“It is not the strongest of the species that survives, nor the most intelligent, it is the one that is most adaptable to change” – Charles Darwin: the arid adapted Hot-rod ants (Ocymyrmex) in southern Africa

15/12/2022

By Nokuthula Mbanyana-Nhleko, Simon van Noort, Theresa Wossler, Johannes Le Roux, & Bonnie Blaimer

Deserts are one of the most inhospitable habitats, due to low precipitation and high temperatures.

Figure 1: Road between Grunau and Fish River Canyon (Namibia) © Hamish Robertson.

These regions present intense desiccation constraints for the organisms inhabiting these dry habitats. Desiccation resistance, i.e., the ability to regulate water loss is an essential trait for species living in dry regions, especially insects, as they are at high risk of dehydration.  Insects mostly lose water through respiration and excretion, with reduced cuticular water loss related to desiccation resistance. Some insects living in these dry regions have evolved physiological adaptations that enable them to survive the harsh conditions or develop behavioural responses, such as retreating to cooler microclimates or shifting activity to avoid the hottest part of the day to avoid desiccation, for example, Ocymyrmex ants climb up sticks or grass stalks to alleviate heat stress when foraging at high temperatures  (Figure 2); or move their brood deep under the ground when the temperatures are very high.

Figure 2: Ocymyrmex fortior – Venetia Limpopo Nature Reserve © Simon van Noort

Ocymyrmex ants possibly originated from the arid regions of southern Africa and diversified to the tropical and subtropical habitats of this region. These ants are mainly distributed in the dry semi-desert regions of southern Africa (Namibia and north-western South Africa and Botswana), a few species also occur in the eastern part of this region.

Figure 3: Map showing the distribution of Ocymyrmex species across the different biomes in southern Africa. Occurrence records (black records) were extracted from the Iziko Museums of South Africa Specify6 database.

These thermophilic ants are regarded as desert specialists as they are most active during the hottest time of the day. These ants love sun exposure and are extremely fast moving. Some of the species in this group have been observed running at speed of 23 m/min at ground temperatures above 50ºC.

Several insect groups mainly restricted to the dry regions, display traits such as high critical thermal limits, black coloration and thermoregulation that contribute to their success. The heat intolerant ant species tend to forage during certain periods of the day when temperatures are below their critical thermal maxima.

Figure 4: Ground temperatures measured in Tswalu, before excavating Ocymyrmex ants, which were active on the surface © Nokuthula Nhleko.

In some ant species such extreme temperatures may limit foraging activity but, in the case of desert specialists, including Hot-rod ants, adaptations to the extreme thermal niche allow exploitation of resources (dead and heat stressed arthropods).

Figure 5: Ocymyrmex resekhes carrying a beetle – between Grunau and Hardap – Namibia © Hamish Robertson.

These thermophilic ant genera have long legs that usually hold the head, thorax and gaster high above the substrate, which helps with body temperature regulation.

Figure 6: Ocymyrmex velox © Hamish Robertson

These ants nest directly in soil on the ground and nests are sometimes found in the open or in rocky soils, usually with a crater of soil and small stones around the entrance.

Some of the species in this group can nest very deep under the ground.

Figure 6: Ocymyrmex nest – Leon Taljaard Nature Reserve © Simon van Noort
Figure 7: Nokuthula Nhleko digging up an Ocymyrmex nests in Namaqua National Park – © Simon van Noort
Figure 8: Nokuthula Nhleko digging up an Ocymyrmex nests in Namibia – © Hamish Robertson

Much of the diversity of Ocymyrmex ants may be related to their adaptations to cope with the extreme thermal conditions they experience in their habitats. These ants have evolved various traits to deal with these harsh environmental conditions. Hot-rod ants have relatively high thermal limits and are desiccation resistant, and also have long chained cuticular hydrocarbons, and greater abundance of linear alkanes which are believed to be good for waterproofing. The thin layer of hydrophobic lipids covering the insect cuticle plays an important role in waterproofing. These lipids act as a barrier to water loss and prevent insects from desiccation. A relatively high thermal tolerance, long chain hydrocarbons and high abundance of alkanes (which act as a waterproofing agent of the insect’s cuticle, and therefore play an important role in preventing desiccation) are physiological adaptations for all Ocymyrmex species irrespective of where they occur. The extreme desiccation resistance and thermal tolerance are possibly conserved traits across the Ocymyrmex genus, meaning, these traits are possibly ancestral in the Ocymyrmex genus, as these traits are also evident in the mesic species – i.e., Ocymyrmex species occurring in the subtropical humid regions.

Ocymyrmex ants may have experienced strong selection for arid-adapted traits. This inherent adaptability provides benefits for these species to occur almost everywhere in this region as critical thermal maximum and desiccation tolerance are not detrimental traits to have in cooler wetter climates. These ants, especially in cooler and wetter regions of southern Africa, might be able to withstand predicted future climate change, with elevated temperatures.

Recent work has suggested that the thermal tolerances of ant species living in environmental conditions close to their physiological thermal tolerance limits can predict their response to future climate changes, and that their physiological traits may give an indication of how they will respond to climate changes. Understanding the thermal tolerance limits of Ocymyrmex ants might be useful in predicting the response of these ants to climate change.

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