Some ant species start hunting for food as soon as they reach adulthood. Researchers at Boston University found that Dracula ants do not follow typical age-related roles. Frank Azorsa said, "In social insects, adult workers typically change social roles as they age." These ants show unique brain development patterns instead.
Some insects, including honeybees and ants, live in highly organized colonies where individuals take on specific roles. In many ant societies, young adult workers typically care for developing young inside the nest before beginning to forage, or search for food, outside it. Some ant species, however, start hunting for food as soon as they reach adulthood, without undergoing a transition period.
One example is Stigmatomma pallipes, a species belonging to a group commonly called Dracula ants because adults can pierce the skin of their developing young and drink a bloodlike fluid called hemolymph. The age-independent behavior of these ants was first reported in a Science paper published in 1978 by Dr. James Traniello, who is now a professor of biology at Boston University.
Researchers at Boston University recently tried to better understand how the brain development of S. pallipes workers contributes to this early readiness for work. Their findings, published in Proceedings of the Royal Society B, suggest that regions in the brains of S. pallipes reach their largest size during the pupal stage, when immature ants transition into adulthood.
"In social insects, adult workers typically change social roles as they age—progressing from nursing tasks to foraging—a transition accompanied by changes in neuroanatomy," Frank Azorsa, first author of the paper, told Phys.org. "Young honeybee workers, for example, nurse the brood and transition to foraging as they age. However, some species exhibit precocial behavior."
This precocial behavior (i.e., the ability to perform foraging tasks soon after emerging as an adult) is reported in at least six species of ants. One of these ants (S. pallipes) exhibits social and ecological traits thought to resemble those of early ants.
"The larvae actively feed," said Azorsa. "Secondly, workers sting and paralyze centipedes, which are then brought to the nest to directly provision larvae. Foraging and nursing tasks are thus coupled in this ant. Workers also feed on larval hemolymph (hence the name 'Dracula ant'). This species has small subterranean colonies of fewer than 20 workers." Finally, this species does not show temporal polyethism, indicating the absence of age-related task specialization.
Exploring the brain development of S. pallipes
Azorsa and colleagues wanted to better understand how the brain organization of S. pallipes changed during development. They also investigated whether the time different ant species take to develop was associated with workers emerging as adults either less ready to perform tasks such as hunting and foraging (altricial) or already capable of performing them (precocial).
"We collected ants in forested areas within several miles of Boston, Massachusetts (S. pallipes is widely distributed in the eastern United States)," explained Azorsa. "After dissecting their brains, we stained them for synapsin—a protein found in synapses—which allowed us to visualize the internal structure of brain tissue."
The researchers captured detailed, three-dimensional (3D) images of the ants' brains using confocal microscopy, a technique that uses a laser to image thin slices of tissue at different depths. They then analyzed these images using a software tool called AMIRA to measure the volumes of different brain regions and the density of clusters of connections between nerve cells.
"Our sampling included workers at four different stages: pupae (stage 1 and stage 2), young and mature workers," said Azorsa. "Workers in pupal stage 1 were white, while workers in pupal stage 2 were light brown, gray-brown and brown-black, the final pigmentation class approaching eclosion.
"Young workers differ from older workers only in slightly lighter pigmentation of their legs and gaster (the ant's 'abdomen'). Behaviorally, they are equivalent to mature workers and perform all tasks."
The brain development of S. pallipes was found to differ from that of altricial ant species. Specifically, the workers' brains as a whole and most of their specialized brain regions reached their maximum size during the pupal stage, before the ants emerged as adults.
Although brain size stayed relatively stable during adulthood, the density of tiny clusters of connections between nerve cells differed between young and mature workers. This indicates that brain connections continued to reorganize themselves after adulthood began.
"Synaptic remodeling (reduced neural connection over time) in the mushroom body (the brain compartment associated with higher-order processing) during adult development appears to be conserved across species," explained Azorsa.
"Age-related brain organization in S. pallipes follows a different pattern than those found in derived ant species that display age-related division of labor. Finally, we found a correlation between development time and behavioral development. S. pallipes, as an example of a precocial species, exhibits long development time, precocial behavior and precocial brain development."
Understanding ant brain development and age-based division of labor
The results of this study offer some clues about why some ant species, including S. pallipes, exhibit precocial brain development. They suggest that more brain growth occurs before adulthood in S. pallipes than in ant species whose workers become ready to forage later.
"Human infants are famously altricial, requiring extensive care during a long period of behavioral and neurobiological maturation," said Azorsa.
"Other vertebrates have precocial young that do not need care. As in vertebrates, ants with altricial or precocial behavior show distinct brain development paths. In addition, longer development time enables ant workers to eclose (emerge from pupal case) with a fully mature brain to support complex tasks."
This team's recent efforts could inspire further studies exploring the brain development of different species exhibiting altricial and precocial behaviors. Azorsa is now collaborating with Dr. Joel Vizueta at the University of Copenhagen to identify candidate genes that causally regulate caste-specific brain development and produce differences in brain structure between ant castes, such as workers and queens.
"We will apply comparative macroscopic, cellular and genetic approaches to explore the neurobiological underpinnings of the evolution of division of labor in ants," added Azorsa.
"Specifically, we will test if the evolution of elaborated social traits and division of labor are associated with divergent trajectories in brain development that involve coordinated shifts in brain compartment investment, neural cell-type composition and cell-type-specific gene regulation.
"Ultimately, this project will establish the mechanistic foundation for future research into the neurobiological basis of age-related division of labor in social insects."