Open-air concerts: the call of frogs and toads

Well into mid-spring, when the nights get warmer, it’s in the more temperate latitudes where we can start hearing the songs of the frogs. If we get close to any humid area in summer we’ll hear the frog’s and toad’s choirs which sing to attract a mate and proclaim their territories. In this entry we’ll explain the functioning and secrets hidden behind the different calls and songs of the anuran world.

CALL ANATOMY

Anurans are the amphibian order with the greatest vocal abilities. Practically all species make different kinds of calls which they use to communicate and transmit information to their own kind. That’s why frogs and toads have developed a much more specialized vocal systems than the rest of lissamphibians to generate their famous calls.

New Granada cross-banded tree frog (Smilisca phaeota) in the midst of a call. Photo by Santiago Ron.

Anuran calls originate when the air passes from the lungs through the larynx where the vocal cords are found. Anurans are the only lissamphibians with true vocal cords, while urodeles and caecilians don’t have them. Lissamphibians must pump air to their lungs to breath (although they also breathe through their skin) and in most frogs the call is generated during exhalation.

The oriental fire-bellied toad (Bombina orientalis) differs from the rest of anurans in that it emits its call both during exhalation and inhalation. Photo by Flickpicpete.

Most frogs and toads also present vocal sacs that amplify the sound of their calls, some of which can be heard up to one kilometre away. Anurans may have one vocal sac in their throat, or two vocal sacs in the corners of their mouth. To emit their famous calls they must have their mouths and nasal openings closed, to direct the air to the vocal sacs. Even if some species do not have vocal sacs, most species emit calls in some form or another.

The marsh frog (Pelophylax ridibundus) is an example of a frog with two vocal sacs in the corner of its mouth. Photo by Xavier Robin.

THE REASON WHY THEY SING

Toads and frogs use their calls for one main reason: to mate. In anurans singing is a method to distinguish animals of their own species, to help males and females find each other and to detect receptive individuals. Normally the males are the ones who sing to attract females and that’s why there’s a sexual dimorphism in the vocal sacs, with males having more developed sacs than females and more elaborated calls.

Even if it’s hard to appreciate, here we can see how in oak toads (Anaxyrus quercicus) the males (left) present a bigger skin fold corresponding to a more developed vocal sac than the females (right). Image by Eric Shashoua.

It is thought that during the evolution of anurans a process of sexual selection has taken place with females selecting the males with the more adequate calls. As a general rule females prefer males with louder and deeper calls. Probably, this is due to the fact that the bigger males (which generally have the deeper voices) are usually the stronger and older ones, indicating that they have been able to survive for a longer time and that they have better genes to transmit to their offspring.

In this video by Pocketbattleship we can hear the song of the American bullfrog (Lithobates catesbeianus), which is deep and powerful.

Yet there are some species with very high-pitched calls in which the selection by females is focused on other factors. Most anuran females also prefer very frequent (with many repetitions of the sound) and longer calls (long-lasting sounds). This is because singing is a really intense activity that requires a lot of energy, indicating the males that have been able to store enough energy to carry out such an exhausting activity.

The call of the golden poison frog (Phyllobates terribilis) is really high-pitched and is characterized by its high frequency, as we can see in this video by Mavortium.

The mating season usually comes after some rainy weather in the more arid habitats and during the summer nights in the colder latitudes. Males usually form what we call “choirs” near bodies of water, as it’s in those where mating will take place. Anuran species can be separated into two groups based on their reproduction strategy: explosive breeders and continuous breeders.

Explosive breeders are usually found in dry habitats, where water availability is scarce most of the year. After some heavy rains, males congregate in the recently-formed water zones and form the choirs, singing for one or two nights. In these species females arrive simultaneously. This brings great numbers of males and females to congregate in one night and in the same area and, once the females arrive, the males quit singing and start competing energetically to make sure they mate.

Couch’s spadefoot toad (Scaphiopus couchii) is a desert living amphibian from the south of the United States, which is characterized by its explosive mating. Image by CaliforniaHerps.

The most complex behaviours occur in species which breed continuously (the majority of the anurans). In these species the breeding season can last for six months and, while males come first to the mating spots and start to form the choirs, females arrive sporadically, mate and then abandon the mating ponds. This implies that when a female arrives there are many males in the mating spot, creating a strong selection of males by the females.

Common toads (Bufo bufo) are one of the best examples of continuous breeders. Photo by Janek.

Instead of chasing the females like the explosive breeders, these use different calls both to stand out from the rest of the males and therefore be chosen by the females, and to warn male rivals not to approach their territory. Even if usually the males that are able to maintain their territories for the longest time are normally the ones that will have more offspring, there are also are the so-called “satellite males” which instead of singing, stay close to the males with the more powerful calls and intercept the females attracted by them.

A CALL FOR EACH SPECIES

Obviously, the calls also allow the females to differentiate the individuals of their own species from others. This can also help us, as anurans are usually secretive and nocturne animals and their calls allow us to identify which species we have around us, even if darkness covers it all.

