Dynamic changes in home-ranges of the subterranean rodent: a case study on Myospalax baileyi

Background Subterranean rodents, as ecosystem engineers, excavate and inhabit burrow systems. Their underground space-use mobility is poorly recorded. There is conflicting evidence regarding that burrow system of subterranean rodents, once established, are relatively stable because of high-energy costs of digging. For monitoring data at different stages of a year’s life cycle. We predict that mating and habitat characteristics might influence home-range size and locomotion. Methods The present study implemented successive radio-tracking in order to quantify the locomotion and overlap of the plateau zokor ( Myospalax baileyi ) home-range throughout the year. Home-ranges were calculated according to the 100% minimum convex polygon (MCP) methods. We also examined the relationships of home-range sizes with body mass, vegetation biomass, and soil compaction, respectively.

resource. Regarding locomotion of home-range, subterranean plateau zokors to affirm that their home-ranges are dynamics and males shift in space with one breeding cycle.

Background
Basic theoretical studies on the use of animal space are all based on the concept of home-range, which was initially defined as an area where animals to meet its daily needs [1]. But more recently, the home range has been defined as an area that an animal repeatedly traversed [2,3]. Most resident mammals limit their movements into a fairly well-defined area, rather than just wandering randomly.
The original range has been explored in a variety of ways, including empirical and theoretical. The fact is that the home-range of animals is dynamic in size and position in the spatiotemporal sequences has been overlooked in most studies [4]. Only in the last 20 years, researchers realized that the dynamics of home-range have essential effects on population dynamics and other aspects of animal ecology [5]. Moreover, studies of these home-range dynamics have been restricted to relatively few subterranean rodent groups [6]. Most studies have focused on factors that influence the size of the home-range at a given point in time.
Subterranean rodents are convergent ecological groups [7,8]. Compared with ground-dwelling species, its scope of activity and dispersal are greatly restricted by the underground environment.
Most of their activities take place in underground burrow systems that are occupied by solitary and society individuals. Subterranean rodents mainly rely on plants' underground storage organs for food.
They must constantly extend burrow systems to ensure adequate food resources. Begall et al. proposed burrow systems of subterranean rodents must be continuously transformed by excavating new tunnels while part of the older tunnels will be abandoned [9]. In addition, some subterranean rodents maintain an optimal size of burrow systems through backfilled tunnels with soil [6]. If the home-range size of animals is too large, it will disadvantage other competitors. But this can be even more detrimental to the domain's occupant because the cost of defending the territory increases dramatically as the home-range expands [10]. The food resource and soil characteristics can directly influence the home range sizes and burrow systems dynamic of subterranean rodents in different habitat types [11,12]. Home range size is usually linked to the animal body size, where larger animals require larger areas to encounter potential mates or recruiting. Cutrera et al. observed the intraspecific variation of home-range size between two different sites in the South American Talas tuco-tuco (Ctenomys talarum) [13]. Home range size influenced by body size and soil characteristics (mainly soil hardness and soil humidity) between the study sites [14]. For those small mammals inhabiting the underground environment, live trapping and radio-tracking become a valid tool to collect data systematically on their characteristics of movement and space use. At present, a few species of Bathyergus and Ctenomys, and Spalax studies on home range size have been carried out by radio-tracking [15][16][17][18][19]. Surprisingly, there are few studies on the long-term space utilization of subterranean rodents. Although there have been studies of seasonal changes in home-range dynamics [20,21], these studies were conducted in different populations of the same species at different seasons. Nevo indicated that all underground mammals' home-ranges [22], once established and use for one breeding season, which is essentially permanent for life. However, in solitary subterranean rodents, blind mole rat (Spalax ehrenbergi) constantly shifting home ranges were described (but not quantified) [23]. Šklíba et al. kept tracking silvery mole-rat (Heliophobius argenteocinereus) for three months in non-breeding season and found their home ranges were dynamic and continuous change in space [6]. Based on all the facts as mentioned above, we assume the home range is dynamic, patterns of animal space use, resulting from life-history strategies or interactions between individuals and the external environment.
In this study, we analyzed the use of space and stability of home range in the plateau zokor Myospalax baileyi (Rodentia: Myospalacinae), a widely distributed solitary subterranean rodent from the Qinghai -Tibet Plateau [24]. Adult females of plateau zokors give birth once a year between April to July. The gestation period and lactation period last about 50 days, respectively [25]. Myospalax baileyi occurs various habitats with different food supply and soil properties in an environment with an alternate change of cold and warm season, and it exemplifies a successful adaptation to an extreme subterranean environment [8]. We investigated individual home-range using a radiotelemetry. We tracked M. baileyi from June 2016 to May 2017, because it contains different physiological changes of plateau zokor during the year, as well as changes in food supply and soil characteristics. Here, it occupies various habitats with varying characteristics of soil and food supply in a seasonal environment with regular inter-change of the dry and rainy season. The main objectives were: (i) to quantify home-range dynamics, overlap degree between individual home ranges, the quantity of new mounds bulldozing, movements of single individual home-range position, and changes of the nest; (ii) to assess how these animals explore the underground environment in a full year.

