Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

John Carroll

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Andrea Cupp, John Bensoon, Kari Morfeld, Larkin Powell

Department

Natural Resource Sciences

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Natural Resource Sciences

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

Populations of zoo-managed African elephant (Loxodonta Africana) exhibit substantial individual- and population-level variation in reproductive success, yet research has largely focused on females, leaving male fertility poorly understood. Subfertility in bulls, characterized by low libido and inconsistent semen quality, poses significant challenges to genetic management and long-term population sustainability. This study a) systematically evaluates semen quality, b) identifies physiological, behavioral, social, and environmental determinants of fertility, and c) advances ex situ conservation through the expansion of the African elephant sperm bank. Between January 2023 and July 2025, 209 semen collection events were conducted from 13 sexually mature bulls across ten North American zoos. Semen samples were evaluated using macroscopic and microscopic metrics, including volume, concentration, total count, motility, forward progression, morphology, viability, acrosome integrity, agglutination, appearance, and pH. Samples were classified into low (0–29%), moderate (30–59%), and high (60–100%) motility groups, revealing substantial variability both within and among individuals. High motility was consistently associated with greater viability, acrosome integrity, normal morphology, and reduced agglutination, while volume and concentration were less predictive of motility. Physiological analyses revealed that testosterone, the glucose-to-insulin ratio, and triglycerides, were positively associated with high motility, whereas high insulin was negatively associated. Management and environmental factors further influenced semen quality: high motility was positively associated with female elephant luteinizing hormone surges, as well as increased exercise, forage-based enrichment, time outdoors, and higher ambient temperatures, whereas a greater percentage of time spent indoors was negatively correlated with semen motility. Mean semen motility also differed according to reproductive history: bulls that had never sired offspring exhibited the lowest motility, whereas bulls that had sired via artificial insemination or natural breeding had progressively higher motility. Finally, the African elephant sperm bank was expanded to store 13,070 straws from eight bulls, including 10,125 straws from previously unrepresented individuals, increasing genetic diversity and supporting artificial insemination and long-term population management. Collectively, this work establishes the most detailed multi-institutional dataset on African elephant bull semen quality, elucidates key physiological, behavioral, and environmental determinants of fertility, and demonstrates the critical role of cryobanking in ex situ conservation. These findings provide evidence-based guidance for reproductive management, health monitoring, and genetic sustainability in zoo-managed African elephants, ultimately supporting the long-term conservation of this iconic species.

Advisor: John Carroll

Download

Included in

Zoology Commons

Share

COinS