Surreal illustration of a dolphin skeleton highlighting bone defects in Rio de Janeiro.

Unveiling Congenital Bone Defects in Guiana Dolphins: What Does It Mean for Coastal Health?

Soraya Malik in Science & Nature September 2025 • 4 min read.

"A Deep Dive into Skeletal Anomalies and Environmental Factors Affecting Dolphin Populations in Rio de Janeiro."

Along the north-central coast of Rio de Janeiro, a unique group of marine mammals, the Guiana dolphins (Sotalia guianensis), navigate the estuarine and coastal waters. These dolphins, also known as 'boto-cinza' in Portuguese, are a vital part of the South American Atlantic coast ecosystem, ranging from Santa Catarina in Brazil to Nicaragua in Central America.

In a detailed study conducted between 2001 and 2006, researchers examined the skeletal structures of these dolphins, uncovering congenital bone defects that provide critical insights into their health and environment. This research, focusing on vertebral anomalies, specifically cervical ribs and cleft neural arches, opens a window into the developmental challenges faced by these marine creatures.

This article delves into the findings of this research, exploring the types of skeletal anomalies observed, their potential causes—ranging from genetic predispositions to environmental factors—and what these defects can tell us about the overall health of dolphin populations and the coastal regions they inhabit.

Decoding Dolphin Bones: What Anomalies Were Discovered?

Surreal illustration of a dolphin skeleton highlighting bone defects in Rio de Janeiro.

The research team carefully examined the vertebral columns of 20 Guiana dolphin carcasses, ensuring that at least 60% of the vertebrae were preserved to allow for accurate analysis. The collected skeletons predominantly belonged to immature animals (75%), indicating a focus on developmental stages. Notably, all skeletons displayed minimal taphonomic changes, which helped to maintain the integrity of the bone structures for examination.

Two primary categories of congenital anomalies were identified, both linked to the development of the paraxial mesoderm, the tissue responsible for forming the vertebrae:

Cervical Ribs: The presence of cervical ribs on the seventh cervical vertebra (C7) was observed in 45% of the dolphins. These ribs, varying in size and shape, indicate irregularities in the development of the rib cage.
Cleft Neural Arches: Also found in 45% of the individuals, cleft neural arches represent an incomplete closure of the vertebral arch, leaving a gap in the protective bony structure around the spinal cord.
Co-occurrence: Interestingly, some individuals (15%) exhibited both cervical ribs and cleft neural arches, compounding the skeletal irregularities.

In all instances, the vertebrae adjacent to those with anomalies appeared normal, suggesting localized developmental issues rather than widespread skeletal malformations. The frequency of these anomalies was notably higher in the Rio de Janeiro series compared to other Sotalia populations studied in Amazonas, Ceará, and Santa Catarina, suggesting unique regional factors at play.

What's Next for Dolphin Research?

The study emphasizes the need for continued research to fully understand the implications of these congenital anomalies. Future investigations should focus on differentiating the roles of genetic and environmental factors in causing these defects. By understanding the factors contributing to skeletal anomalies in Guiana dolphins, conservation efforts can be better targeted to protect these animals and their habitats.

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This article is based on research published under:

DOI-LINK: 10.1590/s0100-736x2010000600003, Alternate LINK

Title: Anomalias Ósseas Congênitas Em Sotalia Guianensis (Mammalia, Cetacea, Delphinidae) Da Costa Centro-Norte Do Estado Do Rio De Janeiro

Subject: General Veterinary

Journal: Pesquisa Veterinária Brasileira

Publisher: FapUNIFESP (SciELO)

Authors: Maíra Laeta, Sheila M.F.M. Souza, Salvatore Siciliano

Published: 2010-06-01

Everything You Need To Know

What specific skeletal anomalies were observed in the Guiana dolphins along the coast of Rio de Janeiro?

Researchers found two primary types of congenital anomalies in the Guiana dolphins: cervical ribs and cleft neural arches. Cervical ribs were observed on the seventh cervical vertebra (C7) in 45% of the dolphins examined. These ribs varied in size and shape, indicating irregularities in the development of the rib cage. Also found in 45% of the individuals, cleft neural arches represent an incomplete closure of the vertebral arch, leaving a gap in the protective bony structure around the spinal cord. Additionally, 15% of the dolphins exhibited both cervical ribs and cleft neural arches. The vertebrae adjacent to those with anomalies appeared normal, suggesting localized developmental issues.

What do cervical ribs in Guiana dolphins indicate about their development?

The presence of cervical ribs on the seventh cervical vertebra (C7) in Guiana dolphins suggests irregularities during the development of the rib cage. These cervical ribs, varying in size and shape, indicate a disruption in the normal formation process of the skeletal structure. The formation of cervical ribs stems from issues in the paraxial mesoderm, which is critical for forming the vertebrae. While the study highlights the presence of these anomalies, it emphasizes the need for further research to distinguish between genetic and environmental factors influencing their occurrence and the specific implications for the dolphins' overall health and survival.

Why are cleft neural arches a concern in Guiana dolphins, and what do they suggest about spinal cord protection?

Cleft neural arches are a concern in Guiana dolphins because they represent an incomplete closure of the vertebral arch, leaving a gap in the protective bony structure around the spinal cord. This anomaly can potentially compromise the protection of the spinal cord, making the dolphins more vulnerable to injuries or neurological issues. The incomplete closure stems from issues in the paraxial mesoderm. The presence of cleft neural arches, especially when co-occurring with cervical ribs, underscores the importance of understanding the developmental challenges faced by these marine creatures and the potential impact on their long-term health. It also highlights the need for further investigation into the causative factors, be they genetic or environmental.

What environmental or genetic factors might be responsible for the congenital bone defects observed in Guiana dolphins?

The congenital bone defects observed in Guiana dolphins, such as cervical ribs and cleft neural arches, could stem from a combination of genetic and environmental factors. Genetic predispositions might make certain individuals more susceptible to these anomalies. Environmental factors, such as pollution, exposure to toxins, or nutritional deficiencies during development, can also play a significant role in disrupting normal skeletal formation. The study emphasizes the need for future research to differentiate the roles of genetic versus environmental influences. Identifying these factors is crucial for developing effective conservation strategies to protect Guiana dolphins and their habitats.

How does the frequency of skeletal anomalies in Rio de Janeiro's Guiana dolphins compare to other populations, and what might this suggest?

The frequency of congenital anomalies, specifically cervical ribs and cleft neural arches, was notably higher in the Guiana dolphin population of Rio de Janeiro compared to other Sotalia populations studied in Amazonas, Ceará, and Santa Catarina. This suggests that unique regional factors are at play in the Rio de Janeiro area. These factors could include higher levels of pollution, specific toxins present in the environment, differences in prey availability and nutritional content, or distinct genetic traits within the Rio de Janeiro population. The higher prevalence of these anomalies underscores the need for targeted research and conservation efforts to address the specific challenges faced by these dolphins in this region.