In the present study, inter-specific variation was observed for behavioral responses in an experiment testing novelty. On average, the smallest species in our sample, M. caroli, spent the most time exhibiting non-defensive behaviors, predominantly comprised of exploring. The other mouse species and second smallest, A. agrarius, spent the most time foraging, had the highest consumption ratio and most foraging events during the first trial. Contrarily, the two rat species in our experiments spent considerably more time demonstrating defensive behaviors; R. exulans spent the most time hiding and R. losea was motionless for the longest cumulative period. One possible explanation for these findings is differences in life history strategies conforming to a fast-slow continuum. Despite the species in our study all being relatively small-bodied animals, they do fit a spectrum of size differentiation, as well as exemplify variation in life history. The larger rat species, included in the present study, have longer lifespans and reach sexual maturity later compared to the two mice species [47, 52]. Our results support our prediction of behavioral differences between species and largely adhere to the POLS hypothesis, demonstrated by the smaller mice species performing a higher proportion of riskier behaviors (exploration and foraging) in a novel situation, in contrast to the larger rat species. The smaller species in our study, M. caroli and A. agrarius, prioritized fast exploration or acquisition of resources, as oppose to defensive behaviors. Furthermore, not only did A. agrarius allocate the most time to foraging, they also consumed the most proportional to their body weight during the first trial of our experiments. This species favored the acquisition of resources over exploration or cautious behavior in an unfamiliar situation. These results may indicate a trade-off favoring current fitness returns compared to future expectations [16, 18]. Similarly, Vasquez  studying foraging behavior of three different Chilean rodent species varying in body size found that under increased risk the largest species was the most cautious.
The response variables foraging, foraging events, consumption ratio and concealing were found to differ significantly between sexes. At the rodent community level (all species combined), males spent more time foraging with more foraging bouts compared to females for their first trial. Since male rodents generally have less parental investment and are promiscuous, it can be expected that there will be sex-related differences for trade-offs between risk and reward [20, 54]. Therefore, males may have a higher propensity to expose themselves to risk in return for a reward ; in the case of the present study, risk of exposure in an unfamiliar environment and a reward of access to food. Furthermore, since males are territorial and will often compete over resources , such as food, the opportunity to freely forage (as was the case in our trials) without competition may have prevailed over the perceived risks. Male rodents are generally larger than females [52, 54], therefore, they may also have a higher food intake potential , as was observed in the present study. Overall, the defensive behavior concealing was higher for females, which indicates that in response to a novel, unfamiliar situation females exercised more caution. Adult, sexually mature females may show a preference for defensive behaviors in a novel context since the risks may outweigh rewards as they incur higher costs for parental care . Our results are consistent with other studies examining behavioral responses to high risk situations [21, 25], with female rodents exhibiting a higher proportion of defensive behaviors, such as hiding, compared to males. Even though our results do not demonstrate significant differences between sexes for each species group for the behaviors foraging and concealing, we still observed trends. With respect to time spent foraging, the males of each species group had, at least, somewhat longer durations. Additionally, we detected a trend for females favoring slightly more time concealing for three of the four species in our study: M. caroli, A. agrarius and R. losea.
The presence of leopard cat cues during the second trial did not have effects on the defensive and non-defensive behavioral categories. There was, however, a significant interaction between species and treatment for the defensive behavioral category. Moreover, the only individual behavior that was influenced by predator odor was concealing, which is a defensive behavior. This result can most likely be explained by a consistency in concealing behavior for individuals of the A. agrarius species; they spent similar amounts of time hiding for both trials. Individuals of A. agrarius that were randomly assigned to the leopard cat odor treatment group also were more inclined to hide. We also found a significant interaction between treatment and species for the behavior motionless. In this case, individuals of R. exulans in the control group spent more time motionless during the second trial compared to those in the predator odor group. Therefore, once again this finding can most likely be attributed to intra-individual constancy for this defensive behavior; certain individuals displaying a higher affinity for remaining still. Repeatability of behaviors over time and even across varying situations has been observed in similar experiments [41, 56, 57], and can even outweigh the effects of predator odors .
The predator odor failing to elicit aversive behaviors in the present study conforms with many other studies that have exposed rodent species from wild populations to predator cues both in lab [25, 32, 58] and field [31, 59, 60] contexts. Furthermore, the majority of studies that have found significant effects of predator odors performed their experiments on captive-bred rodents [21, 27, 61, 62]. The domestication process of captive rodents may lead to an inhibition of behavioral variation and adaptability [35, 63], resulting in more pronounced responses to foreign odorous stimuli. There is a growing consensus stipulating that for wild prey populations predator odors alone may not evoke strong antipredator responses [31, 49, 64], but in turn, a combination of factors, including physiology, type of perceived risk, and habituation . Indirect risk factors, such as illumination and vegetation cover, have been found to play larger roles in governing rodent foraging behavior compared to direct predator cues, such as odor [64, 65].
