Squirrels are the animal most frequently blamed for power outages across the United States, and utility data broadly supports that reputation. The American Public Power Association has tracked animal-caused outages for years, and squirrels consistently dominate the tally. But the picture shifts dramatically depending on where in the world you look: in tropical regions, monkeys and snakes take center stage, while in subtropical Florida, an invasive lizard has become a utility’s top animal headache. The real answer depends on geography, climate, and the type of electrical infrastructure in question.
Why Squirrels Are So Destructive to Power Grids
Squirrels cause outages through a combination of gnawing and bridging. They chew on insulation around wires and transformer components, sometimes exposing bare conductors. More commonly, a squirrel climbing across equipment will simultaneously touch a live conductor and a grounded surface, creating a short circuit that trips protective devices and cuts power to nearby customers. The outage itself is usually brief, often restored within an hour or two, but the cumulative toll is enormous simply because squirrels are everywhere that electrical infrastructure is. They thrive in suburban neighborhoods with mature trees, exactly the places where distribution lines run along streets and through yards.
What makes squirrels such a persistent problem is not that any single squirrel is especially destructive but that their population density near human infrastructure is staggering. Gray squirrels and fox squirrels are found across most of the continental United States, and both species are instinctive gnawers. Their incisors grow continuously, and gnawing on hard objects keeps those teeth filed down. Electrical hardware happens to be conveniently located along their travel routes. Utilities have tried everything from plastic guards and spinning baffles on poles to greased wires and ultrasonic deterrents, with mixed results.
Birds and the Surprising Physics of Droppings
Birds are the second most significant animal category worldwide for power outages, but they cause problems in fundamentally different ways than squirrels. One of the more dramatic mechanisms involves droppings. Large raptors and waterfowl often defecate just before takeoff, releasing a streamer that can be one to two meters long. Because bird excrement is highly conductive, that streamer can bridge the insulation gap between a grounded crossarm and a high-voltage terminal, causing a flashover and tripping the line.
This mechanism has been studied extensively on high-altitude transmission lines. On China’s ±400 kV Qaidam-Lhasa DC line, which averages about 4,650 meters in elevation, 95 of 102 recorded short-circuit events over roughly five years were caused by birds, almost entirely through defecation. At that altitude, there are essentially no tall trees or other perching options, so birds congregate on transmission towers, making the problem far worse than it would be in forested lowlands.1Energy Reports. Bird-related fault analysis and prevention measures of ±400 kV Qinghai-Tibet DC transmission line
The physics behind streamer flashover have been studied in detail. Large raptors produce streamers long enough to partially or fully bridge the insulation gap on standard towers, and the high conductivity of the excrement makes the path electrically viable even for high-voltage lines.2International Journal of Electrical Power & Energy Systems. High-risk region of bird streamer flashover in 110 kV composite insulators and design for bird-preventing shield
Birds also cause outages through nesting and collision. Sticks from large nests can short between conductors. A bird carcass draped across two wires does the same thing. In Iran, a study of avian-caused outages found that 96% occurred on distribution-voltage lines, and 91% involved phase-to-ground contact on grounded concrete pylons with grounded steel crossarms.3Bird Conservation International. Electrical Components Involved in Avian-Caused Outages in Iran That last detail matters: the pylon design itself makes certain regions far more vulnerable to bird-caused faults than others.
Monkeys, Snakes, and the Regional Variation Problem
In parts of Southeast Asia, Africa, and South America, monkeys are the dominant animal cause of outages. They are large enough to bridge conductors, curious enough to investigate electrical equipment, and social enough to travel in groups along power line corridors. An analysis of animal-caused faults in the Sumatran power system found that monkeys were responsible for the largest amount of undelivered energy: roughly 629 megawatt-hours, compared with about 391 MWh from birds and 358 MWh from a third category of animals.4International Journal of Electrical, Computer, and Biomedical Engineering. Analysis of Single Phase to Ground Faults Caused by Animals on Energy Not Served (ENS) in the Sumatera System Per-incident, monkey-caused faults tend to be more severe because monkeys are larger-bodied and often damage components rather than simply tripping a breaker.
