A slide from my lecture tomorrow morning on social insects:
Years ago I held the opinion that social insects were too alien, their concerns too remote from our own, to enlighten us about the nature of our human societies. I’ve since come around to a more nuanced perspective.
It is certainly still true that ants’ optimal solutions to their various social conflicts differ from our optimal solutions. Just because ants kill the offspring of cheating workers does not mean we ought practice infanticide, for example.
Yet our society expresses common underlying dynamics to those of insects. We face the same questions of how to balance individual autonomy with sacrifice for the advancement of the group. Our answers cannot be similar to those of the ants, but by studying how ants work through their conflicts we may at least learn something of the topology of the problem.
Giant ants of the South American genus Dinoponera are unusual in lacking a separate queen caste. Instead, colonies comprise outwardly identical workers, a subset of which mate and lay eggs. Are the ants inwardly identical as well? Not at all, according to a new study by Chris R. Smith et al in PLoS ONE. Foraging ants are lean, with low fat reserves, while workers and reproductives deep in the nest have ample body fat:
Eusocial species exhibit pronounced division of labor, most notably between reproductive and non-reproductive castes, but also within non-reproductive castes via morphological specialization and temporal polyethism. For species with distinct worker and queen castes, age-related differences in behavior among workers (e.g. within-nest tasks versus foraging) appear to result from physiological changes such as decreased lipid content. However, we know little about how labor is divided among individuals in species that lack a distinct queen caste. In this study, we investigated how fat storage varied among individuals in a species of ant (Dinoponera australis) that lacks a distinct queen caste and in which all individuals are morphologically similar and capable of reproduction (totipotent at birth). We distinguish between two hypotheses, 1) all individuals are physiologically similar, consistent with the possibility that any non-reproductive may eventually become reproductive, and 2) non-reproductive individuals vary in stored fat, similar to highly eusocial species, where depletion is associated with foraging and non-reproductives have lower lipid stores than reproducing individuals. Our data support the latter hypothesis. Location in the nest, the probability of foraging, and foraging effort, were all associated with decreased fat storage.
source: Smith CR, Suarez AV, Tsutsui ND, Wittman SE, Edmonds B, et al. (2011) Nutritional Asymmetries Are Related to Division of Labor in a Queenless Ant. PLoS ONE 6(8): e24011. doi:10.1371/journal.pone.0024011
Who's that odd ant out?
While in sunny Florida last summer (ah, sunshine! I vaguely remember what that looks like), I spent an hour peering into a nest of little Dorymyrmex elegans. These slender, graceful ants are among Florida’s more charming insects.
Every few minutes, though, the flow of elegant orange insects out of the nest was interrupted by a darker, more robust ant: Dorymyrmex reginicula. Who was this interloper?
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Zootermopsis soldier termite, jaws at the ready.
If you think of termites as pasty white squishy things, here’s one that’ll jar your preconceptions. Zootermopsis dampwood termites of western North America have large soldiers- over a centimeter long- that are muscular and well armored.
Soldiers are deployed not against predators but against other termites, as colonies within a single rotting log fight when they encounter each other. Those jaws are ideal for slicing through an enemy queen, for example, or for protecting their own.
Photo details: Canon mp-e 65mm 1-5x macro lens on a Canon EOS D60
ISO 100, f13, 1/200 sec, diffused flash
Figure 1. Relationship between normalized metabolic rate and body mass for unitary organisms and whole colonies (from Hou et al 2010)
The notion that insect colonies and their constituent individuals are analogous to multicellular organisms and their constituent cells has been a controversial idea for decades. Is it useful, for example, to think of an ant colony as a single individual? Do superorganisms really exist as coherent entities? Or do insect colonies function more as aggregations of individuals?
Last week, PNAS published the first application of empirical methods to test the superorganism concept. This is a significant paper. The researchers, led by Chen Hou, asked whether the set of relationships between mass, energy, and reproduction that govern multicellular organisms show the same patterns when measured across whole insect colonies.
The answer, in most cases, was a resounding Yes: social insect colonies grow and breathe just like regular organisms.
source: Hou, C., Kaspari, M., Vander Zanden, H. B., Gillooly, J. F. 2010. Energetic basic of colonial living in social insects. PNAS early edition.
