The Power Spectrum and Hearing Threshold

In the context of this grasshopper species:

• Power Spectrum (Solid Line): Represents the distribution of energy across different frequencies in the male’s call. The peak indicates the species-specific frequency, where most of the call’s energy lies.

• Hearing Threshold (Dashed Line): Indicates the minimum sound intensity a female can detect at each frequency. The lower the threshold, the greater the sensitivity.

For effective mate recognition and reproductive isolation, the female’s hearing threshold should be lowest (most sensitive) at the species-specific frequency—the same frequency where the male’s call is strongest. At other frequencies, the threshold should be higher (less sensitive), so that even loud calls from males of other species (which peak at different frequencies) are less likely to be detected or responded to by females.

Why This Tuning Matters

• Selective Mate Recognition: Females are most sensitive to the frequency where conspecific (same species) males call, allowing them to find mates efficiently—even if the calls are soft.

• Reproductive Isolation: By ignoring calls at other frequencies, females avoid mating with males of different species, maintaining genetic integrity.

• Energy Efficiency: Males don’t need to call as loudly to attract females of their own species, reducing energy expenditure and predation risk.

What Should the Correct Figure Show?

The correct figure should display:

• A solid line (power spectrum) with a clear peak at the species-specific frequency.

• A dashed line (hearing threshold) with its lowest point (highest sensitivity) aligned exactly at the same frequency as the power spectrum peak.

• At frequencies away from the peak, the dashed line should rise (indicating reduced sensitivity).

This alignment ensures that females are “tuned” to the frequency of conspecific males, while being less responsive to calls at other frequencies.

Visualizing the Relationship

Imagine the following:

• X-axis: Frequency (Hz or kHz)

• Y-axis: Power (for the solid line) and Hearing Threshold (for the dashed line)

• Solid line: Peaks at the species-specific frequency

• Dashed line: Dips (lowest point) at the same frequency as the solid line’s peak, rising on either side

This pattern demonstrates frequency matching between male call production and female auditory sensitivity—a classic example of co-evolution in animal communication.

Conclusion

The correct option is the figure where the female hearing threshold (dashed line) is lowest at the species-specific frequency where the male call’s power spectrum (solid line) is highest. This precise tuning allows females to detect and respond only to the calls of their own species, ensuring effective mate selection and reproductive isolation.

Summary:

• Female grasshoppers are most sensitive to the frequency where male calls are strongest.

• This frequency tuning supports reproductive isolation and species integrity.

• The correct figure shows the dashed line dipping at the same frequency as the solid line’s peak.

Choose the figure where the dashed line (hearing threshold) is lowest at the peak of the solid line (power spectrum).