As a biologist, when I spot a frog, the first thing I try and figure out is what species is it. Relatively quickly after that, I want to know if it’s a male or a female. Below are some pointers in distinguishing male from female frogs- some rules are pretty stricly adhered to in the frog world, while others are a little less definitive.
Male frogs call
Male frogs are the ones you hear croaking away all night from your backyard pond or local stream. Male frogs call from potential breeding sites to attract female frogs- females decide which calling male of her species sounds the most attractive and then approaches him. This is not to say that female frogs can't make any noise- they certainly do (for example, if a snake grabs a female frog you’ll generally hear a ‘scream’), but they don’t advertise themselves all night- they leave that to the males!
Male frogs have vocal sacs
When male frogs call, their throat region usually expands as their vocal sac fills with air, amplifying their call. Because of this, when not calling, males tend to have obviously thin, baggy skin on their throat. This skin is also usually a different colour to their belly (often yellow or black).
Female frogs are generally bigger than males
Although there are exceptions to the rule, female frogs are generally bigger and heavier than males. This makes sense, as females are the ones responsible for holding the eggs, and bigger females can hold more eggs (which potentially translates into having more offspring). Sometimes the size difference can be subtle, but sometimes it's dramatic, with females three times larger than males (Odorous frogs [Odorrana sp.] from Asia are prime examples of this). There are exceptions, though. Particularly where male frogs fight with each other (such as the Australian Tusked frog [Adelotus brevis] and Asian fanged frogs [Limnonectes sp.]), males have become larger than females.
Males and female frogs often have slight differences on their hands and feet
Male frogs often have small differently coloured and/or more roughly textured patches on their hands, especially on the insides of their thumbs. Often tricky to see, in the breeding season they often turn dark and become raised. Most Australian frogs have very subtle nuptial pads (slightly darker and more textured bits on the inside of their thumb), but male Giant Burrowing Frogs (Heleioporus australiacus) sport some pretty serious spikes on their hands in the breeding season. In a few species, it’s females that have the hand or foot adornment, but it’s generally in the form of fringes on the fingers or toes (the fringing is generally used for whipping up their eggs into a foamy nest).
Male frogs sometimes have thicker arms
Male frogs sometimes have thicker, more muscly arms, making them look a little like they’ve been working out (a lot). Muscles probably come in handy when wrestling with other males or when holding on to the females as they carry them to a suitable breeding spot.
Male frogs are sometimes brighter than females
Usually male and female frogs are roughly the same colour. But not always. In a handful of species, males are vastly different than females all the time. In others, males match the females most of the time, but get more colourful in the breeding season. One of the best examples of this is the Australian Stony Creek Frog (Litoria wilcoxii), with males turning from a relatively dull brown to a vivid yellow in the breeding season (especially when they are actually in amplexus).
Male frogs sometimes have rougher skin than females
Sometimes there’s a difference in skin texture between male and female frogs. Most often, males have the rougher or spikier skin than the females. One of the best examples of this is the recently discovered Thorny Tree Frog (Gracixalus lumarius) from Vietnam. Males of this species have conical spikes running down their backs, while females are smooth.
Dr Jodi Rowley,