Could competition for precious water resources between the Mongolian wild ass and Przewalski’s horses have been a factor in the near-extinction of the latter?
The reintroduction of the Przewalski’s horse to the Gobi desert steppe has enabled Chinese and American scientists to examine the possibility.
Acquiring water is invariably challenging for desert-living animals. In recent years, the Przewalski’s horse has been reintroduced to the desert area in China where the last wild surviving member of the species was seen before it vanished from the region in the 1960s.
Its reintroduction placed it within the range of a close evolutionary relative, the khulan, also known as the Mongolian wild ass.
“Determining whether or not these two species experience competition and whether or not such competition was responsible for the extinction of Przewalski’s horses in the wild over 50 years ago, requires identifying the fundamental and realized niches of both species,” Defu Hu and his colleagues reported in the open-access peer-reviewed journal, PLOS ONE.
The researchers remotely monitored the presence of both species at a variety of water points in the Kalamaili Nature Reserve, in Xinjiang, China.
Camera traps collected more than 50,000 images over a period of 563 days during the dry seasons in 2010 and 2011. Khulan were responsible for 3385 of those trapping events and the Przewalski’s horse 1056 events. They were the most common species using the water points, the researchers said.
They found that Przewalski’s horses drank twice per day, mostly during daylight hours and at low salinity water sources. Khulan were found to drink mostly at night, usually at high salinity water points or those far from human residences.
Differences in drinking times and the selection of watering holes supported coexistence, the researchers said. However, they also found evidence of the Przewalski’s horses dominating during the rare instances when both species appeared at watering holes at the same time.
There were 23 events clearly captured on camera when both speciess were together in the same frame at the same water point.
Of these 23, only five showed individuals of both species drinking water at the same time and place. In the other 18 instances, only Przewalski’s horses were seen drinking.
“Overall, Przewalski’s horses were seen drinking during all 23 recorded encounters when khulans were present, whereas khulan were standing and waiting during 15 out of 23 such episodes.
“In addition, khulan were also captured retreating or being vigilant in 5 of the 18 events when only horses drank.”
Przewalski’s horses were never seen standing or watching on the five occasions when khulan drank.
“When seen together at water points, Przewalksi’s horse were free to drink when they wanted, whereas khulan appeared to behave as subordinates in approximately 80 percent of encounters when at water points with Przewalski’s horses.”
However, overall, the two species showed different use of common water points and used them at different times of the day during the dry season. It appeared that the two species had evolved distinct fundamental niches in terms of water use.
The researchers noted that the preference for night-drinking by the Khulan still tended to occur at watering holes used only infrequently by Przewalski’s horses.
“Drinking and moving at night either while feeding or migrating would lower heat loading, reduce water loss and reduce the need to drink during the day,” they said.
“Differences in needs and fighting abilities appear to reduce overt interspecific aggression, lessening the incidence of direct competition.
“Therefore, it appears that circadian differences in drinking rhythm at high quality water points emerge from a combination of the khulan’s inferior competitive ability relative to Przewalski’s horses and superior tolerance of high salinity and temperatures.”
Ultimately competition may be the prime driver of these niche differences rather than physiological traits.
“Initially, increases in Przewalski’s horse aggressiveness may have emerged because of its need to drink repeatedly during the day. Hence when challenged by khulan, Przewalski’s horses escalated contests in order to maintain control of water when most needed.
“That khulan were able to forage and drink at saline sites during the day or drink at high quality water points during the night enabled them to respond to the challenges arising from the appearance of Przewalski’s horses.”
They concluded: “Although the two equids both shared heightened needs for water in the Kalamaili desert, the species co-existed by frequenting different water points at different frequencies and at different times of day.
Varying tolerances for water saltiness and human activity – the khulan tended to avoid water sources with much human activity – might have played a role allowing the species to coexist, the research team said.
But these conditions alone were not sufficient to explain the different times the species used water points.
“Ultimately, differences in water needs have apparently led to differences in the value of consuming water, which in turn have intensified potential competition between the species.
“Direct competition, however, is minimized because khulan physiology allows it to adopt an alternative and energetically efficient strategy of nocturnal drinking and daytime feeding that is unavailable for the Przewalski’s horse.”
Although both species can co-exist because of their niche segregation, costs associated with competition were not eliminated, the research team said.
Zhang Y, Cao QS, Rubenstein DI, Zang S, Songer M, Leimgruber P, et al. (2015) Water Use Patterns of Sympatric Przewalski’s Horse and Khulan: Interspecific Comparison Reveals Niche Differences. PLoS ONE 10(7): e0132094. doi:10.1371/journal.pone.0132094
The full study can be read here.