City lovin’: Citizen science reveals frog breeding seasons are longer in the big smoke

By Gracie Liu, Research Assistant and FrogID Validator, Herpetology, Australian Museum Research Institute; PhD candidate, UNSW Sydney.

August 24, 2022


Human activity causes frogs to reproduce earlier and longer. What does this mean for the future of frogs?

For humans, buildings, traffic noise and streetlights are part of everyday life. But the changes we make to the environment can have serious consequences for wildlife. As habitat is cleared to make way for cities or farms, the animals living in these environments must adapt or risk declining. We used data from the Australian Museum’s FrogID citizen science project to reveal that frog breeding seasons start earlier and last almost 3 weeks longer in heavily human-modified areas, including our cities. . Could this be a sign that they are adapting to city life? It’s possible, but it’s a bit more complicated than that.

Frogs, like most animals, breed at specific times of the year. When the frogs are breeding, it’s hard to miss as there’s often a hoarse chorus of croaks, clicks, cackles and bleats from male frogs desperately trying to woo a female. For many species of frogs, this breeding period is during the warm spring and summer months, but for some it is during the winter, and for others it is all year round. . Ultimately, all of these species have one common goal: to breed at a time that maximizes the chances of their offspring surviving. Usually this is a time when food is plentiful, there is water and the weather is not too harsh.

Related: How to Make Your Garden Frog Friendly

Timing is everything. If frogs try to breed too early or too late in the season, it may be too hot, too cold, or too dry for successful breeding. Their eggs may dry out or their offspring may starve and never reach adulthood.

So how do frogs know when to breed? Frogs connect to their environment, which holds important clues as to when conditions are right for breeding. The right temperature, humidity levels and plenty of food will trigger the start of breeding. And when the food drops, when it gets too cold (or too hot), or their breeding pond or substrate dries up, their breeding attempts (and calling) stop. But if frogs rely on their environment to tell them when to breed, what if the environment changes?

Cities and suburban gardens are dramatically different from the natural habitats of frogs, which range from forests to grasslands. They are often hotter, noisier, artificially brighter (due to street and building lighting) and more polluted. Could frogs adapt their reproductive behaviors to cope with these conditions?

To answer this question, we need a huge amount of data. Specifically, we need repeated records of frogs over time (throughout the year) in many different areas to establish when breeding is taking place in different habitats exposed to varying levels of human disturbance, from cities to the suburbs, from farms to forests. Collecting this data is a challenge in itself and this is probably why no one has explored this question – until now.

Citizen science data, data collected by members of the public, is a game-changer and allows us to investigate large-scale ecological questions like this for the first time.

Using the FrogID App
Submitting repeated records of frogs over time to the FrogID app can help determine frog breeding seasons. Photo credit: Gracie Liu © Gracie Liu

We analyzed calling patterns in over 226,000 frog records submitted to the Australia-wide citizen science project, FrogID. These data covered 42 species (the number of species with enough records) and, surprisingly, we found that all 42 species had longer-breeding species along a gradient of human modification. In other words, species inhabiting heavily modified habitats (such as cities) reproduced longer than species inhabiting unmodified natural habitat. This difference was about 23 days, or just over 3 weeks. This was almost always due to frogs in modified areas starting their breeding seasons earlier in the year (rather than ending their breeding seasons later in the year).

Three weeks might not seem like a long time, but producing those ad calls is incredibly taxing. It requires a lot of energy and alerts potential predators to their location. Considering the risks, are the benefits worth it?

At best, changes in their breeding seasons could give frogs an adaptive advantage in modified areas. Perhaps they are taking advantage of warmer temperatures in cities to breed more. There may be more food in urban areas to support their breeding efforts. Perhaps they capitalize on garden and city ponds that are full year-round. These can be reliable breeding habitats for pond-breeding frogs because the risk of their eggs or tadpoles drying out is low.

A pair of Peron's tree frogs (Litoria peronii) trying to mate next to a suburban pond
A pair of Peron’s tree frogs (Litoria peronii) attempting to mate at the edge of a suburban pond. Photo credit: Gracie Liu © Gracie Liu

But there’s also the more ominous possibility that these changes aren’t adaptive at all, that the frogs are actually wasting their effort. Males might call females more in modified habitats, but those calls might fall on deaf ears. Or the males may outgrow the first stage and find a mate, but their offspring may succumb to the pressures of city life and not survive to adulthood.

If frogs waste energy engaging in costly reproductive behaviors in human-modified habitats, it could be a huge blow to frog populations, which have already been hit hard by habitat loss and degradation. , diseases, climate change, fires, droughts and floods.

Our research is the first step in understanding how frogs respond to human-imposed pressures. Now that we know that frogs respond by reproducing earlier and longer, we need to look at the consequences. Do these changes allow frogs to successfully adapt to human-modified landscapes? Do they mate more? Do they lay more eggs? Do their offspring survive? These are important questions that need to be answered and, with the help of citizen scientists, will go a long way in helping us better understand the human impacts on frogs and inform appropriate management actions.

Related: “I’ve only seen one in my life”: the mystery of the blue tree frog

This article was first published on the Australian Museum blog.