Dracula ant workers reach peak brain size while still pupae

In many ant species a new adult's first job is indoors: it tends the brood before it ever forages outside ^[4]. Frank Azorsa, the paper's first author, told Phys.org that in social insects this progression is "a transition accompanied by changes in neuroanatomy" ^[18][17]. Stigmatomma pallipes workers do both jobs at once. They sting and paralyze centipedes and carry them back to the nest to feed the larvae directly ^[7]. "Foraging and nursing tasks are thus coupled in this ant," Azorsa said ^[8]. Colonies live underground and hold fewer than 20 workers ^[9].

The young workers are the comparison that matters most in this design. The only visible difference from older workers is slightly paler legs and gaster ^[19]. "Behaviorally, they are equivalent to mature workers and perform all tasks," Azorsa said ^[13]. If brain regions were still growing in that group, growth would overlap with the start of hunting, and the pupal stage would explain little. The measurements instead put the peak before emergence for the whole brain and most specialized regions, with size relatively stable across adulthood ^[2][14].

The volumes came from three-dimensional confocal images, which a laser builds up from thin slices at different depths, and were measured in AMIRA software ^[11]. The software also measured the density of clusters of connections between nerve cells ^[11]. That second measure differed between young and mature workers even though volume held steady. The researchers take this as a sign that connections keep reorganizing after adulthood begins ^[14].

The thing this doesn't tell you is the size of the effect. The Phys.org account does not give the number of brains imaged, the volume differences between stages, or the answer to the team's second question: whether the time a species takes to develop predicts how ready its workers are when they emerge ^[15]. The researchers report that S. pallipes brain development differs from that of altricial species, whose workers emerge less ready to hunt and forage ^[16][15].

We think the timing result is a sound description of when this ant's brain grows. It is also a plausible account of day-one hunting in a species where hunting is how the brood gets fed. S. pallipes also has social and ecological traits thought to resemble those of early ants ^[6]. That makes it a reasonable species to start with in asking whether a brain that peaks in the pupa is the older pattern in ants.