A field study by Nagoya University yields results

A research group led by Graduate Student Hanami Suzuki and Professor Hisashi Kajimura of the Graduate School of Bioagricultural Sciences at Nagoya University announced that they have found that field mice, which feed mainly on seeds in forests, prefer the seeds of Bambusoideae (Sasa borealis) to those of other trees (Quercus crispula and Fagus crenata) but not to those of and Lindera triloba. They found this by examining the preference of field mice for seeds from each of the four tree species and Sasa borealis. The discovery provides a clue for evaluating the impact of Bambusoideae mast seeding in forests. The results were published in the October 19 issue of the British journal Ecology and Evolution.

After decades to a century of vegetative (asexual) reproduction, Bambusoideae undergo large-scale sexual reproduction during which multiple individuals reproduce synchronously, produce a large number of seeds (mast seeding), and then die off. The mast seeding of Bambusoideae is known to cause large outbreaks of field mice, which are typical seed eaters. Previously, the research group had shown that field mice living in forests forage and store seeds of Sasa borealis. However, the extent of their food preference was unclear.

In this study, field experiments were conducted in Inabu Field, where mast seeding of Sasa borealis occurred from 2016 to 2017. (Inabu Field, which is affiliated with the Field Science Center Division of the Graduate School of Bioagricultural Sciences at Nagoya University, is a forest in Toyota City, Aichi Prefecture.)

A cafeteria system was used in which a certain amount of Sasa borealis seeds of and those of one other tree species were placed in a shallow mesh container and allowed to be freely selected by the visiting animals. Seeds were collected from four tree species in the same forest: Castanea crenata, Quercus crispula, Fagus crenata, and Lindera triloba. The most frequently recorded foraging scenes captured by the sensor-type automatic cameras installed in the vicinity were those of the field mouse species Apodemus speciosus and Apodemus argenteus.

The foraging rate was calculated on the basis of the overnight decrease in the number of seeds offered. Additionally, two indices of the preference of the mice in terms of foraging order were used.

The first indicator compared the probability that the field mice chose either the Sasa borealis seeds or the other tree seeds first. The second indicator was the percentage of mice that foraged either the Sasa borealis seeds or the tree seeds until all the seeds of either type were gone. The data on foraging rate and order showed that field mice had the highest preference for seeds of Castanea crenata, followed by those of Lindera triloba, Sasa borealis, Fagus crenata, and Quercus crispula (in decreasing order).

Based on the traits of each seed type, it appears that the high preference for Castanea crenata seeds is due to their large size and high number of resources available at one time. By contrast, the low preference for Quercus crispula seeds is due to their high tannin content, which is toxic to field mice.

Identifying the preference of mice for seeds of each tree species coexisting with Sasa borealis is key to understanding how the tree species are affected by the mast seeding of this Bambusoideae species.

Kajimura stated, "This research was mainly conducted by Ms. Hanami Suzuki, a 3rd year PhD student at the Graduate School. She has been fascinated by the interactions between organisms during the mast seeding of Sasa borealis and has been collecting data for six years. We found that field mice affect the predation pressure or dispersal pattern of Sasa borealis seeds, and as part of that, we came up with the idea of comparing the Bambusoideae seeds with tree seeds. The dynamics of forest ecosystems, which involve rare biological phenomena, have made it difficult to verify cause-and-effect relationships, but this novel approach has led to this breakthrough."