Vibrant life in the ground below

 

Few of us spend our days thinking about the forms of life that exist beneath us; even fewer imagine castles of clay and flourishing life in the soils below. Endemic to the soils of Africa is the fascinating small mammal known as the mole-rat that lives underground, excavating burrow systems that may extend for more than a kilometre. These clay castles have areas designated as sleeping parlours, others as pantries to store food and still others strictly designated as toilets.

Mole-rats are nothing if not fascinating – their sociality, body structure diversity, and their ability to increase plant life above ground in the vicinity of the area they occupy underground are just some of the things that keep researchers enthralled. New species are being discovered and added to the already identified 32, with the latest species just found in Tanzania. The differences between species are far greater than their size although the smallest adult, the naked mole-rat, weighs as little as 20 grams and the largest, the Cape dune mole-rat, exceeds two kilograms in mass.

Mole-rat species are either solitary or social, but all feed on geophytes, the underground storage organs of plants. Solitary species are typically found in areas that are mesic, or  moister, as opposed to social species that have adapted to dry environments known as xeric areas. The wonders of nature are evident in these habitat preferences. In the mesic areas, food is evenly distributed so the solitary mole-rat does not have to dig very far to find its food. In arid areas, however, the workforce of the colony has to excavate very long burrows in order to get to the food source and there are only short windows of opportunity when this can occur, namely after a period of good rainfall, which occurs sporadically and is unpredictable. Burrow excavation using (perpetually growing) incisors causes the teeth to wear down, but with up to 30 members in a colony, food is reached quicker, reducing the wear on their teeth. Food is then shared among the colony members and excess items are stored in a carefully crafted pantry.

The incumbent of the South African Research Chair of Mammal Behavioural Ecology and Ecophysiology, Prof Nigel Bennett*, has dedicated most of his career to researching mole-rats. His main area of focus has been to better understand the modes and mechanisms responsible for reproductive suppression in the non-reproductive females of various species.

 The very specific roles of animals within social colonies add to the interest of mole-rats.   Colonies have distinct divisions of labour that are based on reproductive abilities. Exactly why some members of a colony are able to reproduce and others are not remains to be confirmed, but Bennett has found it particularly captivating to try to figure out how the colony’s queen is able to suppress the reproduction of the working non-breeders. By looking at the hormone levels in the different animals, Bennett is trying to unravel where and how suppression takes place. He is currently investigating the different hormone concentrations released from different regions of the brain in order to provide a possible explanation for how the queen inhibits reproduction in some members.

What make these mammals even more complex are the vast differences not only between solitary and social species, but between the different social species. In naked mole-rats, found in parts of East Africa, for instance, both non-reproductive males and females are physiologically suppressed. In the Damaraland mole-rat, found across parts of southern Africa, a completely different pattern is observed. Only the non-breeding females are physiologically suppressed. The non-breeding animals can be divided into frequent workers (those animals that perform large amounts of burrow maintenance) and infrequent workers (those that perform relatively little work).

Bennett is collaborating with a research team from Cambridge University to determine how these divisions occur, using epigenetics (the study of chemical reactions and the factors that influence them). Results have already shown that there are different metabolic rates between the two groups of animals in a colony, for example between the frequent workers and the infrequent workers. In 2006, Bennett and his research team published these findings in Nature, a distinguished international weekly journal of science. The article precipitated a large amount of research into and debate on this area of study and has resulted in a number of scientists becoming eager to solve this evolutionary puzzle of why some animals work unselfishly for the good of the colony.

Solitary species display different reproductive patterns entirely. These mole-rats are described as highly xenophobic. They do, however, seem to believe in courtship before mating takes place. Using seismic communication, the male will signal his intention to mate and make his way to the female by drumming sounds to let her know he means no malice. As soon as they have mated, though, the female tires of him and chases him from her area. Babies stay with their mothers for about 45 days, after which they disperse along new little tunnels, setting up burrow systems of their own.

Bennett conducts his research in the field through a mark recapture system, capturing mole-rats every six months over a number of years and monitoring the changes in colony composition. He also studies them in the laboratory, measuring hormone concentrations and investigating immunohistochemical binding in different parts of the brain. He also collected genetic material from over 2 500 animals over a 13 year period from one population of mole-rats in Namibia and established that the breeding pair is genetically unrelated. He and colleagues in London have shown that colonies may possess litters exhibiting multiple paternity, but of which only one father remains in the colony. This suggests that some males enter the colony transiently, mate with the queen and then exit before they are detected and sent packing. Bennett has contributed significantly to knowledge of the sociality of and reproductive suppression in these fascinating creatures and is recognised as one of the leading experts in his field.

While much is still being discovered about these spellbinding little animals, one thing is certain, the soil is much richer in elements like calcium, nitrogen and phosphorus where mole-rats occur. This results in more diverse plant life. Mole-rats are thus eco-engineers and vital to a healthy and thriving environment.

