The hottest years on record are reshaping how animals act, breed, and survive. A global analysis led by University of Connecticut researchers found that one in six vertebrate species faced unprecedented heat in 2024, with hotspots across equatorial South America and Africa, UConn Today reports.
“It seems to be the same species and regions are getting hit during these really hot years,” ecologist Mark Urban told UConn Today. “We like to say there’s no rest for the wilted.”
Behavior follows the heat. Animals shift foraging to cooler hours, retreat to scarce shade or water, and abandon exposed ground to escape lethal temperatures. When these responses collide—more bodies crowding the last puddles—conflict rises and survival drops, according to an assessment published in Phys.org.
One in six vertebrate species faced unprecedented heat in 2024.
Breeding Seasons, Sex Ratios, and Nesting Choices Are Changing
Reproduction is where heat stress bites deepest—and behavior pivots quickly. Elevated temperatures can flip the typical sex ratios of some reptiles, producing functional females from genetically male eggs in Australia’s central bearded dragon, Noticias Ambientales reports. Parents and hatchlings then face a behavioral squeeze: altered sex ratios can shift mate competition, nesting site selection, and territorial patterns as populations scramble to balance who breeds, where, and when.
Inside the cell, heat is scrambling the machinery that makes sperm and eggs. In Madagascar’s Guibé’s ground gecko, temperatures at both cooler and warmer extremes boosted crossover events—the DNA swaps that generate genetic diversity—potentially affecting fertility and development pathways that guide courtship timing and nesting fidelity, Earth.com reports.
Lead author Laura González Rodelas told Earth.com that the team observed “hyper-CO spermatocytes” at both temperature extremes—evidence that stress is rewiring the baseline for successful reproduction.
Extreme heat is altering reptile reproduction at the genetic level.
From Daily Routines to Community-Level Strain
When a year is hotter than any a species has seen, routine behavior breaks. The rapid bioassessment devised by UConn scientists tallies how much of a species’ range crosses that line, then stacks those exposures across entire communities to see where cascading stress will likely surface first, Phys.org reports.
Assistant Research Professor Cory Merow described the goal plainly: “We are trying to hindcast or forecast species that were recently or are about to be at greater risk,” to trigger targeted monitoring and mitigation—shade, water, supplemental food—before die-offs occur, according to UConn Today.
These heatwaves do more than exhaust individuals, Phys.org reports. They compound across years. Eighty percent of species exposed in 2023 were hit again in 2024, an accumulating burden that erodes resilience and can force animals into riskier movements, crowded refuges, and altered feeding schedules.
Species are shifting feeding and mating behaviors to cooler hours.
What Comes Next in a Warmer Wild
As heat pushes animals to feed at night, nest deeper, or abandon traditional ranges, the margin for error shrinks. Research tracing genetic impacts in reptiles and heat-exposure mapping across continents points to the same takeaway: behavior is the first alarm bell, and it’s ringing now.
“When five out of the last 10 years are the worst… it can have compounding effects on populations,” Merow told UConn Today, calling for proactive checks before thresholds are crossed.
Biodiversity faces a moving target, and animals are already changing their playbooks to cope.