Material And Methods
The study was conducted in the eastern part of the Qinghai-Tibet Plateau in the Gansu province

Radio-tracking
Radio-tracking was carried out in June 2016 to May 2017. We captured 14 adults M. baileyi (7 females and 7 males) in 2016 and 12 (five females and seven males) adults in 2017, respectively (Table 1). In addition, we captured a sub-adult (ID: M981) in 2017, which was not included in the statistical analysis. Animals were captured using a living trap for subterranean rodents [26]. In order to ensure the integrity of the tunnel, we did not re-capture individuals, so we did not obtain weight data for other months of 2016. After capture animals were anesthetized (1% pentobarbital, 0.5 mg/100 g body mass), weighted, sex, fitted with a radio-collar (Ag 357, Biotrack Ltd., Wareham, Dorset, U.K. Battery life is about 9 months). The animals were released only when fully recovered from the effects of the anesthetic and released at their respective trapping locations. We used radio collars that weighed 4.75 g (< 4% of the body weight of the smallest zokor used in this study) [27]. Experimental procedures involving capture, handling, and use of radio-collars in this study was approved by the Institutional Animal Care and Use Committee of the Grassland Science College of Gansu Agricultural University (GSC-IACUC-2018-0011). We used a Sika radio-tracking receiver (Biotrack Ltd., Wareham, Dorset, U.K.) and two-element Yagi antennas (Sirtrack Ltd., Havelock North, New Zealand) to locate zokors. In 2016, radio fixes were taken in 24-h sessions for 10 days each month (the 15th to 25th of each month). To ensure the independence of data collection, there was a 2-h time interval between fixes [19]. In 2017, radio-tracking began 36-48 h after animal release. All individuals continued to be tracked until May 25. Radio fixes were taken in 12-h sessions (8:00-20:00) and 2-h time interval between fixes. 13 zokors (7 females and 6males) from June to August and 11 zokors (5 females and 6 males) in September and October of 2016, and 12 (5 females and 7 males) in April and May of 2017 were successfully radio-tracked. The radio collars of two zokors (ID: F533, F663) fell off in September 2016, and we were not able to recapture them, so we did not obtain any more data after that.
Another radio-collared male (ID: M425) was not radio-tracked, because it left its burrow system after Individual home-range size was calculated using 100% minimum convex polygon (MCP) methods. This method is commonly used to evaluate the home range size of subterranean rodents [3]. In order to accurately record the current position of zokors, we set up a geo-referenced grid (5 m × 5 m cell size) above all burrow systems before radio-tracking. By measuring the vertical distance between the fixes and the nearest tunnel, we estimated the accuracy of radio-tracking points at < 1 m. In the process of radio-tracking, we found that some individuals' home range did overlap. Therefore, the degree of the home range overlapped is calculated by dividing the area of one individual overlap with other intraspecific individuals by the total area of the zokor. We also calculated the MCP using all tracking points from each individual for purposes of comparison with other studies. MCP 100% and overlap calculations were performed using the Ranges 8 version 2.1.6 (Anatrack Ltd., Wareham, U.K.).

Environmental Factors Measurement
To evaluate the soil and vegetation characteristics, we considered every individual nest as a central point. At each of the central points, plant biomass and soil compaction were sampled in the three sampling units situated a distance of 3 meters in the three cardinal directions. To estimate plant biomass and soil hardness, we referred to the method of Galiano et al. [28]. In brief, the vegetation present in a 0.25 m 2 × 0.3 m sample was collected and separated in aboveground and subterranean portions, dried for 24 h at 80℃ and weighted to the nearest 0.1 g. Soil samples were produced from near the capture points. Soil compaction measured 12.5 cm, 15 cm, and 17.5 cm (soil depth) use SC-900 Soil Compaction Meter (Spectrum, USA), we calculated the mean of three depths as the compaction (unit: kg·cm − 2 ) of one sampling point, because the tunnel depth at the capture points of the zokor is between 10-20 cm [29]. For analysis of soil compaction and vegetation variables, we computed the mean of the three sampling units and then the mean in the analysis.

