There is something that baby mammals always do: suck their thumbs (or trunks in case of elephant).

Several benefits have been associated with thumb-sucking: it relaxes babies, it slows down heart and breathing rates and regulates stomach muscle movements.

Thumb- and trunk-sucking is associated with food and mom, and therefore is the most comforting action a baby can do. This comfort sensation is given by the endorphin rush (read here to know more about this hormone).

Babies usually suck their thumbs to release psychological and physical tension, thus demonstrating psycho-emotional maturity. Most of children stop sucking their thumbs between the age of 2 and 4, when they develop other forms of self-comforting.

This comfort action is so addicting that both some adult humans and some adult elephants has been observed to thumb- or trunk-sucking.

References:

Greco et al., “Why pace? The influence of social, housing, management, life history, and demographic characteristics on locomotor stereotypy in zoo elephants”, (2017), https://doi.org/10.1016/j.applanim.2017.05.003

Griffiths, “Can Thumb Sucking Be Addictive?”, (2016), https://www.psychologytoday.com/us/blog/in-excess/201609/can-thumb-sucking-be-addictive

Staufert Gutierrez and Carungno, “Thumb sucking”, (2020), https://www.ncbi.nlm.nih.gov/books/NBK556112/