Pronghorn Fawn Survival
For wildlife populations to sustain or grow, gains (births and immigration) must exceed population losses (deaths and emigration). For pronghorn populations in the Trans-Pecos, immigration and emigration can be considered negligible. Thus, for pronghorn populations suffering from a region-wide decline, pronghorn recruitment is paramount to population recovery. Fortunately, pronghorn have the reproductive potential to recover from catastrophes given that adult does have high incidence for twinning (~98%) including yearling does on a high nutritional plane.
Causes of mortality for pronghorn fawns monitored in the Trans-Pecos, 2011.
Since 2008, annual pronghorn surveys have revealed exceptionally low fawn recruitment. In fact most of the Trans-Pecos experienced fawn recruitment of less than 10% for two consecutive years in 2010 and 2011. These low fawn crops are a major contributing factor to the overall pronghorn decline in the Trans-Pecos. Several hypotheses emerged regarding pronghorn recruitment including fertilization rates, conception rates, fawning rates, incidence of twinning, and fawn survival.
To address fawn recruitment in the Trans-Pecos, we initiated a fawn survival study. Fawns were capture prior to 2 weeks of age using hoop-nets and spotlights at night. Upon capture fawns were weighed, aged, ear-tagged, and a light-weight, expandable radiocollar was affixed. Fawns were monitored from afar to document their behaviors and fate.
In 2011, we captured and radioed 26 neonate fawns over 4 study sites throughout the Trans-Pecos region. We documented high predation rates of fawns in our study where only 2 of the 26 fawns survived to 6 months old. Mortalities were attributed to bobcats (29%), coyotes (25%), and unknown predators (42%). One fawn was inadvertently trampled by cattle.
One of the more alarming statistics we documented in our study was the extremely low body weights of fawns. The abnormally low body weights suggest that the nutrition of fawns was compromised. We speculate that the drought conditions experienced in 2011 depleted the nutritional reserves of pregnant does during their last trimester of pregnancy and during lactation following parturition. The nutritional condition of pronghorn were further jeopardized by high concentrations of Haemonchus. Fawn production across the Trans-Pecos was consistent with our malnutrition hypothesis in that many pregnant does were documented, but few fawns survived to 6 months of age.