Social organisation and pathogen transmission in African ants: at what point do social immunity benefits diminish?
Date
2020-04
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Eusocial insects, and ants in particular, encounter a range of pathogens, often
generalist entomopathogenic fungi that profit from their hosts’ dense living conditions
and high relatedness. Ants exploit a range of individual behaviours that ameliorate pathogen
impacts on the colony, collectively termed “social immunity”. Species with different life
histories and ecologies combat fungal infections using different approaches. This study assessed a
range of social immunity mechanisms employed by three South African ant species,
Anoplolepis custodiens, Camponotus fulvopilosus and Tetramorium sericeiventre.
Self-grooming, allogrooming, trophallaxis, and organisational immunity, as well as the effect of
group size, were assessed through a range of methods including group level observations, colony
level observations, spatial use assessments, interaction networks, and mathematical models.
In assessments of group level effects, I showed that as group size increases ants increased their
investment in grooming to remove conidia. Further, I showed that all three species responded to
pathogen exposure by increasing interaction rates and mitigated exposure to the generalist
entomopathogenic fungus, Metarhizium anisopliae. Mortality did not differ 21 days post-
exposure compared to control treated ants, with access to nestmates and social immune
interactions. Each species drastically lowered the number of conidia on their cuticle if allowed to
groom; however, C. fulvopilosus displayed ineffective allogrooming. Commonly, ants
increased the frequency and decreased the duration of allogrooming in response to exposure. Species
displayed differential response in these behaviours with A. custodiens grooming most frequently
and C. fulvopilosus allogrooming the least. The duration and frequency of trophallaxis
was maintained in response to exposure, with A. custodiens engaging in the most trophallaxis, in
terms of both frequency and duration. Assessing organisational immunity in the form of spatial use
patterns, I showed that all three species displayed clustering within nests, likely limiting
pathogen transmission. Only A. custodiens, however, showed spatial separation between foragers
and the queen and further increased clustering in response to exposure, limiting pathogen
spread. I generated interaction networks for C. fulvopilosus obtaining data from behavioural
recordings of experimental colonies and calculated network metrics before and after pathogen
exposure. Camponotus. fulvopilosus decreased network connectivity in response to pathogen
exposure which limits pathways for pathogen spread. Finally, I generated matrix projection models
based on the data to assess how each of the three species managed
exposure to fungi, by tracking spores as they are managed by self-grooming and allogrooming.
All three species were able to mitigate pathogen exposure, removing all conidia before they could
lead to infection. Anoplolepis custodiens relied primarily on allogrooming whilst C.
fulvopilosus relied only on self-grooming to remove conidia. Tetramorium sericeiventre relied
primarily on self-grooming but also benefitted from allogrooming. Overall, I show that three
African ant species mitigate exposure to a generalist entomopathogenic fungus through a
different combination of behavioural social immunity mechanisms, highlighting the
importance of assessing several pathogen control mechanisms across multiple species. This
represents the first assessment of social immunity in South Africa, showing that three species
use either individual or collective behaviours to mitigate fungal exposure.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
Description
Thesis (PhD)--Stellenbosch University, 2020.
Keywords
Social immunity, Ant communities -- Behavior, Insect societies, Entomopathogenic fungi, Ants -- Ecology, UCTD, UCTD