Uncommon behaviours observed in a family group of captive black rhinoceros Diceros bicornis: nursing of older offspring and redirected aggression

Abstract

Black rhinoceros Diceros bicornis is considered solitary in the wild, but the Hiroshima City Asa Zoological Park, Japan, has housed a family group with good reproductive outcomes over five decades. Together as a group during the daytime, the members are separated at night. During quantitative observations on this group over more than 4 years, focusing on maternal behaviours and social relationships among individuals, we recorded two unexpected behaviours previously reported in some mammal species that live in well-organized social groups such as primates and elephants, but not in black rhinoceros nor, to our knowledge, any other ungulates: 1) A mother sometimes allowed her two older offspring to suckle repeatedly after she again gave birth, but she accepted their suckling attempts less frequently and terminated their suckling bouts more frequently than those of her new calf, suggesting at least partial satisfaction of unusual suckling demands of older offspring, and differential responding to suckling demands by offspring of different ages. 2) Directly following an unfulfilled suckling demand, one older offspring sometimes displayed aggression toward their younger siblings, similar to redirected aggression seen in animals living in social groups when the victim of a conflict then threatens or attacks a third party. These findings show that black rhinoceros living in a family group in captivity can maintain stable dominance relations, with behavioural tolerance and sociality sufficient for coping with various demands and agonistic situations.