Snakes are another major player in warmer climates. They climb poles and enter ground-level switchgear seeking warmth or prey, and their bodies are long enough to bridge between live and grounded parts. A decade-long study of non-avian electrocutions on power networks in Iran recorded 120 incidents, with mammals accounting for 74% and reptiles (predominantly snakes) accounting for 24%. Ground-mounted components such as low-voltage freestanding switchboards were the most common location, responsible for 35% of all incidents. More than half of the documented incidents resulted in outages, and some also triggered fires.5Cambridge University Press. Mammal, reptile, amphibian and invertebrate electrocutions on electric power networks in Iran
The takeaway is that there is no single global answer. In temperate North America, squirrels dominate. In tropical island systems, monkeys and snakes take the lead. In high-altitude treeless terrain, birds are the overwhelming cause. The animal that causes the most outages in your area depends on what lives there and what kind of electrical infrastructure is installed.
Florida’s Iguana Problem
One of the more unusual regional cases is southern Florida, where invasive green iguanas have become a utility’s primary animal concern. Florida Power and Light, which serves much of the southeast part of the state, has identified iguanas as the number one animal-related outage concern. They cause an estimated 15 to 20 outages per year in FPL’s service territory alone. Keys Energy, which covers the lower Florida Keys, has reported an average of four iguana-related outages annually, with individual incidents ranging from 94 to 29,550 customer-minutes of lost power.6Frontiers in Amphibian and Reptile Science. State of knowledge for invasive green iguanas in Florida reveals negative impacts and pervasive research needs
Iguanas are drawn to the warmth of electrical equipment, and their large body size means they can easily span the gap between live and neutral components inside transformers or at substation connections. The severity of each outage varies depending on which component fails, but at the extreme end, a single iguana has knocked out power for tens of thousands of customer-minutes. These numbers are still modest compared with squirrel-caused outages nationally, but for a localized utility in subtropical Florida, iguanas have surpassed every native species as the primary animal threat to reliability.
Small Animals That Cause Big Damage Underground
Not all animal-caused outages happen on overhead lines. Underground cables face their own set of biological threats, and the culprits are often surprisingly small.
Rodents, particularly Norway rats, are well-documented cable gnawers. They chew through the insulation on both power and communications cables, causing short circuits, service interruptions, and occasionally fires.7JOURNAL OF WILDLIFE MANAGEMENT. REPELLENTS TO REDUCE CABLE GNAWING BY WILD NORWAY RATS Because rats are commensal animals, meaning they live alongside humans and exploit human-built environments, they are concentrated in exactly the places where underground utilities run: urban and suburban areas with buried infrastructure. The damage is less visible and often harder to diagnose than an overhead outage, since the gnawing can degrade insulation slowly before a fault develops.
Fire ants present an even stranger problem. Red imported fire ants are attracted to electrical fields and tend to colonize junction boxes, traffic signal cabinets, and other enclosed electrical housings. Once inside, they chew on wire insulation and their accumulated bodies can create conductive bridges between contacts. This has been a particular headache for traffic signal systems in the southern United States, but fire ants also infest utility equipment and have caused documented outages and equipment failures. The ants seem to be attracted to the electromagnetic field or the warmth generated by electrical equipment, though the exact mechanism of their attraction is still debated.
Sharks and Submarine Cables
Sharks biting submarine cables became something of an internet legend in the early 2010s, and the phenomenon is real, though its scale is often overstated. Shark bites have caused documented damage to undersea fiber-optic and power cables, leading to costly repairs and prolonged outages. The vast majority of sharks never interact with cables at all, but certain species appear to investigate cables using their electroreceptive senses, and their bite force is more than sufficient to penetrate cable sheathing.8Annals of Mirce Science. MIRCE Science: Shark Bite as a Mechanism of Motion of Submarine Cables through MIRCE Space
In practice, anchor drags and fishing trawlers damage far more submarine cables than sharks do. But shark-related cable failures are still a recognized risk category for cable operators, and modern submarine cables in shark-prone waters are often armored with additional protective layers. The outages that result from a severed submarine cable are fundamentally different from a squirrel tripping a transformer: they can take weeks to repair, require specialized cable-laying ships, and can affect internet and power connections for entire island communities.
How Utilities Try to Prevent Animal-Caused Outages
The sheer variety of animals involved means there is no single engineering fix. For squirrels and climbing mammals, utilities install wildlife guards on transformers and bushings. These are typically plastic covers that prevent an animal from simultaneously contacting two energized points or an energized point and a ground. Spinning baffles on lines discourage squirrels from crossing, though determined animals sometimes bypass them.
