Tuesday, September 22, 2009

Sexual Selection and Hyla gratiosa: Barking Fast and Barking Long

In regards to sexual reproduction, the selection of potential mates can generally be thought of as functioning along one of four lines; through identifying Good Genes, receiving Direct Benefit, via Sensory Bias and by Fisherian Runaway. However, even though most species have adapted to one of the core strategies listed above, the methods of selection themselves are far from being restrictive. In fact, these strategies of mate choice may operate independently, collectively or in conjunction with other aspects of local ecology and Natural Selection – they’re not by any means exclusive. In a word, the processes under which one chooses a mate can be rather “complex”.

In considering the complexity of this dynamic, evolution and sexual selection have, on the average, placed the female gender in a position to choose between potential male reproductive partners; when it comes to sex, the females are the deciders.


Well, this is so for a few different reasons. One of which is that females are generally the limiting gender in a population. This means that they ultimately control population numbers and take on the added burdens of producing ova and caring for young – both of which can be taxing to available resources. In addition, females also tend to be fewer in number within a given population, often outnumbered by the males.

So, if the power of mate choice belongs to the female, on what grounds is her selection to be made and what criteria are weighed and measured prior to committing to such a costly reproductive venture?

Certainly, any mate is better than no mate at all, but when the opportunity presents itself wouldn’t it be beneficial to capitalize on the availability of the most virile, successful or healthy male – how to choose?

For Hyla gratiosa, the “barking treefrog,” the female’s choice of mate seems to center on the duration and frequency of the boy’s bark.

I took this H. gratiosa snapshot two weeks ago near Wildwood, Fl

Hyla gratiosa is a rather robust looking treefrog that grows to be somewhere between five and seven centimeters as an adult. Found in the southeastern United States, these guys are easily recognized by their spots, which cover large portions of their back and legs.

Back View of above Specimen

Although generally spending the majority of their time high in the trees, they have been known to venture to the ground during the summer months in efforts to cool off in the shade of herbaceous ground plants. Mating season pushes the males to water where they call, or “bark,” for females. The male’s “bark” during mating season (March through August) is loud and explosive. Often repeated in intervals of a second or two, the duration and frequency of their call has the ability to communicate health and virility to the inquiring female.

You can Listen to a Sample “Bark” from the Smithsonian HERE (at Bottom of Page).

In testing how the female places value on the calls of the male, a couple of researchers from James Madison University recorded and played back the calls to females in order to judge the their response to the stimulus. Basically, they went out and caught females (actively engaged in amplexus), took them to a prepped area, placed them behind blinds, and then played them calls of varying structure. Once the calls had been played, the blind was lifted from the female frog and the researchers watched as she either moved to engage her would be suitor – a sound speaker, or took a non-responsive action.

Going into the experiment, the researchers developed four hypotheses to describe possible ways in which the female may assess the calls:

(1) Single-trait Hypothesis. Females may perceive multiple cues as a single trait, even though researchers may identify them as separate traits.

(2) Amplifier Hypothesis. A trait may not be directly assessed by females but may amplify another trait that is assessed directly, thereby allowing females to better discern differences between males in the assessed trait.

(3) Hierarchical Hypothesis. Females may assess traits in a hierarchical fashion, basing their choice on the higher level trait whenever this trait differs between males and using lower-level traits only when higher-level traits are difficult to discern between males.

(4) Simultaneous Hypothesis. Females may base their choice of mates on multiple traits, combining preferences for individual traits. Preferences for traits may be combined additively.

Following the experiment, the researchers concluded that the females consider both the rate and duration of the call independently, but they show an overall leaning towards high call rates over those of an extended duration. However, this preference only stands true with in certain ranges of acceptability, if the frequency of the male’s call rate falls below a certain threshold, the female will additively consider the duration as well.

Hypothesis 4, the Simultaneous Hypothesis, seems to best describe the female barking frog’s call selecting criteria.

BURKE, E., & MURPHY, C. (2007). How female barking treefrogs, Hyla gratiosa, use multiple call characteristics to select a mate Animal Behaviour, 74 (5), 1463-1472 DOI: 10.1016/j.anbehav.2007.02.017

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