Using Astragalus to Model Body Mass, Habitat Preference, and Population Density of Prehistoric Bovid Duboisia santeng (Dubois, 1891) in Eastern Java island in The Early Pleistocene

Astragalus bone is one of the most important fossil records as it can reconstruct the prehistoric life. Respectively, this study aims to model the body mass, habitat preference, and population density of prehistoric bovid Duboisia santeng (Dubois 1891) in eastern Java island in the early Pleistocene. The astragali from 9 specimens were used to estimate the body mass and population density. Likewise regression models are used to analyze the relationship between astragalus lateral length, width, and body mass compared to the astragalus of extant Bovid species. The result revealed the body mass average was 60.3 kg (95%CI: 58.9-61.7) and this indicates the D. santeng belongs to large herbivores. While the population density was estimated at about 5.39 individuals per km2 (95% CI: 3.18-7.6).

Java island is known as one of island with high diversity of Bovids from Pleistocene era. One of Bovids that the fossil records have been found in central and eastern parts of Java island is Duboisia santeng. This antelope like Bovids were originated from south east Asian land and migrated to south to Sundaland during Pleistocene era 1.2-1.5 Ma. The migration routes of D. santeng have resulted in the presence of this Bovid species in several paleohabitats including Satir, Ci Saat, Trinil, Kedung Brubus, Ngangdong, Punung, and Wajak (Leinders et al. 1985, Theunissen et al. 1990, van den Bergh et al. 2001.
Considering the widely distributed of D santeng, this study aims to model the body mass and paleo habitat related population density of D santeng using astragalus studies in eastern part of Java island.

Methods
The study area was a landscape known as Ngawi located in the eastern parts of Java island ( Figure 1). This landscape was bordered by 3265 m mount Lawu (Figure 2) in the south and 84 m width Solo river in the north (Astirin et al. 2019). The landscapes in the north near the river were dominated by patches of lowland forests, plantations, paddy fields, and pastoral areas. Forest areas were common in the southern parts especially near the mount Lawu. The size of forests in mount Lawu is 54 Km 2 . The fossils in the form of astragalus bones were found in Trinil area located near the river bank ( Figure 1) in the north of Ngawi. The bones then were measured following Degusta and Drba (2013). The parameters taken from the bones measurements were width (W) and lateral length (LI) in mm ( Figure 3).
The body mass (kg) estimation using astragalus parameters were done in 2 steps. First, this step is by comparing the D. santeng width and lateral length with other extant Bovid species. By using this comparison, D. santeng can be compared whether the body mass related to which living Bovids. Then, the body mass was estimated by using allometric equation developed by Tsubamoto (2014). The astragalus lateral length (Ll) -body mass linear model was using slope value of 2.789 and intercept of 2.078. In this linear model, the lateral length was used as the predictor and the body mass was the result.
The linear model was also used to estimate the population density of D. santeng. The model was followed model developed by Damuth (1981). The body mass -population density linear model was using slope value of -0.75 and intercept of 4.23. In this linear model, the body mass was used as the  predictor (Tsubamoto 2014) and the population density (individuals/km 2 ) was the result. The spatial model of the population density in Ngawi landscape was the function of forest covers. The whole process of population density using the astragalus bones can be seen in Figure 4.

Results
The LI and W measurements on 9 specimens of D. santeng astragalus have been done. From the obtained LI and W, the estimated body mass (kg) have been calculated. The results of LI, W, and body mass were available in Table 1. From the model, it is estimated that the D. santeng originated from Trinil in Ngawi has average body mass equals to 60.3 kg (95%CI: 58.9-61.7).
The obtained LI and W data of D. santeng astragalus were compared to the extant Bovid species to study in which body mass classes of D. santeng (Ds). Four extant south east Asia Bovids were selected including Bos frontalis (Bf), Bos javanicus (Bj), Bubalus mindorensis (Bm), and Bubalus depressicornis (Bd). From the Figure 5, it can be seen that D. santeng has proximity of body mass to Bubalus mindorensis and Bubalus depressicornis. While D. santeng was separated with large Bovids including Bos frontalis and Bos javanicus.
The Ngawi landscape was consisting of varied forest covers ( Figure 6). The high covers were observed in 2 parts. Those parts including near river banks were the D. santeng fossils were found. Then dense covers were also observed in the southern parts near the mount Lawu. Landscape between the mount Lawu and the rivers was having low covers. The forest covers and body mass then were used to estimate the population density. Population equation (Damuth 1981) Log pop (inds./km 2 ) = Log body mass (g)*-0.75 + 4.23 The population density model of D. santeng is available in Figure 7. In Ngawi landscape the mean density of D. santeng is 5.39 individuals per km 2 with 95% CI: 3.18-7.6. High density was observed in the forest areas near the mount Lawu. While in the lowland between mount Lawu and river, the densities were observed lower than forest areas.

Discussions
Astragalus bones have been used even in the Paleogene era, an era before Pleistocene (Martinez & Sudre 1995 (Mistari 1996). While B. mindorensis weighted from 180 to 300 kg (Cebrian et al. 2014). Based on the estimation, the D. santeng weight average equals to 60.3 kg is much more realistic. This weight is also supported with the fact that the order of LI and W data is D. santeng < B. depressicornis < B. mindorensis. D. santeng body mass is also smaller than 2 other extant Bovids including Bos frontalis and Bos javanicus. Those extant species were having heavy body mass even greater than B mindorensis and B. depressicornis.
The habitat preference of D. santeng was also comparable to the B. depressicornis and B. mindorensis since those species having similar morphology, body mass, and length of LI and W. Among herbivores, body size has been previously implicated as a driver of spatial scales and habitat selection (Fisher et al. 2015). In Anthropocene era right now B. depressicornis was endemic to Sulawesi island and B.
mindorensis endemic to Mindoro island. In Sulawesi island, B. depressicornis inhabited lowland forest with population density 1.6 inds./km 2 . A subspecies B. depressicornis quarlesi was known inhabiting 7 montane forest at up to 2300 m above sea level (Burton et al.2005 The studies about relationships of body mass and population density have been documented (Ebenman et al. 1995, Johnson 1999 Ngawi was related to the body mass as well. This is in agreement that small animals are more abundant than large ones (Peters & Wassenberg 1983). D. santeng was estimated weighing around 60 kg and this allow this species to run escaping from predator and moving to hide. Second is related to the absence of competitors in grazing. Malhi et al. 2016 found that the existence of megaherbivores weighing >1000 kg can alter habitat structure through their high rates of consumption or through breakage and trampling. In Ngawi, there is no record yet about the existence of megaherbivores that can be potential competitors for large herbivore likes D. santeng.

Conclusion
This study has estimated the population density of D. santeng in Ngawi. The population density was interpolated and forest areas have more population than areas with low covers. The estimated body mass and population density are comparable to the extant Bovids. Using comparison with extant species, D.
santeng is grouped as large herbivores.