Dominance relationships in captive female African lions are ameliorated by oxytocin administration

Free-ranging female African lions maintain symmetrical social relationships by respecting each other’s “ownership” of valuable food items rather than by supplanting subordinates according to well-defined dominance hierarchies. However, captivity often skews relationships in captive carnivores, whereas oxytocin elicits prosocial effects in virtually all mammals.  We therefore investigated whether captive female lions demonstrate obvious dominance relationships, and, if so, we hypothesized that oxytocin would reduce asymmetries between dominants and subordinates. We designed two experimental protocols for investigating pairwise relationships.  We first identified dominant individuals by performing neutral trials that allowed each female equal opportunity to possess the food item.  Second, we performed non-neutral trials that biased the opportunity for subordinates to gain possession (“ownership”) of the food and thereby determined whether dominants would still gain access to the resource. The neutral tests revealed that pairs of captive females do display dominance relationships, with one individual possessing the resource more than the other. However, in non-neutral trials, subordinates behaved less submissively by increasing aggression and their possession of the resource after receiving oxytocin compared to when they received saline solution.  Our study not only reaffirms the social dynamics altered by captivity but also highlights the potential for oxytocin to mitigate these disturbances.

Methods
Subjects

All trials were performed at Lionsrock Sanctuary FOURPAWS, in South Africa, between February and October 2023. Procedures were approved by the Institutional Animal Care and Use Committee of the University of Minnesota, and in accordance with ARRIVE guidelines. All subjects (n=20 females, 10 pairs) were healthy female adults in prime condition, housed in greater than 1 hectare, species-appropriate enclosures, and were selected by sanctuary staff based on health and availability. All 20 female lions were included equally across trials and analyses. No studies were performed on females who were lactating, pregnant, or in estrus, and no animals in this study were used for breeding purposes. Pairs were selected for the study that had been housed amicably together since early adolescence or birth (direct levels of relatedness are unclear due to the nature of rescues), and all individuals were similar in age and condition within each pair.

Trial types

Dominance can be characterized as monopolization over a resource (Packer et al., 2001). In this study, we use food in the form of blocks of raw meat as a resource (meat was provided by the sanctuary and was within feeding guidelines for each animal). All animals in the study were pre-trained to take meat blocks from the researcher at the gate prior to study and two baseline trials were performed to habituate the females to the procedure. Asymmetry in resource possession and changes in aggression were measured. 

Trials were conducted to first determine whether dominant behaviors were displayed within a pair of females, then to see whether oxytocin could mitigate these behaviors. Each trial consisted of a two-part test (neutral and non-neutral) and was repeated a total of four times (twice with oxytocin treatment and twice with saline control), 10-14 days apart (differences based on weather conditions).

Trial Protocol

Treatment (10 IU Oxytocin) or vehicle (saline, as control) was administered intranasally 45 minutes prior to trial commencement (presentation of resource), and in alternating order across pairs. During trials, the female pair was brought to the gate being lured by meat. For the neutral trial, the resource (meat) was placed on a stick against the fence at an equidistance between the two females, giving them equal opportunity to approach and possess the “resource.” This was repeated a total of 5 times (except when one female was reluctant to return or excessive aggression occurred, then only repeated 3 times as a humane endpoint) with greater than 30 seconds between each round to allow for a reset. A lioness was considered “dominant” if she monopolized the resource at least 60% of the time. This was followed by a non-neutral trial in which an additional set of rounds was administered (directly following the first set), where the meat was then held closer to the subordinate, as determined by the first set, giving the subordinate a chance to reach the meat first and claim “ownership.” It was then observed whether the dominant female allowed the subordinate to maintain possession, or if they overtook the resource regardless of the “ownership” rule observed in female lions in the wild.

Groups were selected randomly to receive OT or saline in alternating order for a total of 4 trials. The dose was 10 international units (IUs) of OT (Burkhart et al., 2022). Intranasally administered OT bypasses the blood–brain barrier, peaking at 45 min and lasting ≈ 4 h. (Weisman et al., 2012; Lee et al., 2020). Trials therefore commenced 45 minutes post OT administration. Treatments were prepared by research assistant, Dubois, so that the researcher, Burkhart, was blinded to the treatment in order to control for biasing behavior, interpretation, or interaction during trials. All trials were recorded using a Gopro Hero 5 and Samsung s20 5G+.

Behavior was measured during each round to determine dominance as follows: who possessed the resource, as well as aggressive vocalizations and physical actions. Aggressive encounters were then categorized by severity, 1= brief non-contact, 2= brief contact, 3= prolonged contact. A total dominance score was calculated as 1 if vocal + 1 if physical + level of aggression. Also measured was how many body lengths apart the individuals were while resource was consumed, the ultimate body position of the non-possessor, and whether the non-possessor retreated. 

Statistical analysis

Differences in Resource Possession

Successful possession of the food resource by dominant vs submissive lions was our primary indicator for the presence of dominance hierarchies within pairs. We therefore compared the ratio of within-pair resource possession across trials. Binomial generalized linear mixed models (GLMMS) were employed using “glmer” (R-package lme4) to model the likelihood of dominant and submissive individuals to successfully possess the food resource. This “possession” likelihood was modeled as a function of treatment (saline or oxytocin), dominance (dominant or subordinate individual), and the interaction between treatment and dominance. We considered neutral vs non-neutral trials as separate tests and ran these data in separate models. Pair identity was added as a random effect to allow for pair-differences in aggression and dominance.

Differences in Aggressive Behavior

We explored differences in aggressive behavior and intensity across trials using 4 aggression metrics, as changes in aggression for both dominant and subordinate individuals was a likely mechanism for changes in pair-wise resource possession. We first modeled the presence/absence of vocal aggression (1) and physical aggression (2) using binomial GLM models, and then modeled physical aggression intensity level (3) and total aggression count (4) using poisson GLM models. Data from neutral vs non-neutral trials were again modeled separately, as was dominance status for easier interpretation of effect sizes. All aggression metrics were modeled as a function of treatment (saline or oxytocin), with pair identity used as a random effect to allow for pair and individual differences in displayed aggression.