Student Creates First Scientific Scale for “Dangerous” Animals
The Crespo scale defines danger as how likely a species is to cause your death, using two quantifiable factors.

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What’s the world’s most dangerous animal? That’s a question asked by many people; very few seem to specify what they mean by “dangerous”.
This paper introduces a new rating system I created – the Crespo scale – that measures and ranks how dangerous an animal species is based on how likely it is to cause your death. The work was completed during my foundation year at the University of Reading and the paper, which is open access, was published in the Asian Journal of Research in Zoology.
Bad reputation
While working on my final research project in college, I’d keep finding these articles and YouTube videos that supposedly list the top “most dangerous animals in the world”. Species such as the inland taipan (Oxyuranus microlepidotus) are frequent on these lists.
They say it’s “the most lethal” of all snakes because its venom is the most potent. What they don’t tell you is that there hasn’t been a single reported human death caused by it. So here is a venomous but mild-mannered snake – which hasn’t killed anyone – being added to lists of the deadliest animals in the world. This inspired me to find a more consistent and accurate way to assess and measure how dangerous a species of animal really is to humans.
Breaking down the scale
Species are ranked into categories 1–5, with category one being the least dangerous and five the most. The category an animal is placed into on the scale depends on two distinct factors: population size (PS) and mortality rate (MR).
The first factor accounts for how likely you are to encounter a species; the second accounts for how likely you are to be killed by that species. Both factors consider you as a random individual, out of the global human population (rounded to the nearest whole billion), within a particular country.
Each factor is represented by a number of “points,” referred to as degrees of danger (DoDs), with a maximum of five (Table 1). Together, the DoDs of the two factors are multiplied to get a total number of DoDs, with a maximum of 25 (Table 2).
Table 1: DoDs point system.
Table 2: Total DoDs numbers and category types.

The scale was then used to evaluate select species in given countries where relevant data was available at the time of the study (Table 3). Consequently, many medically important species, such as brown rats (Rattus norvegicus) and the puff adder (Bitis arietans), were not included in this analysis. Due to the sparsity of data, information was gathered from a wide array of sources, including traditional scientific papers, articles from organizations and government websites.
The key findings of this study were:
- All six examples of categories three, four and five animals are commonly found in developing countries (Table 3)
- The most deaths caused by category three, four and five species occur in developing countries (e.g. Brazil, India, etc.)
- Predictably, all the category three, four and five examples can be found in countries graded for low or medium human development, according to the human development index (HDI)
Table 3: The five categories of danger classification with examples.

People, poverty and problems
The category a species was placed in was found to correlate with the country it was recorded in and how developed that country is.
Education is one of the HDI’s dimensions in determining a country’s level of development. Poorer education can indirectly cause a species to have a higher MR due to a lack of awareness and knowledge within the general public. For example, not knowing how certain diseases are transmitted can exacerbate the health risk by spreading them more rapidly; not knowing the symptoms of a disease can lead to misdiagnosis and incorrect treatments.
People’s standard of living is another dimension of the HDI. As with education, a poorer standard of living can also be an indirect cause of increased MR for a species.
Cost, distance to available treatment and the means to get there can also be significant barriers in less developed countries, increasing the risk a victim may succumb to injuries or illness related to animal encounters.
This scale provides a framework with which experts can begin to assess the species that pose the greatest risk to human life in given countries. This can help to guide further preventative steps and education programs where needed.
Despite attempts to ensure all statistics are accurate, it should be acknowledged that both the population numbers for the PS and the number of deaths for the MR are estimates made from the data available, which in some cases may be inaccurate, outdated, scarce or non-existent. The scarcity of data also meant that only a select few species were analyzed using the scale and there are therefore still many species that pose a threat to human life that were not included in this study.
Future plans
It’s unknown exactly how many deaths are caused by the species considered in the study, if the death toll is increasing/decreasing or if there are other emerging species of interest that should be included. Further research and collection of data is required to address these limitations and to expand the range of species and species locations that could be evaluated.
Originally, the scale was to be used as a guide to direct experts in deciding which species in which countries require more attention to prevent further loss of human life. However, a comprehensible and easy to interpret scale like this can be useful to both experts and everyday people.
The information gleaned could be used to set up public information displays (e.g. backlit dioramas, information kiosks, posters, signs, etc.) for locals and tourists in countries with category-three, four or five species to help reduce risks. These could be displayed at airports, hotels, hospitals, schools, shopping centers and other public areas as well as on websites. Further information could also be added to the scale such as what steps should be taken to avoid contact with a species.
For instance, tourists arriving at Murtala Muhammed International Airport in Lagos, Nigeria would see a sign telling them that Anopheles mosquitoes are classed a category five in Nigeria due to malaria and offer advice on how to avoid contact.
Table 4, below, serves as a rough conception of what the Crespo scale would look like in Nigeria (language would be translated appropriately).
Table 4: Significant threat examples in Nigeria. Credit: A Feral dog on Navassa Island, Schistosoma mansoni trematodes and Anopheles Gambiae mosquito - all Public Domain images.

* Mortality rate (MR) is the estimated number of deaths (either generally or from a specific cause) within a particular population number and per unit of time (e.g. per year).
* Population size (PS) is the estimated number of mature individuals in the population of a species, wild or feral.
Reference: Duarte Crespo D.The Crespo scale: Categorising “dangerous” animal species with two quantifiable factors. Asian J Res Zoo. 2024;7(2): 42–51. doi:10.9734/ajriz/2024/v7i2148.