Suckling strategies in terrestrial ungulates (Artiodactyla, Perissodactyla) and the factors influencing them: the offspring perspective

In order to survive, ungulate young obtain their mother’s colostrum and milk through a variety of suckling strategies. Here, these strategies and the factors that influence them are reviewed and interpreted primarily from the perspective of the young. Suckling strategies are determined by a variety of factors, some of which play a role even before birth. Neonatal suckling behaviour depends first on the type of placenta and the developmental stage of the young at birth. Ungulates have an epitheliochorial placenta in which transplacental transport of immunoglobulins is impeded. Therefore, they are born in a hypo−/agammaglobulinemic state hence, timely intake of sufficient colostrum is a life-or-death situation and an unsuccessful suckling strategy is likely to result in death. Precociality enables the newborn ungulate to actively participate in the suckling process immediately after birth, which is critically important. After the prenatally defined physiological needs are satisfied, young suckling strategy is influenced by other factors. Continuous milk production requires constant use and stimulation of the mammary gland. Brain lateralization has also been found to play a role in suckling behaviour. Finally, competition between siblings or peers in the social group also influences young suckling strategies. Sibling competition particularly affects polytocous ungulates and leads to suckling order, for example. Similarly, interactions within the social group create the conditions for the emergence of specific strategies, including allosuckling, strategy of suckling a non-filial mother. The young suckling strategies in ungulates basically represent an evolutionary intertwining of offspring selfishness and selection that maximises inclusive fitness.