Have you ever seen photos of frogs stacked on top of one another in strange towers or pyramids? If so, you probably wondered what would possess these amphibians to clamber on top of each other in precarious poses. It’s a peculiarity of frog behavior that invites curiosity.
If you’re short on time, here’s the quick answer: Frogs stack themselves in towers and pyramids during the mating season as part of their breeding rituals. The male frogs do this to improve their odds of mating with females.
In this nearly 3,000 word article, we’ll provide a detailed explanation of this phenomenon. We’ll look at what species exhibit this stacking behavior, why they developed this unusual habit, how the stacking works to the frogs’ reproductive advantage, and what problems it can cause for them.
What Species of Frogs Stack Themselves?
Grey Tree Frogs
The Grey Tree Frog is one of the most well-known frog species that exhibits stacking behavior. During cold weather, these small frogs, with green, grey, or brown skin and bright yellow patches, will congregate together and stack up in large piles on tree branches or other surfaces.
Research indicates the frogs stack themselves to conserve heat and protect against freezing temperatures. These communal blobs can contain hundreds of frogs stacked up to 3 layers deep. The Grey Tree Frog is native to much of the eastern United States and Canada.
Green Frogs
Similar to the Grey Tree Frog, Green Frogs are known to stack on top of one another to stay warm in cold conditions. Ranging from bright green to brown in color, these frogs inhabit marshes, ponds, and other wetlands across most of North America.
When temperatures drop, they will amass in groups of up to 200 frogs. Their communal balls are usually around a foot wide but can sometimes grow much larger. It is believed the frogs huddle together to retain heat and moisture.
Bullfrogs
The large and widespread Bullfrog occasionally exhibits stacking behavior as well. During winter hibernation, they have been observed piling on top of each other at the bottom of ponds and streams. It is thought that like other frog species, the Bullfrogs stack together for warmth, so their vital organs do not freeze.
Though they mostly inhabit permanent water bodies, Bullfrogs require aerial respiration and will breathe at the surface during their hibernation. Their communal balls prevent them from surfacing and losing heat.
Other Frog Species
While less studied, various other frog species around the world have been documented participating in this unusual communal stacking behavior as well. For example, piles of European common frogs have been seen in cold conditions.
Tropical frog species like the paradox frog in Central America will also form balls during particularly cool weather. There are likely many frog species across various climates that utilize huddling and stacking to stay warm during temperature drops.
When and Where Does Frog Stacking Occur?
Mating Season
Frog stacking behavior is most common during breeding season as male frogs compete for mates. The exact timing of the mating season depends on the frog species and location, but generally occurs in spring and summer months.
For example, in North America many frogs breed between March and July when temperatures warm and rains fill seasonal ponds and wetlands. This provides ideal conditions for frogs to lay eggs and for tadpoles to develop.
During this time, male frogs are primed with hormones and gather at breeding sites, calling to attract females. Competition is fierce, with male frogs attempting to climb on top of one another to gain the best vantage point for mating.
Stacking allows them to rise above the fray to be seen and heard by females.
Notable frog stacking occurs in species like the American bullfrog, northern leopard frog, and green frog. Upwards of 6,000 male frogs may crowd a single pond, forming roiling balls and towers of competing suitors.
At Breeding Ponds and Pools
Frog stacking predominantly occurs at temporary fishless ponds and pools that form during wet seasons. These aquatic spots provide ideal frog breeding habitat free of egg and tadpole predators like fish.
Shallow, warm, vegetated ponds allow amphibians to readily stack on top of submerged plants and debris. Conditions where frogs must cluster into a limited space also promote stacking behavior and competition.
After rains, low-lying flooded fields, roadside ditches, and wetlands often teem with breeding frogs. Still backyard ponds, bog gardens, and pool vernal pools formed in the landscape also regularly witness frog stacking during mating time.
Not all frog species join these boisterous breeding balls and towers. Some like treefrogs and narrow-mouthed frogs spread out eggs terrestrially or lay in permanent waters less crowded with other frogs.
Why Do Male Frogs Stack During Mating Season?
During the breeding season, male frogs frequently clump on top of one another in a behavior known as “stacking.” This unusual conduct serves several important purposes that assist male frogs in securing a mate and propagating the species.
To Amplify Their Mating Calls
Male frogs rely on loud mating calls, or croaks, to attract the attention of females during spawning. By stacking together in a group, male tree frogs and other species can increase the overall volume and range of these vocal advertisements.
This helps ensure that nearby females can hear – and subsequently locate – their collective chorus.
To Gain Access to Females
In addition to making their presence known, stacking provides males closer access to any females drawn in by the calls. Rather than remaining scattered around a pond or wetland, piling on top of fellow competitors places each male within striking distance of an incoming female.
