Spring and autumn territoriality in song sparrows: same behavior, different mechanisms?

Area of Science:

• Behavioral endocrinology
• Neuroethology
• Animal behavior

Background:

• Vertebrates exhibit diverse social behaviors, including territoriality, with varying hormonal control mechanisms.
• Testosterone is implicated in territorial aggression in birds during breeding seasons, but its role in other periods is less clear.
• Seasonal differences in hormonal regulation of territoriality raise questions about the underlying mechanisms and their adaptive significance.

Purpose of the Study:

• To investigate the hormonal mechanisms regulating territorial behavior in song sparrows (Melospiza melodia morphna) during both breeding and non-breeding seasons.
• To explore why territoriality exhibits diverse control mechanisms across different seasons.
• To elucidate how hormonal messages for territorial behavior reach the brain differently throughout the year.

Main Methods:

• Field and laboratory experiments were conducted on male song sparrows.
• Plasma hormone levels (testosterone and estradiol) were measured across seasons.
• Experiments involved administering aromatase inhibitors and estradiol implants to assess their effects on territorial aggression and behavior.

Main Results:

• Song sparrows defend territories year-round, with non-breeding season aggression mirroring breeding season behavior.
• Plasma testosterone and estradiol levels were significantly lower in the non-breeding season.
• Aromatase inhibition reduced non-breeding aggression, while estradiol replacement restored it, indicating a role for estradiol.

Conclusions:

• The neural mechanisms for testosterone-regulated territorial behavior remain functional year-round in song sparrows.
• Seasonal variation in territoriality is mediated by differential hormonal signaling, specifically the shift from testosterone to estradiol dominance.
• These findings reveal distinct seasonal pathways for activating territorial behavior, avoiding the costs of elevated testosterone during non-breeding periods.