The desert bighorn sheep was taken off New Mexico’s threatened and endangered species list in 2011. Since then, the New Mexico Department of Game and Fish has funded a project to obtain estimates of lamb survival, recruitment and cause-specific mortality in the Peloncillo Mountains.
Principal investigator James Cain, assistant unit leader for the New Mexico Cooperative Fish and Wildlife Research Unit at New Mexico State University, and graduate student Rebekah Karsch led the research, concluding the three-year project in May 2014.
“Prior to delisting, adult survival and cause-specific mortality was monitored for over a decade in almost all the desert bighorn populations in New Mexico by the NMDGF. However, there were no estimates of lamb survival rates,” Cain said. “The NMDGF has been trying to increase the desert bighorn populations in the state since they were first put on the state’s endangered species list.”
Desert bighorn sheep were placed on the New Mexico endangered species list in 1980. For 20 years, the department attempted to bolster populations by moving sheep from source herds in New Mexico and Arizona to populations that needed more sheep. However, these translocations alone weren’t enough to increase the populations due to high mountain lion caused mortality. Thus, in 1999 NMDGF commissioned mountain lion control. The removal helped stabilize adult population numbers, which led managers to seek the next factor limiting population growth: variable and often high lamb mortality rates.
“The monitoring methods are pretty much the same between lambs and adult desert bighorn; what really differs is how we capture them and how frequently we relocate them in the field,” Cain said.
Using telemetry collars, the sheep are monitored from the ground or from aircraft using a very high frequency receiver and antenna to determine location and survival status.
Karsch’s fieldwork began with catching adult female sheep, fitting them with telemetry radio collars and checking for pregnancy using a portable ultrasound machine. Pregnant sheep were then equipped with a Vaginal Implant Transmitter, a temperature sensitive device that alerts researchers when a female gives birth. In 2012 and 2013, 39 pregnant ewes were fitted with VITs.
“When a lamb is born, the VIT is expelled and the change from the female’s internal body temperature to the outside ambient temperature causes the signal to change, allowing me to know she gave birth,” Karsch said.
Between the two years, 26 lambs were born, captured, and fit with expandable radio collars. Of those, 14 died, 3 in 2012 and 11 in 2013. Karsch’s goal was to determine why such a jump in deaths took place in the span of one year.
“It’s extremely physically taxing for a female to successfully raise a lamb that gets recruited into the population every year,” Karsch said. “So it’s natural to think if there is really good recruitment and survival in one year, survival the next year wouldn’t be as good.”
Ultimately, Karsch said she would like to know what steps managers could take in stabilizing recruitment to see population growth on behalf of NMDGF.
This project provides useful information on desert bighorn sheep, lamb mortality and estimation of survival, as well as data on the birth site characteristics.
“Researchers always assumed where lambs were seen, which tend to be the areas with the highest elevation, most rugged terrain and steepest slopes – where traditionally, you think a mother would take her young to be safest – is also where they were giving birth,” Karsch said. “However, though my sample sizes are small, I have found that the parturition (birthing) sites are actually at lower elevations, with gentler slopes, than where lambs are seen in nursery groups.”
Using geographic information systems, Karsch looked at the elevation, slope and terrain ruggedness variation between parturition and nursing sites. Her findings show the parturition sites are at lower elevations with less rugged slopes and that the lambs are moved within a few days after birth to nursing sites at more steep and rugged areas.
The information attained from the project will be a novel contribution to scientific literature on desert bighorn sheep. NMDGF gains data on birth sites, lamb mortalities and the locations in which it all occurs.
Another important part of such projects is that they provide opportunities for undergraduate students in NMSU’s Department of Fish, Wildlife and Conservation Ecology with hands-on experience in wildlife field research. The projects also give students some insight into what is involved in graduate student research, which can help them decide if they want to try to pursue a master’s degree after completing their undergraduate program.
“The graduate students get training on how to conduct wildlife research including proposal development, grant writing, report writing, study design and data analysis and interpretation,” Cain said. “All this helps prepare them for their careers in wildlife biology.”
Information from: NMSU