Recognition in a Social Symbiosis: Chemical Phenotypes and Nestmate Recognition Behaviors of Neotropical Parabiotic Ants

<div><p>Social organisms rank among the most abundant and ecologically dominant species on Earth, in part due to exclusive recognition systems that allow cooperators to be distinguished from exploiters. Exploiters, such as social parasites, manipulate their hosts’ recognition systems, whereas cooperators are expected to minimize interference with their partner’s recognition abilities. Despite our wealth of knowledge about recognition in single-species social nests, less is known of the recognition systems in multi-species nests, particularly involving cooperators. One uncommon type of nesting symbiosis, called <i>parabiosis</i>, involves two species of ants sharing a nest and foraging trails in ostensible cooperation. Here, we investigated recognition cues (cuticular hydrocarbons) and recognition behaviors in the parabiotic mixed-species ant nests of <i>Camponotus femoratus</i> and <i>Crematogaster levior</i> in North-Eastern Amazonia. We found two sympatric, cryptic <i>Cr. levior</i> chemotypes in the population, with one type in each parabiotic colony. Although they share a nest, very few hydrocarbons were shared between <i>Ca. femoratus</i> and either <i>Cr. levior</i> chemotype. The <i>Ca. femoratus</i> hydrocarbons were also unusually long–chained branched alkenes and dienes, compounds not commonly found amongst ants. Despite minimal overlap in hydrocarbon profile, there was evidence of potential interspecific nestmate recognition –<i>Cr. levior</i> ants were more aggressive toward <i>Ca. femoratus</i> non-nestmates than <i>Ca. femoratus</i> nestmates. In contrast to the prediction that sharing a nest could weaken conspecific recognition, each parabiotic species also maintains its own aggressive recognition behaviors to exclude conspecific non-nestmates. This suggests that, despite cohabitation, parabiotic ants maintain their own species-specific colony odors and recognition mechanisms. It is possible that such social symbioses are enabled by the two species each using their own separate recognition cues, and that interspecific nestmate recognition may enable this multi-species cooperative nesting.</p> </div

Summary of average abundance of the 34 most abundant peaks from the pooled profiles of parabiotic ants.

<p>The percentages indicate the average relative proportion of each compound, as determined by the area under the peak in the chromatogram, +/− SD. The bolded compounds are highlighted in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056492#pone-0056492-g001" target="_blank">Figure 1</a>. The word ‘trace’ indicates compounds only found in trace amounts (<1% of all profiles).</p

Behavior of enslaved and free-living <i>Formica altipetens</i> towards non-nestmates.

<p>Y axis displays the proportion of behavioral trials which focal workers (enslaved or free-living) initiated aggression towards non-nestmates from colonies nearby (<40m from focal nest) or far away (+.150m from focal nest). Negative controls between paired nestmates of focal colonies are not shown here as none of these trials resulted in aggression. *** indicates a p-value of < .0005 when comparing enslaved and free-living categories as a whole (lumping close and far variables within).</p

Summary of published work on chemical phenotypes, and heterospecific and conspecific nestmate recognition behaviors in naturally occurring parabiotic and xenobiotic compound nests.

<p>Summary of published work on chemical phenotypes, and heterospecific and conspecific nestmate recognition behaviors in naturally occurring parabiotic and xenobiotic compound nests.</p

Comparison of average (±SD) genetic composition found within colonies of enslaved (N = 10 colonies; 189 individuals total) and free-living <i>F</i>. <i>altipetens</i> (N = 10 colonies; 182 individuals total).

<p>Na = number of alleles, Ne = effective number of alleles, and H_exp = expected heterozygosity and all other statistics listed are averaged over the 11 microsatellite loci used in this study.</p

Proportion of aggressive behavior by <i>Ca. femoratus</i> in behavioral assays with nestmate and non-nestmate <i>Cr. levior</i> ants.

<p>Black circles are within <i>Cr. levior</i> Type B, green circles are within <i>Cr. levior</i> Type B, and red circles are for between <i>Cr. levior</i> Type A and <i>Cr. levior</i> Type B colony pairs. There was not a significant difference in aggression towards non-nestmates.</p

Proportion of aggressive behavior by <i>Cr. levior</i> in behavioral assays with nestmate and non-nestmate <i>Cr. levior</i> ants.

<p>The boxplot shows the mean +/− standard deviation. Black circles are for colony pairs considered within <i>Cr. levior</i> Type A combinations (n = 2), green circles are for within <i>Cr. levior</i> Type B combinations (n = 5), and red circles are for between <i>Cr. levior</i> Type A and <i>Cr. levior</i> Type B combinations (n = 3). The asterisks indicates there was significantly more aggression to non-nestmates (p<0.05).</p

Nonmetric multidimensional scaling plot of the relative proportions of 45 cuticular hydrocarbon peaks from pooled ant profiles.

<p>Each shape represents the pooled profile of 30 <i>Cr. levior</i> or 5 <i>Ca. femoratus</i> worker ants of a different colony (n = 20 colonies).</p

Map of nest locations showing 18 of the nests used in this study.

<p>Black circles represent <i>Cr. levior</i> Type A, and white circles represent <i>Cr. levior</i> Type B nests.</p

Proportion of aggressive behavior by <i>Ca. femoratus</i> in behavioral assays with nestmate and non-nestmate <i>Ca. femoratus</i> ants.

<p>Although <i>Ca. femoratus</i> was only of one chemotype, coloring is as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056492#pone-0056492-g004" target="_blank">Figure 4</a> for consistency. The asterisks indicates there was significantly more aggression to non-nestmates (p<0.05).</p