September 19, 2019
By Corinne Kozlowski, Ph.D., Endocrinologist
Somali wild asses are considered one of the most endangered wild horses in the world, with fewer than one thousand individuals remaining outside human care. The Saint Louis Zoo is committed to conservation of this species in the wild in Somalia, Eritrea, and Ethiopia, as well as in zoos, and participates in a cooperative breeding program to maintain a healthy population as a guard against extinction. In 2013, all 5 females in our herd gave birth to foals within the span of about two months (three males: Hirizi, Tristan and Rebel, and two females: Farah and Luana). Their births provided Zoo scientists with a unique opportunity to study the biology and behavior of young Somali wild asses.
Behavioral data collection began when the foals were introduced to other herd members (around 2 to 3 weeks of age), and it continued throughout the first year of life. Interns in the Zoo’s Department of Reproductive and Behavioral Sciences worked in teams to document social interactions 6 days a week. Using a pre-defined list of behaviors called an ethogram, interns recorded which foals initiated instances of nursing, lying down, chasing, kicking, greeting, and biting, among other behaviors. Interns also detailed who received the behaviors. This information allowed us to determine whether foals were more likely to interact with their mothers, other foals, or non-related females. Finally, we collected fecal samples for hormone analysis. The fecal samples would tell whether levels of cortisol, a hormone associated with activity, metabolism, and sometimes stress, changed as the foals grew.
The results demonstrated that from 0 to 3 months of age, foals interacted almost exclusively with their mothers. By 6 months of age, however, most social interactions occurred between foals. Interactions with mothers were mostly comprised of nursing and kicking, whereas foals were more likely to chase, bite, and threaten to bite other foals. We saw differences among individuals, with Rebel, the youngest foal, engaging in more social behaviors than the other foals. Similar to studies of domestic horses, we also observed that males were more likely to engage in social interactions than females. Results from our hormone testing revealed that cortisol levels were highest for foals during the first month of life, and then declined with age. We suspect that this decrease might be the result of switching from nursing to foraging, adjusting to their surroundings or others in the social group, or seasonal factors.
As scientists, we are always excited to learn new information from the animals in our care. This study, which is the first to document the biology of young Somali wild asses, provides insights into how social dynamics change as foals grow from infants to yearlings. Currently, we are also analyzing behavioral data from our adult Somali wild asses to learn more about their behavior. Taken together, behavioral and hormone data allow us to support excellent welfare and improve breeding programs for these endangered animals.
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