Nurse sharks (scientific name: Ginglymostoma cirratum) have been known to have trouble reproducing in a captive setting, leading to low reproduction rates. In March 2021, a team of scientists set out to investigate if manipulating the sharks’ sense of smell through olfactory enrichment would encourage reproductive behaviors in a captive setting, specifically aquariums.
Read on to find out the interesting results…
Nurse Sharks Are Vulnerable to Extinction
This study was performed to help understand the environmental crisis that shark populations face, and come up with possible solutions. Nurse sharks in particular are considered vulnerable and face a high risk of extinction. In an effort to explore better management practices involving the reproduction of wild sharks, the scientists chose captive nurse sharks to be their subjects. In general, aquatic species have trouble reproducing in captivity. In this case, olfactory enrichment (sense of smell) was manipulated in the nurse sharks’ captive environment. Their hypothesis is that the release of basil leaf extract in their environment will cause nurse sharks to be more responsive to olfactory cues, and lead to a higher frequency of reproductive success.
Scientific Study: Method Used
The study was conducted in Brazil, at two separate aquariums, and took place during the summer months. Commercial sweet basil extract was diluted in tank water.
How The Study Was Conducted
The study was conducted in three phases:
- Control phase
- Short-term enrichment phase
- Long-term enrichment phase
This study uses “environmental enrichment”, which is placing features in a habitat that are meant to stimulate physical and social behaviors.
Basil Leaf Extract
For the enrichment, they used basil leaf extract to stimulate their sense of smell, and observe any differences in behavior. Basil leaf extract is known to treat diseases in humans and provide powerful medicinal effects. In fact, basil has proven health benefits if eaten regularly, improving humans’ blood sugar, heart health, and neurocognition.
In recent studies, animals have responded well to these herbal scents, with the possibility of stimulating behaviors and reducing stress. For this reason, the scent of basil was picked for this study. In addition, 11 captive Nurse sharks were selected in this study. Their behaviors were captured for a total of 120 hours of observation.
Sweet basil extract was spread at different spots in the tank to avoid overstimulation. Afterwards, the scientists observed the nurse sharks’ reproductive behaviors.
Focal sampling (observing animals’ behavior in a natural setting for a period of time) was used to record the sharks’ reproductive behaviors every two minutes for two hours. This was done over three consecutive days.
A list of reproductive behaviors were made, including “male spinning” and “pectoral biting.” The frequencies of the reproductive behaviors that occurred were then calculated to come to a conclusion.
Data Conclusion
The reproductive behaviors differed in the three phases (1. control, 2. short-term, and 3. long-term).
A Generalized Linear Mixed Model (GLMM) is a tool used in statistics to analyze data by factoring in “mixed effects”, or possible outcomes, and creating a generalized model that could be applied to different types of data. A GLMM was made to analyze the sharks’ behaviors when adding in the basil.
The model showed that there was a much higher frequency of reproductive behaviors during the short-term and long-term enrichment phases, thus proving the hypothesis. During the long-term enrichment phase, males were shown to have a greater increase of reproductive behaviors compared to the females.
Results: Does The Smell Of Basil Boost Reproductive Behaviors?
This study found that olfactory enrichment encourages nurse sharks’ responsiveness to their environment, including reproductive behaviors. The results strengthened the hypothesis that olfactory stimulation would cause nurse sharks to become aware of sensory cues in their environment, and thus perform olfactory-driven reproductive behaviors.
Why This Matters: Encouraging Reproductive Success
This is a big step toward shark conservation as scientists learn how to manipulate natural habitats to encourage the reproductive success of vulnerable and endangered shark species. Captive Nurse sharks were the perfect subject for this study, and will aid in the conservation of all species of sharks. The next step is figuring out how to incorporate olfactory enrichment in reproductive management practices, both in captivity and the wild, to protect vulnerable species.
Article written by Liel Shachr, University of Florida, B.S. Natural Resource Conservation
Scientific Study: Environmental enrichment effects on the reproductive behavior of captive nurse sharks Ginglymostoma cirratum by Helen Colbachini & Cristiane Schilbach Pizzutto & Leonardo Mendes de Souza Mesquita & Otto Bismarck Fazzano Gadig
Photos: Shane Gross, Nature Picture Library, Science Photo Library