Silk weaving ants. That in and of itself is really neat. Then you see this picture of Polyrhachis shattuck...I mean, look at her! How many cool points can one animal rack up? A new study in Behavioral Ecology and Sociobiology takes a look at these arboreal nesting and silk-weaving ants.
Let's begin with sociality. It is one of those subjects in biology that is considered its own discipline. When you think of social animals you probably think of herds of mammals or maybe schools of fish. Sociality reaches its peak in eusociality, a surprisingly complex and truly social organization. These animals live in groups, cooperatively care for juveniles, divide labor, and overlap in generations. Studies of these social systems has shed light onto broader concepts of collective decision making, even leading to advances in our own technology (traffic flow, communications networks, internet searches, etc.). However, as much as we know about the social mechanics, we know very little about the evolution of such systems.
Most eusocial animals are found in the phylum Arthropoda, with the order Hymenoptera being the largest and most well-known, and all ants classified as eusocial species. And ants are the focus of today's chosen study. Ant larvae spin individual protective cocoons of silk and, depending on the species, that silk is either allocated to the colony or sequestered for the larvae’s individual needs. Donated silk is used by the worker caste to weave together leaves into nests. The “lowest grade” of arboreal (a.k.a. tree-dwelling) ants, Dendromyrmex, have larvae that produce silk without any interaction or provocation from the worker ants. In “intermediate grade” ants, Camponotus senex and Polyrhachis ?doddi (re-described as Polyrhachis robsoni (Kohout 2006)), the workers hold larvae at the work site and, with simple ritualized behaviors, the workers collect the silk. In the “highest grade” genus, Oecophylla, the larvae donate their silk supply to the colony. A worker ant will use highly ritualized behaviors - bring the larvae to the work site, straddle a leaf seam, use antennae to tap the head of the larva (telling it to extrudes silk from its salivary glands), use silk to glue together the seam, repeat.
A comparison of ant genera in this way, simple to complex, is thought to represent possible evolutionary steps in nest-weaving behavior. However, molecular sequence data suggests that nest-weaving has evolved independently in each of the four genera in which it occurs. This new study focuses on Polyrhachis ants. This genus offers good within-taxa comparison of multiple life strategies as different species vary in their nesting locations, from intertidal to subterranean to arboreal, the presence of nest construction, and even silk sources.
A total of 37 specimens of ants from all 13 currently recognized (*grr*, an often frustrating term in insect taxonomy) subspecies and five outgroup taxa were used for this study. The researchers isolated total genomic DNA and amplified and sequenced DNA from six fragments using specific primers for each gene region. After they collected the sequences, they analyzed and aligned them using computer programs. In their complicated analysis (they used Bayesian…that always makes my eyes cross) they input the gene level data along with variables of nesting preference (ground = soil, logs, stones vs. arboreal = twigs or leaves in trees) and nest construction (silk weaving vs. no silk vs. other silk). All of this allowed them to construct phylogenies (like an evolutionary family tree) and infer relationships among the species and ancestral states for behavior.
Their results showed robust phylogeny with strong support for the monophyly of the genus Polyrhachis, further supported by the inclusions of nesting preference and nest construction.This is good because it provides a nice, solid ancestral reconstruction for the evolution of the different species and their relationships to each other. It also allows for the comparison of the different nesting strategies within the framework of evolution. The investigation turned up some very interesting results. Simply, their results do not support the stepwise evolution of simple to complex. They found that the production of arboreal silk nests is the ancestral state with at least two transitions to subterranean nesting and the loss of silk weaving as species become more derived. There is also some flexibility and reversal in the behavior. Basically, the ants evolve, abandon and then re-evolve the nest weaving practices. The loss of silk nest weaving seems to occur with the transition from arboreal to terrestrial nesting followed by the re-evolution of silk nest weaving. This suggests a strong but flexible link between nesting preference and nest construction.
To illustrate this evolve-abandon-re-evolve point, the researchers present the example of Hedomyrma, a subgenus within a larger clade of subterranean nesters. This larger clade has already lost both arborality and nest weaving. But there are 2 species of Hedomyrma (Polyrhachis argentosa and Polyrhachis fervens) that have reverted to arborality. The re-evolution of this nesting preference has come with the modification of building nests within the hollow internodes of bamboo sans silk. Another reversal pattern is seen in a third species of Hedomyrma (Polyrhachis turneri), which has larvae that retain all of their silk for their own cocoon-constructing needs. Rather, worker ants steal silk from spiders to build nests on the sides of rocks. So the nest construction characteristic is what has re-evolved, just with a different mechanism. Larval cocoons have been lost in 2 of the arboreal nest-weaving species studied, and the allocation of larval silk to colony rather than individual need is considered a more derived but decoupled characteristic of nest construction.
I think that both the flexibility and the rapid evolution (or re-evolution) of this system is what attracted me to this paper. We know that evolution is a complex concept that we often boil down to from-simple-to-complex, and in many cases it is exactly that. This study almost reads like a sequel, a what-happens-next sort of thing.
Robson, S., Kohout, R., Beckenbach, A., & Moreau, C. (2015). Evolutionary transitions of complex labile traits: Silk weaving and arboreal nesting in Polyrhachis ants Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-014-1857-x
(image of Polyrhachis shattuck, Maliau Basin, Sabah via AntWiki via California Academy of Science Ant Course)