Comparison of mineral concentration between salt-lick water and stream water in this study suggested that the herbivorous mammals could supplement sodium and/or calcium by drinking water at the studied salt-licks; the concentration of sodium in salt-lick water was higher than that of stream water regardless of seasons or sites, and that of calcium was also higher in most cases. On the other hand, those of potassium and magnesium in salt-lick water were not necessarily higher than the stream water, suggesting that animals could not supplement these minerals by drinking the salt-lick water (Table 1).
Some previous studies on wet-type salt-licks also reported the higher mineral concentration (including sodium and calcium) in the salt-lick water than the control-site water in various regions (Table 3). For example, Clayton and MacDonald (1999) who studied wet-type salt-licks visited by babirusa (Babyrousa babyrussa) in Sulawesi Island, Indonesia reported higher concentration of sodium and calcium in the salt-lick water than control water. Matsubayshi et al. (2007a) who studied wet-type salt-licks in Borneo Island, Malaysia also reported higher mineral concentration (sodium, potassium, magnesium, and calcium) of the salt-lick water. Besides, they compared animal visit between two adjacent salt-licks at a distance of only 16 m and reported that the sambars and bearded pigs (Sus barbatus) visited the salt-lick with higher concentration of all four minerals more frequently than the other.
Table 3
Concentration of cations in water-drinking type salt-licks. The unit of mineral concentration is ppm. The asterisks (*) indicate that the mineral concentration of the salt-lick was higher than that of control sites at P < 0.05 (1t-test, 2Mann-Whitney U test, 3Wilcoxon rank-sum test)
| | | Na | (range) | K | (range) | Mg | (range) | Ca | (range) |
Matsubayashi et al. (2007a), Borneo, Malaysia1 | salt-lick | (n = 59) | 801.8 ± 1173.5* | (38.7–2710.2) | 14.4 ± 12.6* | (6.8–29.8) | 21.4 ± 9.8* | (13.6–35.1) | 83.4 ± 50.0* | (41.7–155.9) |
control | (n = 18) | 6.9 ± 2.4 | (4.6–8.7) | 1.6 ± 0.6 | (1.5–1.8) | 2.7 ± 1.0 | (2.3–3.1) | 13.8 ± 8.5 | (5.6–20.4) |
Clayton and MacDonald (1999), Sulawesi, Indonesia | salt-lick | (n = 1) | 259 | - | 1.0 | - | 0.1 | - | 90 | - |
control | (n = 1) | 7.1 | - | < 0.1 | - | 6.1 | - | 25 | - |
Owen et al. (2014), Maine, USA | salt-lick | (n = 2) | 12.6 | (9.4–15.7) | 1.6 | (1.0–2.3) | 1.1 | (0.6–1.6) | 4.5 | (3.9–5.0) |
control | (n = 2) | 1.4 | (0.9–1.9) | 0.3 | (0.2–0.3) | 0.7 | (0.5–0.9) | 4.1 | (3.0–5.1) |
Couturier and Barrette (1988), Quebec, Canada2 | salt-lick | (n = 17) | 172.5 ± 97.1* | (73–472) | - | - | - | - | - | - |
control | (n = 10) | 3.6 ± 2.6 | - | - | - | - | - | - | - |
Bechtold (1996), British Columbia, Canada1 | salt-lick | (n = 20) | 107.8 ± 119.7* | - | 2.7 ± 3.3* | - | 10.0 ± 12.0* | - | 29.3 ± 32.2 | - |
control | (n = 20) | 2.1 ± 2.1 | - | 0.5 ± 0.4 | - | 4.4 ± 3.7 | - | 17.4 ± 12.2 | - |
this study3 | salt-lick | (n = 79) | 34.5 ± 25.2* | (4.7–84.9) | 9.5 ± 20.7* | (0.5–77.5) | 1.5 ± 1.3 | (0.2–8.7) | 3.2 ± 1.5* | (0.5–12.8) |
control | (n = 79) | 5.9 ± 4.3 | (2.2–25.0) | 2.2 ± 0.3 | (1.5–2.7) | 1.2 ± 0.8 | (0.2–2.6) | 1.5 ± 0.9 | (0.2–3.0) |
Some previous studies also suggested that herbivores visited salt-licks mainly for sodium supplementation. Couturier and Barrette (1988) who studied salt-lick use by moose (Alces alces) in Matane Wildlife Reserve, Quebec, Canada reported that among the four wet-type salt licks where they observed moose, the one that had the most visits also had the highest sodium content. Moe (1993) who studied salt-licks in Bardia National Park, Nepal showed that the concentration of minerals except sodium was not higher in the soil of dry-type salt-licks, where animals consume mineral-rich soils, than control soils. He also showed positive correlation between the sodium concentration and visit frequency by axis deer (Axis axis) using the data form 12 dry-type salt-licks in the study area. Holdø et al. (2002) who