According to zoologists, this proximity is critical for males to successfully fertilize eggs ahead of the competition.
To Fight Off Other Males
Finally, clustering into stacks enables males to jointly fend off rivals also attempting to mate with an approaching female. By coordinating their movements and efforts, stacked male frogs essentially work together to block the advances of lone challengers.
Experts theorize such cooperative guarding helps minimize external threats to a resident male’s chance to propagate.
| Statistic | Data |
| Average number of male frogs in a breeding stack | 5 to 10 |
| Height of the largest stacks during peak season | Up to 3.5 feet |
| Number of eggs laid by a female wood frog after breeding | 1,000 to 3,000 |
How Does Stacking Help Frogs Mate More Successfully?
Louder Calls Attract More Females
When male frogs stack on top of one another in ponds and wetlands, their chorus calls become louder and travel farther. This allows them to attract females from a wider range, increasing their chances of mating.
According to research, male bullfrogs in stacks vocalize up to 20 decibels louder than solitary males. Their coordinated calls are also more likely to stand out against background noise.
Additionally, lower-ranking male frogs often cannot produce loud, robust calls on their own. By adding their voices to the chorus from a higher vantage point, they can still contribute and attract mates.
Dominant males tolerate the presence of subordinate frogs in stacks because the larger synchronized chorus brings in more females for all participants.
Dominant Positions Increase Mating Chances
Male frogs that occupy elevated positions atop mating stacks enjoy key advantages when females arrive. As females approach to inspect prospective mates, the male frogs jockey for the best spots to interact with them.
Dominant males typically claim central posts on the summit, providing both heightened visibility and first access to females coming to investigate.
| Solitary Mating | Stacked Mating | |
|---|---|---|
| Average Matings Per Male | 2-3 | 5-8 |
| Mating Duration | ≤ 1 hour | ≤ 2 days |
As shown in the table, dominant stacked males often mate with several more females during a breeding assemblage. Their mating durations also last longer before another male displaces them. Subordinate males must wait their turn, but still fare better than isolated males.
Research on other frog species reveals similar advantages.
In some cases, eager males even clamber onto the female frog’s back in stacked mating balls involving multiple partners simultaneously. Though strenuous for females, this further increases total mating quantity for the energetic dominant males.
What Problems Can Stacking Behavior Cause for Frogs?
Increased Competition and Fights Between Males
When male frogs stack on top of each other, it can lead to heightened competition and aggressive behavior. The frogs are all vying for the attention of females to mate with, so they may fight and wrestle for the top spots in the stack.
This jostling for position can result in injury or exhaustion for the frogs. Dominant males will try to forcefully remove less dominant ones from the stack in order to gain the prime mating positions. This increased competition and fighting due to stacking takes time and energy away from other important activities like feeding and avoiding predators.
Greater Risk of Overheating
Being in a dense stack can cause frogs to overheat, especially on hot summer days. Amphibians like frogs are cold-blooded and rely on the external environment to regulate their body temperature. When many warm frog bodies press against each other in a stack, it can raise their overall temperature to dangerous levels.
Hyperthermia and death from overheating are real risks. Larger stacks with more layers of frogs are at greatest risk as there is less air circulation to cool the animals down. Being trapped at the bottom is worst since the weight of other frogs overhead compresses the body and restricts evaporation.
To avoid overheating, smart frogs will hop out of a stack when they get too hot. But immobile tadpoles and eggs in the middle have no escape, putting the next generation at risk.
Higher Chance of Injury and Death
With so many frenzied frogs scrambling over each other, injuries are bound to happen. Scrapes, bruises, sprains, and broken bones can result from all the pushing, shoving, and unbalanced landings. The risk of being crushed under the weight of other frogs is also significant, especially for fragile eggs and tadpoles on the bottom layers.
Being trapped underneath or wedged into awkward positions can suffocate frogs or stop blood circulation. And if a predator attacks the stack, frogs can get trampled in the panicked exodus. An estimated 10-15% of frogs die each mating season from stack-related accidents and injuries.
So while stacking behavior helps attract mates, it clearly comes with considerable dangers that lower survival rates.
Conclusion
As we’ve explored, the phenomenon of frogs stacking themselves into towers and pyramids is connected to their mating rituals. Male frogs engage in this unusual behavior chiefly to amplify their mating calls and jockey for optimal positions to intercept females coming to breed at seasonal ponds.
While this stacking behavior seems bizarre and precarious to human eyes, it has evolved as an adaptive reproductive strategy for species like grey tree frogs, green frogs and bullfrogs. Yet it can also create problems for the frogs in the form of conflicts, overheating and physical harm.
Understanding why frogs stack gives us a fascinating window into the complex mating biology and behaviors of amphibians.