Down below we share with you the calls and songs of some anurans from the Iberian Peninsula, in case you go on an evening out, to help you identify the most common toads and frogs you can find in humid zones.

The common midwife toad (Alytes obstetricans) normally sings at night and on land usually far from water, using underground shelters as echo chambers because, as the rest of midwife toads (Alytes genus), it has no vocal sacs. The call is a clear and flute-like note which is repeated regularly, as we can hear in this video by The Nature Box.

The call of the Iberian spadefoot toad (Pelobates cultripes) is similar to a hen’s cluck. The deep song of the spadefoot toad is usually hard to hear, because this anuran usually sings underwater, although in this video by Versicolora we can hear it pretty well.

The spiny toads (Bufo spinosus) usually sing alone, sporadically and without forming choirs, with their body half-submerged and their head out of the water. The call consists in a series of harsh and pretty high-pitched sounds as we can hear in this recording by Martiño Cabana Otero.

The natterjack toads (Bufo calamita) sing at night, in very shallow waters, with their body pretty upright and inflating their huge vocal sac. Their call is pulsatile, powerful and boomy, and is repeated without rest as we can see in this video by Florian Begou.

The Mediterranean tree frog (Hyla meridionalis) usually sings at dusk and at night, both in water, on land or, as we can see in this video by Pedroluna, perched in the vegetation. The call consists in a single intense, nasal and monotonous note, which is repeated in long and irregular intervals.

Perez’s frogs (Pelophylax perezi) present a wide range of sounds which go from the typical “croak” to a sonorous call similar to a cackle. The choirs of these frogs are usually numerous and really loud, as we can hear in this video by Martiño Cabana Otero.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
• H. C. Gerhardt (1994). The Evolution of Vocalization in Frogs and Toads. Annual Review of Ecology and Systematics. Vol 25. Pp: 293-324.
• Morgan J. McLean, Phillip J. Bishop, Shinichi Nakagawa (2012). Assesing the Patterns of Evolution in Anuran Vocal Sexual Signals. Evolutionary Biology. Vol 40 (1). Pp: 141-149.
• Sound Water Stewards. The Riveting Life of Tree Frogs.
• Museo Nacional de Ciencias Naturales (MNCN). Comunicación acústica en anuros ibéricos: Influencia del clima, hábitat y el ambiente sonoro.
• Cover photo by Brian Gratwicke.

Maratus sp.: The spider that wants to be a peacock

If I told you that there exists a 5 millimeters Australian peacock, would you believe me? Although we can find a large number of incredible animals in this country, scientists haven’t yet discovered such a small bird. However, we can find a small peacock-like animals: the peacock spiders (Maratus sp. Salticidae Family, also known as jumping spiders), whose ‘abdomen’ or opisthosoma (the posterior part of the body in some arthropods, including arachnids) have a flap-like extensions that they can unfold to the sides of its body as real peacocks do.

The last month we introduce you these organisms at our different websites (Facebook and Twitter). Through this article, you will learn its most relevant characteristics and you’ll find out the hidden function of its drop down opisthosoma.

JUMPING SPIDERS

Peacock spiders are a part of Salticidae family, whose members are also known as ‘jumping spiders’. This family has up to 5000 species (probably, they form the largest and diverse group of spiders known nowadays), and they’re located all over the world (they can be found even at the top of Mount Everest; this is the case of Euophrys omnisuperstes). Even so, most of them inhabit tropical forests.

¿HOW CAN WE DISTINGISH THEM FROM OTHER SPIDERS?

MAIN TRAITS

Usually, spiders from Salticidae family get to be a size of few millimeters as adults (normally they don’t exceed 10mm long). From an anatomical point of view, the members of this group are characterized by its two big, simple front eyes flanked by two smaller ones, plus four eyes more located over them. The size and the position of these eyes give them an excellent vision in comparison with other spiders, and even compared to other group of arthropods its vision is extraordinary.

Look at these big eyes! Can you resist them?

Specimen of Paraphidippus auranticus (Picture by Thomas Shahan (c)).

Besides its excellent vision, these organisms have the ability to cover a distance of 50 times its length in one jump, because of what they received the nickname ’jumping’. Thus, their ability to travel long distances in just one leap and their extraordinary vision are the main traits that make these spiders being excellent predators: they hunt by stalking their prey without making spider webs or silk traps. Moreover, some of their legs tend to be longer than the others, letting them to catch preys way better.

Jumping spider predating a specimen of Diaea evanida or pink flower spider (Picture by James Niland on Flickr, Creative Commons).

Spiders of this family usually present a noticeable sexual dimorphism (that is, remarkable physiognomic differences between males and females). Jumping spider males have bigger oral appendixes (or pedipalps) than females, which they use during mating dance and copulation as much for attracting the attention of females as for giving females their spermatophore (capsule or mass containing spermatozoa) during mating.

Sitticus fasciger male (with dark big pedipalps) (Picture by sankax on Flickr, Creative Commons).

Sitticus fasciger female (Picture by sankax on Fickr, Creative Commons).