studied African elephants (Loxodonta africana) in a Kalahari-sand habitat in Hwange National Park, Zimbabwe analysed the mineral concentration of the food plants, drinking water, and soils of dry-type salt-licks the elephants ate. They estimated that the elephants could not take minimum amount of sodium for their requirements from the food plants and drinking water while they could take enough amount of calcium and magnesium. They also reported that the salt-lick soils contained higher concentration of sodium than other soils, while the calcium and magnesium concentrations of salt-lick soils were lower than the food plants and drinking water, indicating that elephants supplement sodium, not calcium and magnesium, from salt-lick soils.
On the other hand, it is also reported that the salt-licks have an important role of supplementing not only sodium but also other minerals such as calcium for animals. Atwood and Weeks (2003), who studied the salt-lick use by white-tailed deer (Odocoileus virginianus) at Indiana, USA, reported that females visited salt-licks with various mineral contents more frequently than salt-licks with only sodium, pointing out that females during lactation might need minerals such as calcium and phosphorus in addition to sodium.
The calcium concentration in salt-lick water (3.24 ± 1.54 ppm, range 0.51–12.79, n = 79) analysed in this study, however, was much lower in absolute values compared to those that reported by these previous studies (Clayton and MacDonald 1999: 90 ppm; Matsubayashi et al. 2007a: 83.4 ± 50.0 ppm, Table 3). Therefore, the supplementation of sodium seems more important than that of calcium for animals that visited the salt-licks studied in this study. It cannot be denied, however, that the animals supplement some minerals other than sodium which were not measured in this study but critical for herbivore nutrition.
In this study, the herbivores (especially the red muntjacs and sambars, which was the main users of the salt-licks) most frequently visited the Tiang C salt-lick with the highest sodium concentration among the three salt-licks in Tiang area, while the omnivores and carnivores didn’t show such tendency. On the other hand, the calcium concentration of the salt-lick water at Tiang C was not always higher than other salt-licks. The fact also suggests that sodium is more important than calcium in their mineral supplementation at the salt-licks. We cannot eliminate, however, the possibility that the animals supplement calcium at the salt-licks, because calcium concentration in the salt-lick water was higher than that in the stream water in most cases.
In contrast to the red muntjacs and sambars, the Malayan tapirs and the white-thighed langur, the third and fourth most frequent herbivorous visitors to the studied salt-licks in Tiang area, did not visit Tiang C with the highest sodium concentration most frequently. The Malayan tapirs visited Tiang B more frequently than Tiang A and C while sodium concentration at Tiang B was significantly lower than Tiang C. The result indicates that not all the herbivore species necessarily visited the salt-lick with higher mineral concentration more frequently. Thus, other factors which may vary with species could influence the frequency of salt-lick visit by animals in addition to the mineral concentration.
The present authors studied the behavior of wild Malayan tapirs around the same salt-licks as this study, and observed male-female interaction such as chasing and vocalization, suggesting that they used the salt-licks as a place to meet potential mating partners in addition to drink water there (Tawa et al 2021). Thus, such social interaction can affect selection of the salt-licks by tapirs. Matsubayashi et al. (2011), who studied the use of salt-licks by orangutans, also reported that multiple individuals such as two pairs of females with infants were recorded by sensor cameras in the same frame, suggesting that the salt-licks have secondary function as a communication site for them.