Statistical analysis
We applied Student's t-tests or Mann-Whitney U tests (in the case where the assumptions of the t-test were not fulfilled) to compare the differences in home-range size and overlap between sex in same months. We also used the same analysis methods to compare the differences between male and female body mass in two years. The comparations of overlap area of the same individual's homerange between June and October, and the percentage of the overlap ratio in two months were examined using a Student's t-test or a Mann-Whitney U test. A comparison of all variables was implemented in GraphPad Prism version 8.0.1 (GraphPad Software, San Diego, CA).
For multiple comparative analyses, we used one-way analysis of variance (ANOVA) to deal with the repeated measured data (home-range sizes, number of new mounds, soil compaction, and plant biomass) of different months. The ANOVA analyses were performed with STATISTICA StatSoft Inc.
(version 17.0) for Windows. The influences of each row represent matched data (soil compaction and biomass of aboveground or underground) on the mean home-range of females, males and individuals were test using a linear aggressive analysis, as implemented in GraphPad Prism. In all cases, the critical significance level was set at P < 0.05. Results are shown as mean ± SEM. During the mating period, females' home-ranges overlapped those of males by 3.52 ± 1.08% (n = 5, female-male pairs), whereas home-ranges of male overlapped females' home-ranges by 23.52 ± 11.02% (n = 8, male-female pairs; Fig. 3F). No overlapping was found between the home-ranges of females. We observed M697 spread over the aboveground from captured area a to area b (after 72 hours of radio-tracking ) ,and after staying at point b for two days, it spread over the aboveground to area c and built a new home-range. There was no overlap between home-ranges of other adult males.

Results
Only one sub-adult had a small overlap with M198 (Fig. 3F). In addition, we found that the female's nest was within the overlap of the male's and female's home-ranges, and we detected the radiocollars signals of the males that overlapped with the female's nest at the same time.
In June, home-ranges of female overlapped those of males by 11.14 ± 3.82% (n = 7, female-male pairs), and home-ranges of male overlapped females' home-ranges by 9.85 ± 3.79% (n = 6, malefemale pairs; Fig. 3F). Home ranges of 5 of the 7 females radio-tracked overlapped by a mean of 17.83 ± 0.29% (Fig. 3A), whereas home ranges of 4 of the 6 males radio-tracked overlapped by a mean of 4.49 ± 1.02%. In July, there were no overlap between home-ranges of males or females. Only two pairs (M442 -F533 and M357 -F663) of all individuals had small overlaps. The mean percentage of overlap of a single zokor home-range with other individuals was 23.6 ± 8.6% in April May and 28.3 ± 8.3% in June, respectively (Table 1); this difference in the proportion of overlapped was not significant (Mann-Whitney U = 77, P = 0.978). Although the average overlapped areas of single individual home-range (72.0 ± 20.8 m 2 ) in June were larger than in April May (9.85 ± 3.79 m 2 ). No difference was found in two months (Mann-Whitney U = 55, n = 12, exact P = 0.225). In July, it was only 2.7 ± 1.5% (mean overlapped area 2.3 ± 1.0 m 2 , Fig. 3B and Table 1) (Fig. 3E). The overlap area of home-ranges between M104 and F521 was only 0.7 m 2 , accounting for 1.3% and 0.8% of their respective home-range (Table 1). However, the overlap area of home-ranges between M442 and F304 reached 14.2 m 2 , accounting for 8% and 31.2% of their home-ranges, respectively (Table 1)     Home-range movements.-All individuals' home-ranges of June had overlapped with their own homeranges in July. Average overlaps accounted for 37.4 ± 8.5% (females 46.3 ± 13.0% and males 26.9 ± 9.8%, respectively) of the home-ranges in June, whereas average overlaps accounted for 84.7 ± 4.4% (females 82.4 ± 5.6% and males 87.4 ± 7.3%, respectively) of the home-ranges in July (Fig. 4). The home-ranges of 3 males (M357, M288, and M039) in July were completely contained within their own home-ranges of June (Fig. 4). No shifting of the position of home-range of all individuals at this stage. Table 3 The overlap area of the same individual's home-range between June and October, and the percentage of the overlap ratio in each month of 2016. Five out of the seven females, except for the two individuals (F533 and F663) of radio-collar shedding, had overlapping in June and October (Fig. 4). The average overlap area of the home-range was 38.7 ± 18.5 m 2 (range 4.4-104.6 m 2 ). The proportion of overlap area in the home-range size to June was 36.0 ± J3.5% and 41.2 ± 13.7% to October (Table 3). From June to October, the home range movements of four females (F233, F638,F521, and F169) around the respective nest. F304 appears to have been building new nest since July and almost give up its home-range of June after October. In