In addition to these developed pedipalps, males of some species of jumping spiders have a colorful, and even iridescent, opisthosoma (the posterior part of the body in some arthropods, including arachnids). Some of them even have an opisthosoma that can reflect UV radiations which are detected by females thanks to their extraordinary vision (as some studies suggest). In contrast, females use to be more cryptic or darker colored than males (but not always).

Male (right) and female (left) of Maratus mungaich. Male has a blue and red colored ‘abdomen’ or opisthosoma (Picture by Jurgen Otto on Flicker, Creative Commons).

The flashy opisthosoma of males attracts the attention of females, and this is complemented with mating dances composed by different dance moves: they vibrate their opisthosoma and their pedipalps or move their anterior legs, they move from one side to the other, etc. Usually, these mating dances tend to be very complex.

NOW, JUST AS A MATTER OF INTEREST…

There exist some jumping spider genera that mimic ants. This phenomenon is known as ‘myrmecomorphia’, term used to describe the fact that some animal imitate ants or also the organisms that accomplish the mimic (‘myrmecomorph’).

Some of the most famous species resembling to ants are inside the Myrmarachne genus.

Specimen of Myrmarachne sp. (Picture by Jean and Fred on Flickr, Creative Commons).

With this example, and what has been explained above, it becomes clear that the Salticidae family is a very diverse group at various levels.

MINIATURE PEACOCKS: THE PEACOCK SPIDERS

At the end of XIX century, the reverend and zoologist Octavius Pickard-Cambridge, who loved spiders, discovered a surprising organism: a small spider just over 5mm whose males have a flamboyant opistosoma.

Maratus volans male (Foto de Jurgen Otto a Flickr, Creative Commons).

According to this definition, there seems not be any difference between jumping spiders and these new organisms. But the true is that these miniature spiders have an ace in the hole: the opisthosoma of males has lateral and flexible flap-like extensions that they can unfold like wings to the sides of their bodies and over their prosoma (that is, the anterior part of the body in arachnids). Finally, when its function ends they can be folded again.

‘Abdomen’ or opisthosoma of a Maratus mungiach male (1,2,3-Different degrees of expansion of the lateral flaps) (Otto and Hill, 2011).

So, due to its colored and expansible opisthosoma this new spider was named ‘peacock spider’.

Probably, the most famous peacock spider is Maratus volans, one of the first species called as ‘peacock spider’. To date there have been identified up to 30 species of peacock spiders, and each presents subtle variations in the patterns of its opisthosoma. All of them are included in Maratus genus, whose members (but M. furvus) are endemic to Australia. Moreover, Jurgen Otto and David Hill have recently identified two new species of the Maratus genus in Queensland: Maratus jactatus and Maratus sceletus.

Maratus amabilis male (Picture by Jurgen Otto on Flickr, Creative Commons).

Maratus harrisi male (Picture by Jurgen Otto on Flickr, Creative Commons).

Maratus volans male (Picture by Jurgen Otto on Flickr, Creative Commons).

Although in the beginning it was been suggested that these flap-like expansions let them fly (because of what they were called ‘flying peacock spiders’ at first place), later studies revealed that these expansions have a more ‘romantic’ function. Let’s discover it in the next section.

A SURPRISING COURTSHIP

As well as male peacock do when they see a female, these Australian spiders use their opisthosoma in order to impress the females during courtship (that usually takes place on september-november).

Throughout this process, males perform a mating dance which is a little different from the ones performed by other jumping spiders: at the first place, they make vibrate their opisthosoma and their pedipalps; then, they raise their third pair of legs and shake them vigorously in order to attract nearby females; finally, they raise their opisthosoma and unfold the lateral flap-like extensions showing its colored pattern. Then, they make vibrate their opisthosoma to show its iridescence and they begin to move from one side to the other while shaking their legs.

But, as everybody knows, an image is better that any word, so I invite you to see the next video (made by Jürgen Otto) where you can see a male of Maratus volans dancing for his ladies:

As you have been able to appreciate while reading this article, two animals that belong to a not related groups (peacocks and spiders) have developed a quite similar courtship strategy. And it seems to yield good results in both cases. Nature is surprising us again!

 

REFERENCES

• Lim M.L.M, Li D. (2005). Extreme ultraviolet sexual dimorphism in jumping spiders (Araneae: Salticidae). Biological Journal of the Linnean Society 89: 397–406.
• Otto J.C., Hill D.E. (2015). Two new peacock spiders of the calcitrans group from southern Queensland (Araneae: Salticidae: Euophryinae: Maratus). Peckhamia 121.1: 1-34.
• Otto J.C., Hill D.E. (2014). Spiders of the mungaich group from Western Australia (Araneae: Salticidae: Euophryinae: Maratus), with one new species from Cape Arid. Peckhamia 112.1: 1-35.ç
• Otto J.C., Hill D.E. (2011). An illustrated review of the known peacock spiders of the genus Maratus from Australia, with description of a new species (Araneae: Salticidae: Euophryinae).
• Salticidae, Jumping spiders.
• Two New Species of Peacock Spiders Discovered in Australia. Sci-knews.com.
• The extraordinary courtship dance of Australia’s peacock spider. The Guardian.

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