The role of middens in white rhino olfactory communication

1. What they did

White rhinos leave information about their sex, age, territorial status (males) and oestrous state (females) in the smell of their dung, and they defecate in shared dung heaps called middens. The authors asked who uses these middens to leave and to read that information, and for whom it is meant.

In Hluhluwe-iMfolozi Park in South Africa, they set motion-triggered infrared video camera traps about 3 m from 10 middens, each in the territory of a different territorial male, from November 2014 to August 2015. The cameras recorded 30-second clips with no visible flash.

• Identifying the rhinos. From 2403 videos they built an identity profile for each rhino (233 individuals), using differences in horn shape and size, and classed them by age from body size and horn development.
• Sorting the adult males. Territorial males were adult males that kicked their dung and spray urinated at the midden. Other adult males were split into subordinate males (frequent visitors, on more than 5% of camera days, without territorial behaviour) and potential challengers (rare visitors that sometimes showed territorial behaviour). Females could not be sorted by oestrous state from the videos, so all adult females were grouped together.
• Recording behaviour. For each visit they recorded whether the rhino defecated and whether they sniffed a dung pile (standing still, nose within 20 cm of the ground, nostrils flaring). After each sniff they looked back through the videos to see whose dung it was.
• Mapping where dung was dropped. For 433 defecations they recorded whether the dung was dropped in the centre, middle zone or edge of the midden, or outside it.
• Weighing dung. They followed 40 adult rhinos on foot (one researcher and one armed guard) until they defecated, collected the whole fresh dung pile, dried it in the sun for 72 hours and weighed it.

2. What they found

They recorded 115 adults across the 10 middens: 10 territorial males, 13 subordinate males, 13 potential challengers and 79 females. Subadults and calves used the middens in much the same way as each other, so the main results are for adults.

Visits and defecation

• Territorial males visited middens far more often than any other adults (on 20% of camera days on average), and defecated there more often.
• Subordinate males visited more often than potential challengers and females. Potential challengers visited about as often as females.
• Subordinate males, potential challengers and females all defecated at middens at about the same rate.

Sniffing dung

• Territorial males sniffed dung on a larger share of their visits than subordinate males and females, and about as often as potential challengers. Females sniffed less often than potential challengers.
• All groups sniffed male and female dung equally. Territorial males’ dung was sniffed most often overall.
• Unexpectedly, territorial males mostly sniffed their own dung, then female dung, and least the dung of other males.
• Subordinate males, potential challengers and females sniffed territorial male dung and female dung about equally, and other males’ dung much less. Females sniffed other females’ dung almost as often as the territorial male’s.

Dung weight

Territorial males’ dung piles were significantly lighter (about 2 kg dry weight on average, read from the figure) than those of other adult males and of females in oestrus or not in oestrus (about 3.3 to 4.5 kg), which did not differ from each other.

Where in the midden

• Territorial males defecated only in the centre (100%), and subordinate males mostly in the centre (65%).
• Females and potential challengers defecated mostly towards the edge or outside the midden (females: 52% edge and 22% outside; challengers: 63% edge and 6% outside).
• Subordinate males and potential challengers did not differ statistically.
• Up to 16 individuals used one midden.

The authors often saw a territorial male kick aggressively and overmark when he found a potential challenger’s dung in the centre of the midden, and return up to three times the same day to check it, but they had too few such records to draw conclusions.

3. What they concluded

• Middens act as information centres for white rhinos. Territorial males are the main users, leaving information on territory ownership and picking up information from females and other males.
• Territorial males may sniff their own dung to check how strong their scent signal still is, since kicking spreads it out and may make the smell fade sooner.
• Territorial males defecate more often but in smaller amounts, which lets them spread a limited supply of dung across many middens.
• Where dung is dropped adds a spatial message to the smell: dung in the centre may signal residency, and the kicking there territory ownership. Challengers defecating at the edge may be signalling that they are not a threat, while dung in the centre may be a challenge.
• Other males may use middens to check female reproductive state. Females may assess the quality of all males and keep track of the other females whose home ranges overlap theirs.
• Because many mammals use communal middens, this role is probably not limited to white rhinos.

6. Other technologies, skills or innovations

• Infrared, motion-triggered video camera traps (Cuddeback Black Flash E3 and Cuddeback Attack Black Flash 1194) placed about 3 m from the midden edge and attached to a nearby tree, with batteries and memory cards changed only once a month to limit disturbance.
• Identification of individual rhinos on video from horn shape and size.
• Fresh dung (less than 5 minutes old) was told apart from older dung in the midden by its temperature, colour and moisture.