Size constrains oxygen delivery capacity within but not between bumble bee castes

Area of Science:

• Zoology
• Insect Physiology
• Comparative Physiology

Background:

• Bumble bees exhibit distinct worker and queen castes with significant size and life-history differences.
• Tracheal system volume scales hypermetrically in workers, but queens have smaller tracheal volumes relative to their size, potentially impacting oxygen delivery.
• Size and caste differences may influence hypoxia tolerance due to varying oxygen demands and delivery capacities.

Purpose of the Study:

• To directly assess size- and caste-related differences in hypoxia tolerance in bumble bees.
• To measure critical oxygen levels (Pcrit) during rest and flight for both queen and worker bumble bees.
• To investigate the relationship between body mass, metabolic rate, and oxygen delivery limitations during extreme conditions.

Main Methods:

• Measurement of critical PO2 (Pcrit) to determine hypoxia tolerance during rest and flight.
• Comparative analysis of Pcrit values between worker and queen bumble bees of varying sizes.
• Assessment of mass-specific metabolic rates during flight under low oxygen conditions.

Main Results:

• Queens and workers exhibited similar Pcrit values during both resting and flight conditions.
• During flight near Pcrit, mass-specific metabolic rates decreased significantly with increasing mass within and across castes.
• These findings indicate size-dependent limitations on oxygen delivery, particularly under high-demand, low-oxygen scenarios.

Conclusions:

• The smaller tracheal systems in queen bumble bees do not generally limit oxygen delivery, except under extreme physiological stress.
• Size, rather than caste, appears to be the primary factor influencing oxygen delivery limitations during strenuous activity in bumble bees.
• Queens can optimize body space for reproduction without compromising respiratory function under typical conditions.