December 06, 2010
Sex, Frogs, and Rock & Roll
Acoustic signals are not the only way we humans communicate, we also rely on visual signals. The way we dress and how we carry ourselves play a big part in the mating game. Likewise diurnal frogs draw attention to themselves with an array of visual signals; from toe-trembling, arm waving, limp-shaking, to name a few. Think about Elvis.
However, most frogs prefer to be out at night, in the cover of darkness, as do juvenile homo sapiens out on the hunt. For frogs at night visual cues may not do the trick. For nocturnal frogs, acoustic communication is therefore fundamental for mate-recognition and the reproduction process.
Notwithstanding the occasional Madonna or Lady Gaga, as talented as so many of them are, and as hard as they try, human musicians who happen to be female have a harder time attracting an audience than male musicians do. Beautiful, classically trained women with angelic voices, who can crank out tunes that rival songs by John & Paul, usually cannot compete with demonic looking males who may only know two or three guitar cords, cannot sing in tune if their life depended on it, whose songs are rather uninspiring, and louder than they ought to be; such is the reality of the music industry. This may not be just a socially instituted bias against female musicians, it might be a biological convention dating back eons.
The best way for males (frog or human) to get the attention of females is by standing out in a crowd. For frogs the more effort a male puts into calling, and the louder he calls, the more effective the calls will be. Consequently the volume of noise put out by these little guys is amazing.
A successful rock star has a following of groupies; some of them may be lucky enough to get backstage passes. Other fans just come to the concert without getting close to the star at all. An entire hierarchy surrounds the concert scene.
Back at the frog pond, status is equally important. During the breeding season, many different frogs from many different species may be calling at the same time, and when large groups assemble competition is tough. Each species has its own unique mating call, but when many species chorus at the same time, it sounds as if several orchestras are playing different symphonies at once. Females single out the calls of their own species from the cacophony and then choose a suitor.
For frogs at the pond, male calls may be interfered with by competing members of the same species or by the calls of different species; the larger the chorus the greater the acoustic interference. This poses a problem for females. They can't possibly check out all the males in a large chorus, and females that move around investigating different suitors are more likely to run into predators. Most females therefore prefer to choose a mate from a relatively small sample group.
A female frog usually tracks the call that is the most noticeable—whether it is the loudest, the longest, or the most complex—so males need to make their calls as district as possible. Often, the call of a male is fairly distinct until a rival comes along and ruins everything. When interrupted by the call of a competitor, a male may change his own call by changing or adding notes, or by increasing the intervals between notes. A tree frog becomes attentive when a competitor arrives on the scene, and may ignore all interruptions except those of the nearest male. Since it is not feasible for one frog to outdo the entire chorus, a male simply distinguishes himself from his nearest or loudest competitor.
Occasionally a male strikes out on his own, stagging a solo performance, but this can be a chancy business. Unless he is prudent, this could take too much energy and allow predators to zero in. Therefore, compared to males in a chorus, an isolated male is often more conservative with his energy. The calls of an isolated male are shorter, which makes him less attractive to females.
Males that call more loudly and more frequently than their competitors should be able to attract more females, but life is not always so simple. Dense vegetation may prevent the sound of a frog's calls from traveling very far, so the quality of his perching site is significant.
Breeding success for the North American gray tree frog (Hyla versicolor) seems to depend more on good perching sties more than any other variable. Ideal perches are horizontal and relatively free of vegetation, so that sound can travel far and undisturbed. When the call is made on a horizontal perch it spreads uniformly and is less likely to be reflected—a primitive but effective sound stage.
Male frogs claim their territory by sound—the farther the call can travel, the larger the territory. A male with a good perch will have fewer male rivals, and when a female approaches she will be less likely to lose auditory contact.
A male must compete for good calling locations either by vocalization or by fighting. Tree frogs presumably defend their territories with aggressive calls, but in some species these calls are not appealing to females. Thus if a male bickers too long or too often with other males he may forfeit his chance to mate for the night. Sound familiar? Male golden palm tree frogs (Hyla ebraccata) of Central America manage not to alienate females by finishing their aggressive calls early in the evening, establishing their territories before the females arrive. The later evening is reserved for courting calls. Eleutherodactylus coqui have solved this problem in a different way. They devised a combination call, which has the duel function of attracting females and deterring males. Since the call has notes for both purposes, the females don't seem repelled.
A common strategy of subordinate males is to lie low near the perch of a dominant rock star, rather than relocating. When the dominate male is busy mating, his perching spot is temporarily unguarded, and a subordinate male may snatch the post. If the subordinate is lucky he may attract a female before the dominate male is finished mating. Another tactic is to intercept a female on her way to the dominate male. This may or may not work, females have been known to reject such overtures.
My favorite rock stars among the anurans are the North American bull frogs (Rana catesbeiana). Bullfrogs are loners, but during the spring, these anurans can congregate in large numbers. Males arrive first before the females. The mating season is quite long because not all females are receptive at the same time. Some will be ready to mate in May others not until July. In addition, females may be gravid for only a day; thus in the pond males outnumber females.
The absolute best location for a male is at the center of the chorus; usually the largest and oldest males are found there. Young males are pushed out to the edges of the chorus or chased away altogether, but if they assume a subordinate posture, they are allowed to remain in the area.
A chorus usually stays at the same location for several nights but eventually dissolves, only to reform again at another location. Choruses form when dominate males call and are joined by other males. During the first few nights activity intensifies and peaks. After several successful males have departed and the chorus has lost it lead vocalists, it tends to dissipate. Males don't know where or when a female will be receptive so by frequently changing location they are more likely to encounter one. When a dominate male leaves for a different location other bull frogs waste no time and quickly follow him.
Females remain inconspicuous as they move from one male to another and from one chorus to another. They submerge under the water until only their eyes show. Hardly causing a ripple, they make their rounds swimming through the crowd. Females enter into the males territory, briefly emerge to check out the crowd, and then submerge again to continue the search. Females may actively move around the pond for several days and the males pay little attention to them. When a female has her mind made up she touches the male of her choice. As soon as she gives this clue, the male follows her. The pair leaves the chorus to mate, and once this task is complete the female departs.
As a young man I often spent as much time observing the behavior of the people who attended concerts and parties as I did listening to the music, and when I got older and obsessed with nature, in particular the nature one finds around wetlands, I couldn't help but notice the similarities. So the next time you venture past a crowd of teenagers at a concert or party, observe their posturing and consider how old the game is. The biological imperative is as ancient as life itself, and some aspects of its expression haven't changed much.
Posted by Wayne Ferrier at 12:08 AM | Permalink