Data from: Sperm production characteristics vary with level of sperm competition in Cataglyphis desert ants

Under polyandry, males are selected to produce more competitive
ejaculates. Theoretical models have explored how the mechanism of sperm
competition drives males to partition investment within an ejaculate
between sperm quantity and quality. The raffle-based competition model
predicts that increased level of sperm competition selects for larger
numbers of sperm in ejaculates. Sperm competition is also thought to
promote the evolution of longer sperm, because longer sperm could be
faster. In eusocial Hymenoptera, the mating system imposes unique
selective pressures on male ejaculates. Males are short lived; they reach
adulthood with a finite amount of spermatozoa, and they mate typically
with a single or a few females and die. The actual number of spermatozoa
stored in their accessory testes at emergence is thus a reliable measure
of total investment into sperm production. In a comparative study of 15
species of Cataglyphis desert ants, we used phylogenetically controlled
analyses to investigate relationships between levels of sperm competition,
sperm production and sperm length. We measured sperm production by
quantifying the number of spermatozoa present in testes, instead of using
a proxy measure such as size of testes. Multiple queen mating is the
ancestral state in the genus but reduction in mating frequency evolved
secondarily and independently in some clades, providing a unique
opportunity to examine how reduction from multiple to single mating
influences sperm traits. Our results provide phylogenetically robust
evidence that species experiencing greater levels of sperm competition
produce more sperm. After controlling for male size, investment in sperm
production decreases significantly according to the sequence obligatory
multiple queen mating > multiple–single queen mating >
single–double queen mating. Furthermore, the number of spermatozoa
produced per male decreases significantly with reduction in paternity
frequency for each species. In contrast, neither sperm length nor male
size was significantly associated with the mating system classes or the
number of patrilines. Our measures of sperm number provide the first
direct evidence that sperm production covaries with the level of sperm
competition in a eusocial insect. Given the reversal from multiple to
single mating in Cataglyphis, our comparative analysis also shows
convincingly that reduction in sperm competition influences sperm traits.