Yes, animals have hormones, and they’ve had them for a very long time. Hormone signaling is one of the most ancient biological systems in the animal kingdom, with insulin-like molecules and growth-signaling peptides found even in sponges, the most primitive animals on Earth. From insects timing their transformation into adults to birds knowing when to migrate, hormones orchestrate nearly every major process in animal life.
How Far Back Hormones Go
The oldest known hormone families are the insulin superfamily and a group of growth-signaling molecules called TGF-β. Both have been identified in sponges, which sit at the very base of the animal family tree. A species of sponge called Geodia cydonium produces an insulin molecule with roughly 80% similarity to human insulin in its genetic sequence. Sponges also have the receptor proteins needed to detect and respond to that insulin, meaning the full signaling loop was already in place hundreds of millions of years ago.
Moving slightly up the evolutionary ladder, placozoans (tiny, flat organisms with no organs) have four insulin-like peptides, and hydra, simple freshwater animals with tentacles, have three. These aren’t identical to the insulin in your pancreas, but they belong to the same molecular family and share the same core structural features, including the arrangement of chemical bonds that gives the molecule its shape.
Insects Run on a Different Hormone System
Insects don’t use the same hormones mammals do for many processes, but their endocrine systems are just as sophisticated. Two hormones dominate insect life: ecdysteroids, which are steroid hormones that trigger molting and metamorphosis, and juvenile hormone, which prevents a larva from maturing too early.
Here’s how it works. A pair of neurons in each side of the insect brain produces a signaling molecule that stimulates a gland in the larva’s body to manufacture ecdysone. The insect builds ecdysone from dietary cholesterol through a chain of chemical modifications. Once ecdysone enters the body’s tissues, it gets converted into its active form, which flips the switch on molting or metamorphosis. Juvenile hormone acts as a counterweight. As long as juvenile hormone levels are high, the larva keeps molting into a bigger larva. When juvenile hormone drops, the next surge of ecdysone triggers the dramatic transformation into a pupa or adult. Experiments in fruit flies confirmed this directly: when researchers blocked the juvenile hormone receptor in the ecdysone-producing gland, the larvae produced ecdysone too early and pupated ahead of schedule.
Stress Hormones Vary by Species
Mammals rely on cortisol as their primary stress hormone, but most amphibians, reptiles, and birds use a closely related molecule called corticosterone instead. Both do essentially the same job: mobilize energy, suppress inflammation, and shift the body into a state that prioritizes immediate survival.
The picture can be more nuanced than a simple either/or, though. Research on axolotl salamanders revealed that these animals use both cortisol and corticosterone, with each one dominating under different circumstances. When axolotls were physically handled (a form of acute stress), cortisol surged higher. But after a limb amputation, corticosterone was the dominant response. The two hormones appear to be activated through different pathways: corticosterone through the classic brain-to-adrenal signaling chain, and cortisol through a faster route involving direct nerve signaling. This suggests that even within a single species, the stress response is more layered than previously thought.
Invertebrates Have Their Own Adrenaline
When you’re startled, your body floods with adrenaline (epinephrine) to prepare for fight or flight. Insects and other invertebrates experience a similar response, but the molecule doing the work is octopamine. Octopamine serves as the primary fight-or-flight signal in invertebrates, increasing heart rate, mobilizing energy stores, and priming muscles for action. When mammals evolved, epinephrine took over that role, though octopamine is still present in trace amounts in mammalian brains.
Insulin Beyond Blood Sugar
Insulin is perhaps the most widely shared hormone across the animal kingdom, but its job description has expanded far beyond blood sugar regulation. The roundworm C. elegans has 40 different insulin-like peptides. Fruit flies have eight. In these invertebrates, insulin-like signaling controls not just metabolism but also learning, memory, and behavior.
In fruit flies, specific insulin-like peptides are involved in forming memories of threatening smells. In roundworms, one insulin-like peptide is required for learning to associate salt concentration with food availability, and another suppresses avoidance behavior toward dangerous bacteria. A snail called Lymnaea stagnalis was one of the first invertebrates where researchers confirmed an insulin-related peptide in the nervous system, providing early evidence that the functional importance of these molecules has been conserved across vast evolutionary distances. An insulin-like peptide in silkworms shares about 40% of its amino acid sequence with human insulin and has the same signature arrangement of six structural anchoring points.
Hormones That Drive Seasonal Behavior
Birds offer some of the clearest examples of hormones coordinating complex seasonal behaviors. Testosterone plays a well-documented role in breeding, but its influence extends to feather replacement (molting) as well. Studies have shown that testosterone implants can delay, prevent, or interrupt molting, which makes sense because birds need to finish breeding before they invest energy in growing new feathers.
Prolactin, a hormone better known for its role in milk production in mammals, turns out to be the key signal for molting in birds. In starlings and other species studied in the wild, prolactin levels peak right around the time molting begins. But the trigger isn’t reaching a certain prolactin level. Instead, molting starts when prolactin begins to decrease from its peak. Researchers confirmed prolactin’s role by blocking its release in starlings, which prevented molting entirely. Prolactin could be uncoupled from reproductive hormones, but molting always tracked with prolactin changes, confirming it as the primary driver.
Melatonin and the Internal Clock
Melatonin regulates sleep-wake cycles across the animal kingdom. It acts on receptors in the brain’s master clock region to synchronize an animal’s internal rhythms with the light-dark cycle of the environment. This circadian clock governs not just sleep but also body temperature, feeding patterns, and the timed release of other hormones. Melatonin’s role in circadian regulation has been documented across mammals, birds, fish, and reptiles, making it one of the most broadly conserved hormonal functions in vertebrates.
Pheromones: Hormones That Work Between Animals
Hormones work inside an animal’s body, but a related class of chemical signals, pheromones, work between animals. Pheromones are substances secreted by one individual and detected by another member of the same species, triggering a specific behavior or developmental change. The concept was originally described in insects, where pheromones can signal alarm, mark trails, or attract mates with remarkable precision.
In mammals, the line between hormones and pheromones blurs in interesting ways. Gonadal steroids like testosterone and estrogen don’t just regulate reproductive physiology internally. They also regulate the production of social odors, sometimes serving as chemical precursors for the pheromones themselves. The brain areas that respond to these social odors are, in turn, shaped by the same steroid hormones. So the hormonal state of the sender determines what chemical signal gets produced, and the hormonal state of the receiver determines how that signal gets interpreted.
Hormones in Egg Production
In fish, reptiles, and amphibians, egg production depends on a hormone-driven process where the liver produces a yolk precursor protein called vitellogenin, which is then taken up by developing eggs. This process is controlled by multiple hormones working together, with estrogen playing a central role in switching on vitellogenin production in the liver. The system is so sensitive to estrogen that environmental pollutants mimicking estrogen can trigger vitellogenin production in male fish, a phenomenon that has become a widely used marker for chemical contamination in waterways.