What are the most commonly used animals in experiments?

Scientific research has historically relied on the use of animals on the premise that they can serve as models for human biological processes, particularly in the evaluation of medical treatments and potentially toxic substances. But which animals are used most often?

Among them, rodents stand out due to their well-characterized genome, ease of handling, and rapid reproduction, which make them one of the most widely used models for studying complex diseases. Birds and fish are also used in studies of genetics and embryonic development, as are nematodes and amphibians. In addition to these species, rabbits and pigs are used in immunology and transplant technologies due to their physiological similarities to humans. This use has sparked ongoing ethical debates, both regarding the limitations of extrapolating results and the welfare of the animals involved.

When did animal testing begin?

The earliest evidence of vivisection dates back to ancient Greece, with figures such as Aristotle and Galen. Over the centuries, the practice became systematized and spread to fields such as biomedicine. In the 17th century, René Descartes argued that animals were incapable of feeling pain, which helped justify their use for a long time. In the following century, advances in physiology and medicine began to be validated through animal testing, such as the experiments conducted by Louis Pasteur in the development of immunization.

Subsequently, legislation on animal cruelty was passed in the United Kingdom, and organizations such as the National Anti-Vivisection Society emerged. Between the 1950s and the 1970s, public criticism of these practices grew. Despite this, transgenic animals were developed in the 1980s, and 1996 marked one of the most controversial milestones with the cloning of Dolly the sheep.

Why are animals still used?

The use of animals in research is based on ethical, biological, and practical considerations, in accordance with the 3R principles and regulations that seek to balance scientific progress with the reduction of suffering. In theory, it is justified only when there are no viable alternatives, such as computational or cellular models.

The choice of species depends on its biological characteristics and the objectives of the study. In this context, it is recommended to avoid using species with greater cognitive abilities, replacing them with others that, although they respond to harmful stimuli, are believed to experience pain differently. However, this criterion raises a persistent ethical debate: whether a difference in the perception of pain is sufficient to justify the use of animals in research.

Most Commonly Used Models

Mice and Rats:

Mice (Mus musculus) and rats (Rattus norvegicus) are the species most commonly used in biomedical research, accounting for an estimated 70% or more of the animals used worldwide. They are used because of their small size, rapid reproduction, ease of handling, and the fact that their genome is well understood and can be manipulated. They have been genetically modified for decades to study diseases such as cancer, diabetes, and neurodegenerative disorders.

Rats, on the other hand, differ evolutionarily from mice and have a physiology more similar to that of humans in aspects such as the cardiovascular and neurological systems. In addition, they allow for more precise physiological monitoring and facilitate procedures such as surgery and drug administration.

Rabbits:

Rabbits (Oryctolagus cuniculus) are used in toxicology and safety studies, particularly because of the characteristics of their ocular physiology, as well as in research on immunity and antibody production for vaccines. They have also been used in cardiology and laser surgery studies.

In recent decades, its use has declined due to social pressure and the development of alternative methods, such as eye organoids and other experimental models.

Birds:

Birds, particularly chickens (Gallus gallus), are used because of their well-defined brain structure and their ability to learn. Their eggs are also used as bioreactors to produce proteins of interest to humans. In addition, they have been genetically modified to increase their resistance to disease and improve their productive efficiency. Other species, such as Taeniopygia guttata (the zebra finch), are used in studies of speech disorders associated with genetic mutations.

Pigs:

Pigs have become important in research because of their physiological similarities to humans. They are used in studies focused on protein production and organ development through gene editing, with the goal of advancing treatments and transplants.

Fish:

The zebrafish is one of the most widely used model organisms because of its external fertilization and rapid, transparent embryonic development, which allows for the observation of biological processes in real time. It shares genetic similarities with humans, which is why it is used in studies of cardiovascular and neuromuscular diseases, as well as mutagenesis.

Fruit fly:

The fruit fly (Drosophila melanogaster) is one of the oldest and most widely used model organisms in genetic studies. Its short life cycle, ease of cultivation, and genetic similarity to humans have made it a popular choice for research on embryonic development, the nervous system, and behavior.

Wax moth:

The wax moth (Galleria mellonella) has been used in studies of the immune system, toxicology, and infections. Its immune response is observed through melanization: the darker the larva becomes, the more advanced the infection. Cocoon formation can also be affected by the presence of pathogens or genetic alterations. Although its use has declined, it remains of biotechnological interest due to its ability to degrade polyethylene.

Nematodes:

Caenorhabditis elegans was the first multicellular organism to have its genome fully sequenced. Its short life cycle, ease of handling, and genetic similarity to other organisms make it a widely used model organism. Its transparency and well-characterized nervous system allow it to be used in studies of aging, diabetes, and developmental genetics.

Amphibians:

The African clawed frog (Xenopus laevis) is a key model organism in developmental biology, used in embryology and genetics studies because of its ability to produce numerous eggs and the ease with which microinjections can be performed. Its cell extracts are also used in research on cell division, replication, and apoptosis.

Other species, such as the American bullfrog (Rana catesbeiana), are used in studies of physiology, toxicology, and metabolic adaptation. Salamanders, such as Ambystoma maculatum, are notable for their regenerative capacity, which allows for research into tissue repair and limb regeneration. Other species are also used to study adaptation and resistance to environmental stress.

Nonhuman primates:

Nonhuman primates have central nervous systems similar to those of humans, with high cognitive and emotional capacity. These characteristics raise more complex ethical dilemmas, since their suffering and perception of pain are considered comparable.

For this reason, its use is subject to stricter regulations and is limited to cases where no alternatives exist, primarily in neuroscience research and in the development of vaccines against high-impact diseases, as was the case during the COVID-19 pandemic.

Key Observations

From an ethical perspective, it is questioned whether animal suffering is an acceptable cost for scientific progress, especially given animals’ capacity to experience pain and emotions. Despite the 3R principles, concerns persist regarding the diversity of species used and the limitations of their regulation. The choice of species is often influenced by ease of handling, physiological similarities to humans, and the perception that organisms with less complex nervous systems suffer less, which has favored the use of fish, birds, and insects over mammals.

From a scientific standpoint, although animal models have been key to numerous advances, their use has limitations. In drug development, many compounds that pass the preclinical phase in animals do not yield successful results in humans, due to biological differences that make it difficult to extrapolate the data.

In this context, the Food and Drug Administration has introduced new guidelines to reduce the use of animals in preclinical stages, promoting alternatives such as cell cultures and computational models, which offer greater accuracy and applicability. However, this transition requires not only technological advances but also cultural and regulatory changes that prioritize respect for animals used in research.

Explore new technologies that are advancing toward the replace animals in science.

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