Since leopard cats have been absent in Eastern Taiwan for several decades, and therefore numerous generations of the local rodent species, it is possible these respective rodents have lost the ability to discriminate the odor. Additionally, other small carnivores in eastern Taiwan that are capable of predating on rodents, such as the lesser civet (Viverricula indica) and feral cats (Felis catus), occur at low densities (I. Best, unpublished data) or rodents are not a main prey item for them [66, 67]. Antipredator responses are very costly  and if a given trait no longer serves a purpose it is likely that it will be selected against and lost . Furthermore, according to the naiveté hypothesis prey are not expected to discriminate and respond accordingly to novel predators due to no previous encounters . In Australia, the invasion of cane toads (Rhinella marina) prompted the relocation of native Northern quolls (Dasyurus hallucatus) to predator-free islands, and they have lived in these conditions for multiple generations . Jolly et al.  compared responses of quolls from both the predator-free island population and mainland Australia to native predator cues. Opposite to the mainland quolls, the island population showed no aversion to the predator odors. For the current study, despite the possibility that rodents inhabiting leopard cat-free regions are naïve to the predator and are unable to recognize their odors, further research testing rodents in areas where leopard cats are present is necessary to affirm this prediction. Additional studies could also be conducted to test the responses of these populations of wild rodents to other mammalian carnivores that do occur in the region, considering that the predation pressure is likely weak.
Given the lack of predator odor effects on rodent behavior, we were able to examine behavioral responses across trials. Even though trial did not have a significant effect on the behavioral categories (defensive and non-defensive), specific responses representative of behavioral plasticity and habituation [29, 41] did change. Two out of the four species showed significant increases in amount of time foraging, and M. caroli did spend more time foraging despite the difference not being significant. M. caroli and R. exulans also significantly decreased the amount of time spent motionless during the second trial. Therefore, the three species, M. caroli, A. agrarius and R. exulans, demonstrated a trade-off in defensive behaviors, as well as exploration, for access to food resources. Even though exploring can be constituted as a non-defensive behavior conferring boldness, individuals are still able to keep some level of vigilance [16, 49] whereas with foraging, vigilance is sacrificed to a much higher degree . Our results provide further support that wild populations of rodent species can have behavioral plasticity, as habituation can be linked to phenotypic plasticity . In the case of the present study, the increase in foraging activities and exploitation of the food patch can reflect learning and also be a measure for information processing . We find it unlikely that the increases in foraging behaviors observed were stress-induced or a product of our experimental procedure. Animals were food deprived for the same amount of time on both days of testing, and were provided with ample food upon return to their housing cage after completing the first trial. Moreover, high levels of acute stress on rodents may actually inhibit food intake and prompt defensive behaviors [73, 74]. The variation between individuals that was observed in our experiments could also indicate not only behavioral plasticity, but also large behavioral repertoires . In a study conducted by Holmes et al. , wild mice in plus-maze experiments were found to demonstrate a greater selection of behaviors compared to captive strains. A broad behavioral repertoire could also have implications for fitness under a changing environment – increased human activity and disturbance. A species with a wider behavioral range (boldness-shyness) may be more resilient to disturbances [38, 77].
The largest species included in our study, R. losea, had contrasting responses compared to the other species during the second trial. Namely, the species failed to exhibit significant increases in any of the non-defensive behaviors. These results may indicate different rates of habituation between the species. On average, R. losea ranged from three to ten times larger in size than the other species. With the predictions of the POLS, R. losea would be expected to be the most cautious species in our study, therefore, it could also be possible that this species would habituate to novelty at a slower pace. Larger species with slower life history traits have a tendency to be more cautious with stronger neophobia responses [16, 38, 49], therefore habituation to a novel situation with associated risk maybe slower compared to smaller species.
The invasive species, R. exulans, somewhat surprisingly spent the most time concealing during the first trial. However, the second trial for this species comprised a drastic reduction staying motionless with an increase in foraging. The average amount of time spent hiding was also lower, though the difference between the two trials was not significant. These results demonstrate the plasticity and habituation potential of the rat, which may be characteristic of an invasive species [39, 78, 79]. Additionally, the initial caution the species exercised could also be somewhat indicative of their strategy for occupying novel environments – not overly bold to a degree of recklessness. The species could benefit from processing information and assessing risk about the new environment from a safe refuge in addition to exploration [80, 81]. To better understand the habituation potential and rate of invasive species, further studies adopting a comparative approach involving multiple invasive and native species will be necessary.
Interestingly, in tandem with the range expansion of R. exulans in Eastern Taiwan, A. agrarius has been experiencing population declines [82; I. Best, unpublished data]. In the present study, we observed A. agrarius to be the most voracious foragers exposing themselves to risk for the longest periods of time. The lack of defensive behaviors to the simulated cues of risk in our experiments (novel environment and objects) may suggest that they have an increased vulnerability to predators, biological enemies and other disturbances in the wild.