Let me preface this post by saying that Christian Peeters is one of my absolute favorite myrmecologists. If lost in a remote African jungle and stalked by ravenous leopards, for example, Christian is the first ant guy I’d pick to help get me out of the predicament.
Having said that, this paper in Insectes Sociaux is so bad I nearly gouged my eyes out and ran around in little circles screaming and flailing my arms.
Nonetheless there exist extant ants with relatively simple societies, where size-polymorphic workers and large queens are absent. Recent phylogenies show that the poneroid subfamilies Amblyoponinae and Ponerinae are basal (e.g. Brady et al., 2006), i.e. closer to solitary vespoid wasps.
Ten points to the first person who can explain what’s wrong with it.
Trophallaxis- the social sharing of regurgitated liquids- is a fundamental behavior in the biology of most ant colonies. One ant approaches another, asks for a droplet of food, and if her partner is willing the two spend anywhere from a few seconds to several minutes in what is best described as a myrmecological french kiss. The behavior is so central to the life of ants that the insects have an entire stomach, separate from their digestive gut, devoted as a reservoir for social sharing.
Although the act involves a transfer of food, it would be a mistake to think of the behavior as primarily a nutrient-dispersal mechanism. Ants do it far more frequently than nutrition requires. Trophallaxis also transfers chemical signals among nestmates, regulating a singular colony odor and sharing information about the needs of the colony. Think of it as the colonies’ own internet.
Over the weekend I set out to rectify my rather embarrassing lack of decent trophallaxis photographs. We have a laboratory colony of Formica obscuripes that are ideal subjects, as these large ants are not only charismatic but engage in trophallaxis with what seems to be a nearly pathological frequency. Here are a few of the better shots.
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Crematogaster lineolata queen with a retinue of workers. (Vermillion River Observatory, Illinois)
This weekend we took a trip with some entomology students to the Vermillion River Observatory. The astronomical function of the observatory has long been abandoned, but the site remains as a lovely nature reserve and one of the closest patches of decent forest habitat to where we live in Champaign-Urbana.
The acrobat ant Crematogaster lineolata was one of many ants we encountered, and in this nest the queen was right up near the surface. She lingered long enough for me to get a few shots before she disappeared into the labyrinth of tunnels.
photo details: Canon mp-e 65mm 1-5x macro lens on a Canon EOS 50D
ISO 100, f/13, 1/250 sec, flash diffused through tracing paper
Achenbach, A., Foitzik, S. 2009. FIRST EVIDENCE FOR SLAVE REBELLION: ENSLAVED ANT WORKERS SYSTEMATICALLY KILL THE BROOD OF THEIR SOCIAL PARASITE PROTOMOGNATHUS AMERICANUS . Evolution, Online Early, doi: 10.1111/j.1558-5646.2009.00591.x
Abstract: During the process of coevolution, social parasites have evolved sophisticated strategies to exploit the brood care behavior of their social hosts. Slave-making ant queens invade host colonies and kill or eject all adult host ants. Host workers, which eclose from the remaining brood, are tricked into caring for the parasite brood. Due to their high prevalence and frequent raids, following which stolen host broods are similarly enslaved, slave-making ants exert substantial selection upon their hosts, leading to the evolution of antiparasite adaptations. However, all host defenses shown to date are active before host workers are parasitized, whereas selection was thought to be unable to act on traits of already enslaved hosts. Yet, here we demonstrate the rebellion of enslaved Temnothorax workers, which kill two-thirds of the female pupae of the slave-making ant Protomognathus americanus. Thereby, slaves decrease the long-term parasite impact on surrounding related host colonies. This novel antiparasite strategy of enslaved workers constitutes a new level in the coevolutionary battle after host colony defense has failed. Our discovery is analogous to recent findings in hosts of avian brood parasites where perfect mimicry of parasite eggs leads to the evolution of chick recognition as a second line of defense.
Those of you who were into ants in the early ’90s might remember SimAnt, a simulation game where you control the decisions your ants make to steer a colony to dominance over a competing species in a suburban lawn.
The game is based, in part, on the optimality equations summarized in Oster & Wilson’s 1978 text “Caste and Ecology in the Social Insects“. The book lays out mathematical foundations for determining the investments a colony should place in workers, queens, and males in order to optimize Darwinian fitness over a range of ecological conditions. If you knew the equations, SimAnt quickly becomes boring as you’d win every time. (warning: game spoiler below) Continue reading →