Variation in size and sperm content of sexuals in the leafcutter ant Atta colombica

Abstract

We studied a population of the Panamanian leafcutter ant Atta colombica and found significant between-colony variation for several male and gyne size parameters. Male sperm complement size was highly variable between individuals and was negatively correlated with head width and thorax weight, the latter itself positively correlated with wing length. This suggests a negative covariance between two likely fitness traits of males: flight ability and insemination potential. Young queens on average stored only slightly more than one male's worth of sperm. We discuss our results in the context of reproductive strategies and mating systems.

Multiple mating increases the sperm stores of Atta colombica leafcutter ant queens

“…Our earlier work has shown that queens of this species mate with a high and variable number of males (average 3, range 1±5) (Fjerdingstad et al 1998) and store very large numbers of sperm (previously reported range: 57±255 million) (Fjerdingstad and Boomsma 1997). The variation among queens in the numbers of sperm stored is substantial (coecient of variation, CV 0.42) (Fjerdingstad and Boomsma 1997) and much larger than in an obligatorily single mating ant species, Solenopsis invicta: (CV 0.08) (Glancey and Lofgren, 1985).…”

“…This reduces worker-queen ®tness con¯ict over the sex ratio (Moritz 1985), and may enhance the ®tness of the queen (Ratnieks and Boomsma 1995;SundstroÈ m and Ratnieks, in press). So far very few empirical tests of these hypotheses on multiple mating have been carried out (only Pamilo et al 1994 andFjerdingstad et al 1997) and no support for them has been reported.…”

“…The hypothesis was criticised by Crozier and Page (1985) who argued that males should be selected to have enough sperm to satisfy the sperm requirements of a queen. However, this criticism cannot a priori invalidate the``mating-for-sperm'' hypothesis, since male sperm content might be constrained by selection on other components of male ®tness (Fjerdingstad and Boomsma 1997), and since males may parcel out their sperm between dierent queens (Boomsma 1996;Reichardt and Wheeler 1996;Boomsma and SundstroÈ m 1998). Across ant species with single-queen colonies, the number of mates per queen is positively correlated with the species-speci®c size of mature colonies (number of workers) (Cole 1983;Boomsma and Ratnieks 1996), as would be expected under the``mating-for-sperm'' hypothesis.…”

The ejaculatory biology of leafcutter ants

“…A first explanation could be that ejaculate sizes are variable and cannot be equalized by the queen in the short time frame between insemination and storage. Indeed, very large differences in sperm complement size have been found in A. colombica, both at the colony level and across individual males (Fjerdingstad and Boomsma, 1997;Stürup et al, 2011). Second, ejaculates could achieve different competitive success in being stored, depending on some intrinsic measure of quality (potentially influenced by the queen's reproductive environment).…”

“…The clear sequence of AG secretion first, followed by sperm, AT secretion and a small mating plug in A. colombica, suggests that the first male to mate with a queen might have an advantage over subsequent males as his sperm will not meet rival AG secretion already present in the spermatheca and his mating plug might delay the arrival of such harmful fluids when the queen mates a second time. In addition, being the first male to mate with a queen might be advantageous as a spermatheca has the capacity to store only a fraction more than what a single ejaculate can provide (Fjerdingstad and Boomsma, 1997;Stürup et al, 2011). inferred that a first male to mate with the queen stores on average twice the number of sperm than any subsequent male.…”