Lazarus et al. (2019) conducted camera trapping surveys at the same three salt-licks in Tiang area as this study. Their results on the record frequency of red muntjac was not consistent with ours, while the results on sambar and Malayan tapir were consistent with this study. They reported that the red muntjacs were recorded significantly more frequently at Tiang A than other two salt-licks based on the research of six months (the exact survey date was not described), while in this study the red muntjacs were estimated to visit Tiang C most frequently, followed by Tiang A and Tiang B based on the research of two years. Although they did not conduct mineral analysis of salt-lick water, they discussed that the red muntjacs might preferred Tiang A because of the thick, dense underbrush and vines around Tiang A where they could quickly hide from predators. They also speculated that the sambars were observed significantly more frequently at Tiang C because they can visit there in group due to the large open area in front of the water source in Tiang C.
Although it is still not clear why their results on red muntjacs are different from ours, the difference in survey season and studied period, however, could cause the difference because the red muntjacs could seasonally change the visit frequency to the salt-licks. In addition, the physiological demand for minerals could also seasonally change by pregnancy and lactation for example. Holdø et al. (2002) reported that the females elephants spent more time than males for soil-eating at the dry-type licks. In Borneo, Matsubayashi et al. (2007b) reported that female sambars visited the wet-type salt-lick more frequently in rainy season than dry season while visit frequency of males did not seasonally change. Couturier and Barrette (1988) showed that the various age-sex (yearling or adult, male or female) classes had different seasonal patterns of salt-lick use, suggesting possible differences in mineral needs related to molting, antler grows, calf growth, lactation, or estrus.
Some previous studies indicated that the seasonal changes in visit frequency to salt-licks are also relevant to the needs for alleviation of gastrointestinal problems, as shown in Ayotte et al (2008). In the other study by the present authors, the red muntjacs and sambars visited the salt-lick significantly more frequently than the annual mean in April, the beginning of rainy season (Tawa et al. 2022). If the sprouting season in this area mainly occur in the beginning of the rainy season, it is possible that these deer species visited the salt-licks more frequently to alleviate gastrointestinal acidosis caused by their seasonal dietary change, while the concentration of bicarbonate in salt-lick water was not analysed.
Although we did not examine the effects of factors other than mineral concentration of the salt-lick water in this study, many other factors, such as topography, vegetation, predation risk and social interaction at the salt-licks could affect the pattern of animal visitation to the salt-licks. The effect of the difference in topography and vegetation, however, seemed small in our results because no remarkable differences in these factors were not observed among the studied salt-licks in Tiang area, which were located in close proximity to each other.
As for predation risk, Griffiths et al. (2020), who conducted camera trapping survey at 52 salt-licks in the northeastern Peruvian Amazon, showed that three mammals (the paca Cuniculus paca, Brazilian porcupine Coendou prehensilis, and red brocket deer Mazama americana) were less likely to visit salt-licks during nights with bright moon probably due to a heightened risk of predation at the salt-licks when visibility was better for predators. In this study site, the present authors recorded some carnivores: the tiger (Panthera tigris jacksoni) and the dhole (Cuon alpinus) etc. at the salt-licks using sensor cameras (Tawa et al. 2022). Although the lunar cycle seems not to be related to the difference in visit frequency of herbivorous species among three salt-licks in Tiang area because of the same study period, it remains to be studied whether the visit frequency of prey mammal is affected by lunar cycles also in this study area.
Mineral analyses of the salt-lick water by this study showed that the wet-type salt-licks studied in BTFC could provide the sodium and/or calcium supplementation for the herbivorous mammals. Especially, the high concentration of sodium was consistent with the hypothesis that the herbivorous mammals visit the salt-licks mainly for sodium supplementation. However, the results on relationship between the mineral concertation of salt-lick water and visit frequency by mammals suggest that the factors other than sodium concentration might also influence the selection of salt-licks by each species.