The question of which animal ranks second in intelligence after humans has long fascinated scientists and the public, yet it remains a complex and debated topic. Assigning a singular rank to non-human intelligence is difficult because “smartness” is not a single, measurable trait but a collection of specialized cognitive abilities. A species’ true intelligence relates to how effectively it solves the problems posed by its specific environment, meaning a direct, linear comparison across different biological groups often proves misleading.
Defining Non-Human Intelligence
Scientific efforts to measure animal intelligence rely on a suite of physical and behavioral metrics, moving beyond simple brain size. One historical measure is the Encephalization Quotient (EQ), the ratio between an animal’s observed brain mass and the brain mass predicted for its body size. A higher EQ suggests a greater proportion of the brain is available for complex cognitive tasks beyond basic body maintenance. However, physical measurements alone are insufficient, as the number of neurons in the forebrain may be a better predictor of intelligence across vertebrates than overall size or EQ.
For a more comprehensive assessment, researchers rely on behavioral observations and experimentation, focusing on adaptive behaviors and problem-solving abilities. This includes testing a species’ capacity for innovation, social learning, memory, and its ability to respond to novel situations. The complexity of a species’ communication signals and its social structure are also factored in, as navigating a large, dynamic social group requires sophisticated cognitive processing.
The Leading Candidates for Second Place
Based on high encephalization, complex social structures, and demonstrated cognitive flexibility, three groups of animals consistently emerge as contenders for the top tier of non-human intelligence.
The first group is the great apes, specifically chimpanzees and orangutans, which are our closest living relatives. Their intelligence is characterized by advanced social strategies, long-term memory, and a capacity for sophisticated learning within their communities.
The second highly ranked group is the cetaceans, particularly bottlenose dolphins and various whale species. They possess some of the largest brains in the animal kingdom and high Encephalization Quotients. Their intelligence is shaped by a cooperative marine environment, leading to complex communication, cultural transmission of behaviors, and intricate social bonds.
The third group is the corvids, including crows, ravens, and jays. These birds possess small but densely packed forebrains, giving them cognitive abilities that rival those of primates. Corvids are known for their behavioral flexibility and capacity for innovation, often exploiting novel solutions to access food resources.
Cognitive Benchmarks
Specific, repeatable experiments serve as benchmarks for advanced cognition. One famous test is the mirror self-recognition (MSR) test, which indicates a degree of self-awareness; success has been documented in chimpanzees, orangutans, dolphins, and some corvids.
The complexity of tool use is a strong indicator of planning and abstract thought, going far beyond simple stick use. Chimpanzees, for example, use sticks to “fish” for termites and stones as hammers and anvils to crack open nuts, demonstrating a planned sequence of actions. New Caledonian crows elevate this benchmark further by demonstrating meta-tool use: the ability to use one tool to obtain a second, more effective tool. This behavior requires a high degree of logical reasoning and an understanding of tool properties.
In the social realm, experiments suggest the capacity for “theory of mind”—the ability to attribute mental states, such as beliefs or desires, to others. Apes and corvids have shown evidence of tactical deception and understanding what others can or cannot see, suggesting they can model another individual’s perspective to manipulate a situation.
Specialized Intelligence and Ranking Complexity
A definitive ranking of animal intelligence is complicated by the existence of highly specialized or “ecological” intelligence, which often falls outside the traditional benchmarks. This type of intelligence is finely tuned to an animal’s particular survival needs and environmental niche, making cross-species comparisons difficult.
For instance, the Clark’s nutcracker demonstrates extraordinary spatial memory by accurately recalling the location of thousands of seeds it has cached for winter. Similarly, octopuses, invertebrates with a completely different neurological structure, exhibit remarkable problem-solving skills and manipulative intelligence. They have been observed opening screw-top jars, navigating complex mazes, and even using coconut shells for protection.
This highlights that intelligence can be decentralized, as two-thirds of an octopus’s neurons are located in its arms, allowing for complex, independent action. These examples demonstrate that different evolutionary paths can lead to equally effective, yet distinctly different, forms of mental sophistication, making a single “second smartest” title an oversimplification.