Five decades of crocodilian ecotoxicology: a global scientometric analysis of environmental biomonitoring research ​

Crocodilians, the ancient reptiles that share our waterways, are more than just formidable predators; they are living archives of environmental health. Because they live for decades, stay in one place, and sit at the very top of the food chain, they act as natural sponges for the chemicals that drift into rivers and lakes. As they eat other animals, they accumulate toxins like heavy metals and pesticides in their bodies, often in higher concentrations than the water around them. This process, known as bioaccumulation, means that a crocodile's tissues tell a story about the quality of its entire ecosystem over many years. Scientists have long used these animals as "sentinels," or early warning systems, to detect pollution before it causes visible harm to wildlife populations. By studying what is inside a crocodile, researchers can understand what is happening in the water long before the damage becomes obvious to the naked eye.

For the past five decades, scientists have been collecting data on these reptiles to track how the world's waterways are changing. However, until now, no one had taken a step back to look at the entire collection of this research as a whole. A new study by Rayssa Lima dos Santos from the Federal University of Paraíba in Brazil has done exactly that. By gathering and analyzing nearly two hundred scientific papers published between 1971 and 2026, the researcher mapped out how the field of crocodilian ecotoxicology has grown, where it is focused, and what it has missed. The work reveals a field that has matured from simple chemical counting into a complex science of biological effects, yet it remains heavily concentrated in just a few countries and on just a few species.

The study began by searching a massive global database of scientific literature, filtering through hundreds of records to find only those that specifically investigated crocodilians in the context of environmental pollution. After a careful review, 187 studies were selected for detailed analysis. The first thing the data showed was a clear timeline of growth. For the first thirty years, from 1971 to 2000, research on this topic was slow and steady, with only a handful of papers published each year. But after the year 2000, the number of studies began to climb sharply. By the 2010s, scientific interest had surged, with the highest volume of research occurring between 2011 and 2020. This boom suggests that the field has moved from a niche interest to a central part of how scientists monitor the health of our planet.

When looking at which animals were being studied, a striking pattern emerged. The research was not spread evenly across all crocodile species. Instead, it was dominated by two specific animals: the American alligator and the broad-snouted caiman. The American alligator appeared in nearly half of all the studies, while the broad-snouted caiman was the subject of about one-fifth of them. The other species, including the Nile crocodile and the Morelet's crocodile, were studied far less frequently. This imbalance meant that our global understanding of crocodile health was largely built on the backs of just two species, leaving many others in Asia, Africa, and Oceania almost entirely unexamined in the scientific literature.

The location of the research told a similar story of concentration. The United States was the clear leader, producing more than half of all the studies, followed by Argentina, Brazil, and South Africa. Each of these regions had its own focus, shaped by the local environment and the types of pollution present. In the United States, particularly in Florida, researchers focused heavily on heavy metals and chemicals that disrupt hormones, driven by decades of monitoring in places like Lake Apopka. In Argentina, the focus shifted to pesticides, as scientists used the broad-snouted caiman to test how agricultural chemicals affect reptiles. Brazil's research was centered on heavy metals, reflecting the intense mining activities in its tropical rivers, while South African studies looked closely at metal exposure in Nile crocodiles.

As the years passed, the way scientists studied these animals also changed. In the early days, the work was mostly about finding and measuring how much of a specific poison was inside a crocodile. Researchers would take samples of blood, liver, or muscle and count the chemicals. Over time, however, the questions became more sophisticated. Scientists began to look not just at how much poison was present, but at what that poison was actually doing to the animal's body. The research shifted toward looking for "biomarkers," which are early signs of biological stress. These signs could include changes in hormone levels, damage to DNA, or weaknesses in the immune system. By the 2010s, it became common for studies to use multiple of these markers at once, creating a more complete picture of how pollution affects the animal's health, rather than just listing the chemicals found in its tissues.

Despite this progress, the study highlighted significant gaps that remain. While the field has grown, it has not grown evenly. Many crocodile species living in biodiversity-rich tropical regions are still missing from the scientific record. These are often the areas facing the fastest environmental changes due to mining, farming, and urban development. Because so much of the research relies on just a few model species in a few countries, it is difficult to know if the findings apply to crocodiles everywhere. The author suggests that future work needs to broaden its scope, looking at more species in more places, and paying closer attention to newer types of pollutants that are becoming common in the environment but have not yet been widely studied in crocodiles.

The paper concludes that crocodilians have proven to be invaluable sentinels, offering a window into the long-term health of aquatic ecosystems. The journey of this research field over the last fifty years shows a move from simple observation to a deeper, more mechanistic understanding of pollution. Yet, to truly protect these animals and the ecosystems they inhabit, the scientific community must expand its reach. By filling in the gaps in geography and species diversity, and by standardizing how these studies are conducted, researchers can build a more accurate and global picture of environmental risk. The work done so far provides a solid foundation, but the next chapter requires looking beyond the familiar to the many other faces of the crocodilian family that have yet to tell their stories.