Divergent Life-History Strategies of Temporal Cohorts of Indian Rice Frogs (Fejervarya limnocharis) in the Two-Crops Rice Fields in Taiwan

Amphibian larvae have high plasticity in life history traits such as metamorphic age and size to cope with the varied aquatic environments. However, among the plentiful studies, few have investigated the effects of seasonal environmental changes. On the other hand, amphibian populations in diverse habitats or at different locations may face considerably distinct environmental variations. To better understand the life history responses of amphibian larvae to the spatial and temporal environmental variations, I used common garden experiments to investigate the metamorphic strategies of the rice paddy frog, <i>Fejervarya limnocharis</i>, tadpoles. I asked the following four questions: (1) Do <i>F. limnocharis</i> tadpoles living in two crops rice fields show cohort-dependent (spring vs. summer cohorts) metamorphic strategies in response to different temperature regimes in spring and summer? (2) How does food availability alter the responses of <i>F. limnocharis</i> spring and summer cohorts to seasonal temperature regimes? (3) Is cohort-dependent plasticity a common life-history strategy among different populations of <i>F. limnocharis</i> residing in two-crops rice fields? (4) Do <i>F. limnocharis</i> populations living in cultivated fields with different irrigation regimes exhibit differential cohort-dependent metamorphic strategies? I performed four experiments to tackle the four questions, respectively. In experiment 1, I collected spring (May) and summer (September) <i>F. limnocharis</i> tadpoles from Ankang rice fields. The tadpoles underwent a 2 x 2 temperature (22 vs. 29 ºC) by cohort (spring vs. summer cohort) factorial experiment. I found that both spring and summer tadpoles have higher growth rate when raised under their respective field temperatures. Moreover, spring and summer tadpoles used ''size advantage'' and ''rate advantage'' as cohort-dependent metamorphic strategies in response to seasonal temperatures, respectively. That is, spring tadpoles responded to low temperature with larger metamorphic weight, while the summer tadpoles did not. On the other hand, while both spring and summer tadpoles responded to high temperature with accelerating growth rates, summer tadpoles grew significantly faster than the spring ones. In the second experiment, I manipulated an additional factor: food availability (<i>ad libitum</i> vs. restricted) in a 2 x 2 x 2 temperature by food by cohort factorial experiment. The results were similar to those of experiment 1 when larvae were raised under <i>ad libitum</i> food. However, under restricted food, both spring and summer cohort reduced their metamorphic size. Spring tadpoles still had size advantage than summer ones at low temperature, whereas summer ones lost their ''rate advantage'' at high temperature and had slower growth rates than spring ones. In the third experiment, I collected tadpoles from rice fields at three locations: northern (Ankang and Daxi) and central (Changhua) Taiwan to examine their similarity in cohort-dependent plasticity. The tadpoles underwent a 2 x 2 x 2 temperature by cohort by location factorial experiment. The results showed that all three populations had cohort-dependent plasticity. Inter-population variations exist: the most northern population, Ankang, showed the clearest cohort-dependent plasticity likely due to being at a strongly seasonal temperatures and largely isolated location. The other northern population, Daxi, displayed a moderate, while the central population, Changhua, showed the least cohort-dependent plasticity probably due to the reduced seasonality in temperature toward the south. In the fourth experiment, I collected tadpoles from rice paddies and water bamboo fields that differ in irrigation regimes (disrupted vs. constant irrigation). The tadpoles underwent a temperature by cohort by irrigation system factorial experiment. The results showed that tadpoles from disrupted irrigation populations had cohort-dependent metamorphic strategies, while tadpoles from constant irrigation populations did not. Overall, the four experiments reveal that <i>F. limnocharis</i> tadpoles have high plasticity, specifically strong cohort-dependence, in metamorphic traits. Spring and summer tadpoles used ''size advantage'' and ''rate advantage'' strategies, respectively in response to seasonal temperatures. Such strategies vary depending on food availability, geographic location, and irrigation system.