For birds, the mitigation strategies depend on the mechanism. Nesting deterrents, such as spikes or platforms placed away from live components, redirect nest-building to safer locations. Anti-perching devices keep large raptors off crossarms where their droppings could cause streamer flashover. Some utilities install insulated covers over energized conductors near towers where bird activity is heaviest. In regions with severe bird-related faults, reflective or spinning devices have been tested to discourage birds from landing on specific towers, though their long-term effectiveness is mixed.
For snakes and reptiles, ground-mounted equipment is the primary vulnerability. Enclosures can be sealed more tightly, and snake barriers made of smooth vertical surfaces have been tested in tropical regions. Iguanas in Florida present a harder problem because they are large, abundant, and drawn to the warmth of energized equipment. Some utilities have experimented with physical exclusion devices, but the animals are resourceful climbers.
Underground cable protection against rodents typically involves harder sheathing materials or physical conduit, but retrofitting existing buried cable is expensive and rarely done unless the damage rate justifies the investment. For fire ants, insecticidal treatments around equipment enclosures are the most common approach, though regular maintenance is required because the ants recolonize quickly.
When the Outage Kills the Animal Too
Animal-caused outages are a two-way problem. The animal that trips a circuit often does not survive the encounter. For common species like squirrels and rats, the population-level impact is negligible, but for endangered raptors and eagles, electrocution on power lines is a genuine conservation crisis.
The endangered Bonelli’s eagle in France provides a well-studied example. A long-term capture-recapture program running from 1990 to 2009 found that insulating dangerous power lines increased survival across all age classes, with the strongest benefits for juveniles and immature birds. The predicted population growth rate rose sharply after insulation programs were implemented, jumping from a declining trajectory to near stability.9Journal of Applied Ecology. Retrofitting of power lines effectively reduces mortality by electrocution in large birds: an example with the endangered Bonelli’s eagle That study was one of the first to demonstrate at a population scale that power line retrofitting actually moves the needle on survival.
In Argentina, the endangered Crowned Solitary Eagle faces a similar threat. Despite being relatively rare in the landscape, the species is disproportionately represented in electrocution records. Researchers estimated that electrocution could kill a minimum of about 3% of breeding adults per year, and possibly as high as 10% depending on how many breeding territories are active. Roughly 11% of fledglings produced each year were estimated to die from electrocution.10Bird Conservation International. Electrocution risk for the endangered Crowned Solitary Eagle and other birds in semiarid landscapes of central Argentina For a long-lived species with low reproductive output, those losses are devastating.
This conservation dimension creates an unusual alignment of interests. Utilities want fewer animal-caused outages because every outage costs money in repairs and lost revenue. Conservationists want fewer electrocutions because they kill vulnerable species. The same insulation and guard technologies that reduce outages also reduce wildlife deaths, which is why bird-safe power line designs have gained traction even in regions where regulators have not mandated them. The engineering is not cheap, but many utilities have found that the reduction in outage-related costs offsets much of the investment, especially on lines with chronic animal-fault histories.
Why the Numbers Are Hard to Pin Down
One persistent frustration in this field is that animal-caused outage data is inconsistent and often underreported. When a breaker trips and a lineworker is dispatched to restore power, the cause is frequently recorded based on whatever evidence is found at the scene. A charred squirrel on a transformer makes the diagnosis obvious. But many outages are simply labeled “unknown” because the animal’s carcass fell to the ground, was carried off by a predator, or left no visible trace. Some experts believe that animal-caused outages are substantially undercounted for this reason.
Reporting standards also vary between utilities and countries. A utility in the southern United States may track every squirrel contact meticulously, while one in Southeast Asia may lump all animal causes into a single category. Comparing squirrel outage rates in Ohio with monkey outage rates in Sumatra requires accounting for differences in grid design, reporting methodology, customer density, and what “an outage” even means in terms of duration and severity. The Sumatran data measured impact in megawatt-hours of undelivered energy, which captures severity more meaningfully than a simple count of events. A single monkey-caused fault that takes down a transmission line for hours in a densely served area looks very different from a squirrel tripping a neighborhood transformer for twenty minutes.
Climate change adds another layer of uncertainty. Warmer winters are allowing species like green iguanas to expand their range northward in Florida. Rat populations tend to increase in mild winters. Bird migration patterns are shifting, potentially changing which species interact with power infrastructure and where. Utility planners trying to forecast animal-related risk in 2035 or 2045 are working with a moving target, and the animal that tops the outage list in a given region a decade from now may not be the same one that tops it today.