Olfactory communication of the white rhinoceros (Ceratotherium simum)
Marneweck, Courtney Jade.
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Many mammal species communicate olfactorily via specialised scent glandular secretions, urine and/or dung. Despite a large body of work on olfactory communication, the extent to which mammals communicate via dung odours, and what information is transmitted, is unknown. White rhinos (Ceratotherium simum) have poor eyesight but an acute sense of smell and therefore rely heavily on olfactory signals. Moreover, white rhinos of all ages and sex defecate communally in middens, thus it is possible that these middens act as olfactory information centres for male-male, female-male, male-female and female-female communication. To explore these possibilities, I first analysed the odours emitted from the dung of free-ranging white rhinos. In doing so, I identified distinct odour profiles that indicated an individual’s sex, age, male territorial status, and female oestrous state. Once I had identified the information transmitted, I then explored how long these signals lasted. In order for an olfactory signal to be effective it must persist in the environment for an extended period. To determine signal longevity I analysed the temporal changes of white rhino dung odours. I found that over a short period male dung odours had shorter longevity than female odours. Within males, territorial odours had shorter longevity than non-territorial, while non-oestrous female odours had a shorter longevity than oestrous odours. The high temperature and humidity of the wet season decreased the longevity of all adult dung odours. However, white rhinos did not adjust their visitation or defecation frequency during the wet season to counteract this decrease in longevity. Having identified the odours and how long they lasted, I then investigated the behaviour of white rhinos at middens to determine which individuals were primarily transmitting information and who were the intended targets. I found that middens were utilised predominately by adults. Moreover, the primary function of middens was for territorial males to transmit and obtain information (male-male and female-male communication), with secondary functions for non-territorial males to also assess female reproductive state, and females to assess the quality and number of potential mates (male-female communication). In addition to olfactory signals there was a spatial aspect to defecating in middens, where territorial males defecated in the centre of the midden and other individuals around the periphery. Further, territorial males regulated their dung output, with a higher defecation frequency and smaller dung volume than any other adult. Finally, I conducted an experiment to investigate the purpose of territorial male dung kicking. Using non-territorial adult male dung as a surrogate, I found that the dispersal of male white rhino dung caused olfactory signal amplification by increasing the emission of hydrocarbon acids. However, despite the benefits of odour amplification, dung dispersal also carried a cost of decreased odour longevity, ultimately decreasing signal longevity. Territorial males likely counteract this by defecating in middens during peak visitation times by other individuals. Ultimately, my results highlight the mechanism behind olfactory communication in white rhinos and the importance of middens in this communication system. Moreover, as many other mammal species defecate communally, olfactory communication via dung odours is likely a widespread phenomenon.