 

*Prof Nigel Bennett is also the incumbent of the Austin Roberts Chair of Mammalogy in the Department of Zoology and Entomology in the Faculty of Natural and Agricultural Sciences at the University of Pretoria (UP).  

Prof Nigel Bennett

February 19, 2015

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Researchers
  • Professor Nigel Bennett
    Professor Nigel Bennett has been at the University of Pretoria (UP) for 26 years. He holds a BSc (Hons) in Zoology, which he obtained at Bristol University in the UK, and undertook his PhD studies at the University of Cape Town.

    His research focus is animal physiology and behaviour using the African mole rat as his model animal. His work is directed primarily at studying the social regulation of reproduction in mole rats.

    Prof Bennett’s research record ranks him among the best researchers studying social regulation of reproduction in any group of mammals in the world. He has investigated cooperative breeding in mammals from a variety of perspectives. This multi-faceted approach has led to an integrated understanding of reproductive suppression in mole rats of a type that has not been achieved for any other taxa. His research has set the benchmark for our understanding of phylogenetic and ecological constraints that regulate reproductive success and social evolution in mammalian species.
    Prof Bennett has always been interested in why some organisms adopt a social lifestyle and others do not. As a young boy, he was fascinated by how wood ants worked for the common good of a queen. His interest in mole rats came about while he was an undergraduate at Bristol University, after he had read a seminal paper by scientist Jennifer Jarvis on cooperative breeding in the naked mole rat. Upon obtaining a position as a doctoral candidate, Prof Bennett wanted to see if this was a feature common to other African mole rats. He went on to study the Damaraland mole rat, and found it to have incredible social organisation similar to that of social insects and termites.

    Prof Bennett is now the world leader in African mole rat biology, particularly in reproductive physiology. A research milestone for him was discovering that breeding female naked mole rats orchestrate non-breeding males and females in the colony to exhibit high prolactin levels. This inhibits the release of hormones that stimulate the development of reproductive activities in the gonads, as evidenced by a lack of follicular development in ovaries and a reduction in numbers and motile sperm in testes. Prolactin also results in individuals exhibiting helping behaviour and cooperative care of the young.

    After nearly three decades of research on the reproduction of social African mole rats, Prof Bennett has not been able to determine how the breeding female actually inhibits reproduction in physiologically suppressed animals. This would be the magic bullet for potential contraception in humans.
    He leads a research group that strives to unravel how social evolution arose in African mole rats – solving this puzzle has important implications as to how social evolution arose among hominids. Essentially, it comes down to food acquisition and protection from predators, which is a central theme in social evolution in most mammalian groups.

    Two people influenced his career: Prof Brian Follett – who supervised Prof Bennett’s honours project and whose infectious enthusiasm for science and incredible lectures fired up Prof Bennett’s imagination – and Prof Jennifer Jarvis, who drove his passion to work on mole rats.

    In 2021, Prof Bennett was made an honorary member of the American Society of Mammalogists, a title bestowed on fewer than 100 luminaries in a century. He has been a visiting professor at the School of Chemical and Biological Sciences at the University of London’s Queen Mary College since 2005. More recently, he was a visiting professor at the Department of Zoology at King Saud University in Saudi Arabia.

    He is a member of the Academy of Science of South Africa, and a fellow of the Zoological Society of London, the Royal Society of South Africa and the African Academy of Sciences.

    Prof Bennett was awarded the UP Chancellor’s Medal for his research on three occasions and has received the Exceptional Academic Achiever Award for the past 14 years. He was also the recipient of the Zoological Society of Southern Africa’s gold medal and received the Havenga Prize for outstanding contributions to Life Sciences, awarded by the Academy of Science and Arts of South Africa. UP awarded him the University of Pretoria Commemorative Research Medal for being one of the top 100 scientists in 100 years of its existence.

    Prof Bennett has served as president of the Zoological Community of Southern Africa for two years. He is also editor-in-chief of the Journal of Zoology and a past editor of Proceedings of the Royal Society B: Biological Sciences. In 2013, he was the handling editor of Biology Letters, another Royal Society of London journal. He has published 433 papers in international peer-reviewed scientific journals, co-authored a specialist book published by Cambridge University Press and has penned 15 chapters in books.

    In his spare time, Prof Bennett travels to different countries in Africa to explore the wildlife. He particularly enjoys visiting the mountain gorillas in Rwanda and Uganda, and the eastern lowland gorillas of the Democratic Republic of the Congo. He is also an avid collector of African art and frequently visits markets to add to his collection.

    If he were not a researcher, Prof Bennett would have liked to have been a game warden in one of East Africa’s national parks to contribute to the protection of the incredible African fauna from poaching.

    ORCID ID: 0000-0001-9748-2947
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