Desiccation limits recruitment in the pleometrotic desert seed-harvester ant Veromessor pergandei

The desert harvester ant Veromessor pergandei displays geographic
variation in colony founding with queens initiating nests singly
(haplometrosis) or in groups (pleometrosis).  The transition from
haplo- to pleometrotic founding is associated with lower
rainfall.  Numerous hypotheses have been proposed to explain the
evolution of cooperative founding in this species, but the ultimate
explanation remains unanswered.  In laboratory experiments, water
level was positively associated with survival, condition, and brood
production by single queens.  Queen survival also was positively
influenced by water level and queen number in a two-factor
experiment.  Water level also was a significant effect for three
measures of queen condition, but queen number was not significant for any
measure.  Foundress queens excavated after two weeks of
desiccating conditions were dehydrated compared to alate queens captured
from their natal colony, indicating that desiccation can be a source of
queen mortality.  Long-term monitoring in central Arizona, USA,
documented that recruitment only occurred in 4 of 20 years.  A
discriminant analysis using rainfall as a predictor of recruitment
correctly predicted recruitment in 17 of 20 years for total rainfall from
January–June (the period for mating flights and establishment) and in 19
of 20 years for early plus late rainfall (January–March and April–June,
respectively), often with a posterior probability > 0.90. 
Moreover, recruitment occurred only in years in which both early and late
rainfall exceeded the long-term mean.  This result also was
supported by the discriminant analysis predicting no recruitment when
long-term mean early and late rainfall were included as ungrouped
periods.  These data suggest that pleometrosis in V. pergandei
evolved to enhance colony survival in areas with harsh abiotic
(desiccating) conditions, facilitating colonization of habitats in which
solitary queens could not establish even in wet years.  This
favorable-year hypothesis supports enhanced worker production as the
primary advantage of pleometrosis.