ID
October, the home-ranges of only three male individuals (M008) overlapped with those of June, and the position of the respective nest did not move (Fig. 4). The average overlap area was 89.6 ± 6.8 m 2 .
The average proportion of overlap areas in a home-range was 27.5 ± 6.5% and 90.6 ± 7.3% in June and October, respectively (Table 3) In our study, exceeding 10 zokors were continuously radio-tracked for more than 10 months for the 1st time without breaking their burrow systems. The radio-collars did not appear to influence the zokors' normal activities, including mating, excavating ,and building new surface mounds. The radiocollars could be precisely located the locomotor position of plateau zokor by Yagi receiver, because we used the receiver to accurately find two radio-collars that the zokors (F521 and F663) had dropped off. Although we did not dissect burrow systems of radio-tracked zokors, their home-range size in the breeding season was 123.0 ± 50.92 m 2 , not significantly different from the average home-range size (122.7 ± 53.75 m 2 , mean ± SEM) found by Zhou and Dou [20] radio-tracked in the same reason.
Studying the secretive habits of subterranean rodents in their natural state is often challenging.
Originally by dissecting the entire burrow systems for studying their home-ranges [30,31]. This method only provides "snapshots" of the respective individuals or colonies tunnel system geometries used to estimate the size of home-range [14]. Longer period space utilization of subterranean rodents relies on registering of new surface mounds and animals' capture sites, but this method may be not appropriate for M. baileyi, because three females (F233, F663, and F169) of expended their homerange in August (Fig. 3C), but none of the individuals produced new mounds in this month. Although an anatomically complete tunnel can provide detailed information about the size and structure of the animal's actual tunnel system. The most effective method for long-term monitoring of home range dynamics seems to be radio telemetry for subterranean rodents in their histories [6].
Plateau zokors show dynamic changes in sizes and overlaps of home-ranges between intraspecific individuals in their natural habitat. A number of factors may explain the larger home-ranges of male M. baileyi during the mating period. Many solitary subterranean rodents have a polygynous mating system [32]. Males try to gain access to multiple potential estrus females by expanding their burrow systems [5], suggesting that females are a finite resource that likely affects the males' home-ranges.
Although the mating system of plateau zokor is still controversial [24], we found a male zokor (M929) entering the nest of two females (F147 and F616), respectively (Fig. 3F), suggesting that M. baileyi has a polygynous mating system. Intersexual differences in home-range sizes of M. baileyi during the mating season also have been reported by Zhang et al. [33]. Our findings suggest that home-ranges of males remain relatively independent, which also avoids competing with each other for mates in the mating period. Similar findings were described by Zhou and Dou [20]. Solitary subterranean males may search for a spouse across the aboveground except for expanding underground burrows. A study reported that long distance between the home-ranges of mating partners of silvery mole-rats was found to rule out an underground looking for mates [34], revealing that males probably seek estrus females surface. In the present study, a male (M697) transformed across the aboveground to two new locations and eventually established a new home range (the area reaches 110 m 2 ) within 32 days (Fig. 3F). Capture-mark-recapture rates of male plateau zokors recapture lower than males over three years during the mating seasons [33], and zokor's skulls were found in vomits of many raptors [35], shows that males may be spreading out from the ground in search of opposite sexes even though they will be at a high risk of predation. This may be another factor in the changes in their homeranges.
In the present study, one month after the mating period, home-ranges of females frequently overlap multiple home-ranges of neighbors, which were rare among solitary and aggressive subterranean rodents [36]. M. baileyi seems to show great tolerance for the closed neighbors. At this stage, the average size of the female's home-range was 2.6 times larger than during mating, which may be related to the large food consumption during pregnancy and lactation; the size of the male's homerange did not significantly change compared with the mating period. However, we estimated the core area of plateau zokor using 60% cores method by Cooper et al. [32], and found that most individuals had no overlap, with only F233 and F663 overlapping 7.6 m 2 ; their core areas remained relatively independent and exclusive. Because the core areas are the location of cache and food storage [20,24], the defense of the cache and food resource may be a major factor in the subterranean rodents preferring to solitude. Compared with the above two stages, the home-ranges of plateau zokor were significantly reduced at the end of reproduction and the non-breeding season, and there was no overlap between the home-ranges of most individuals. The home-range at this time can even be synonymized to the territory [22]. Because maintaining and defending a large territory could pay out huge costs (e.g., predation risk and energy expenditure). In addition, approximately half of the individuals did not extend new space in August and September of the non-breeding season, which activities were mainly concentrated on their own nests, similar to what was observed by Zhou and Dou [20]. Accordingly, the deceased activity intensity of plateau zokor during this period was also reported by Ji et al. [37]. Space utilization patterns of subterranean rodents may reflect an adaption of tunnel ambient temperature [8]. This kind of narrow range may be an adaption for the avoidance of overheating during the warm season [21].
Another evidence of the dynamic of home-ranges of the plateau zokor is a relatively low overlap between a single individual's successive home ranges within five months in 2016 (Fig. 4). Females usually had a fixed nest, with the expansion of new tunnels surrounding the nest. Besides childrearing, the home-range changes may also reflect a foraging strategy against the costs of traveling [9]. As Bandoli [38], Zuri & Terkel [23] and Smallwood & Morrison [39] proposed baileyi refilled burrows with excavated soil, which has been described (but not quantified) by Zhou and Dou [20]. Based on radio-telemetry data and new surface mounds, we suppose that male plateau zokors have a more efficient rate of digging. Compared with the home-ranges of females, the homeranges of the males is not stable in the long-term. It would be shifting in space, especially during the non-breeding season. Three males built completely new burrows in October (non-breeding ) in our study (Fig. 4). essentially permanent for life [22]. With some solitary subterranean rodents, shifting home-ranges were reported for the blind mole-rat [23] and silvery mole-rat [6]. Shifting in male home-ranges may be to avoid too much inbreeding [24], or to reduce the competitive pressure for food resources and space caused by the increased number of newborns in the future. We did not capture two zokors simultaneously in the same burrow systems of females, suggesting that individuals born in the previous breeding season had built their own nests. In addition, the migration of the home-ranges in solitary subterranean rodents may be a population density self-regulation strategy [41], but Šklíba et al. thought the changes of home-ranges more likely reflect a defensive strategy [6].
Most subterranean rodents, including plateau zokors, inhabit seasonal environments [9]. Seasonal food availability probably influences the space use of subterranean rodents [28]. Although the underground storage organs of plants are the main source of food for subterranean rodents. Plateau zokor is known to consume aerial part of the vegetation in addition to subterranean plant organs, and even they feed on the stems and leaves of grass and sedge [42], which may affect their space utilization. Instead, the burrowing and foraging activities of plateau zokor M. baileyi has great influence of soil properties and plant community [43]. Some of the overgrazed areas in the alpine meadows of Qinghai-Tibetan Plateau, with the population of plateau zokor increases rapidly, which leads to increased grassland degradation [44]. In our study, we found that there was a significant positive correlation between the size of home-ranges of plateau zokors in different months and the underground biomass. Soil compaction is also considered an ecological constraint in extending burrows of subterranean rodents [45,46]. Although we found no significant difference that soil harness was between two peaks (June and October) in the activity of the plateau, the average size of home-ranges in June was significantly (Mann-Whitney test: U = 20, P = 0.002) larger than that in October. This may be due to the abundance of underground biomass in June. But in October, the plateau zokor is supposed to gather a great deal of food from underground (the aboveground parts of the plant are almost inedible) for the long winter. The plateau zokor did not further expand its homeranges to obtain more food when the underground biomass was relative deprivation. They could be by digging more branching tunnels for foraging. Despite this has not been proven in our study because we did not open the tunnel for long-term monitoring. Exploration of the surrounding of permanent primary tunnels by excavating short and branched foraging tunnels is a typical feature in many subterranean rodents [6,31,36].