Bawan Amin, Dómhnall J. Jennings, Adam F. Smith, Matthew Quinn, Srivats Chari, Amy Haigh, Devorah Matas, Lee Koren, Simone Ciuti
The field of animal personality has been flourishing in the past decades, focusing on individual differences in behaviour. Although it is quite evident that individuals behave differently from each other, the origins of these consistent differences are still unclear. It is challenging to study personality in wild animals, and most of our knowledge is based on studying animals either in captivity or in laboratory conditions. Adult individuals may display among-individual differences that are affected by experience, while younger individuals do not have this life-experience and so may display innate personality.
Our study investigated whether individual differences are present shortly after birth in new-born fallow deer fawns living at the edge of a metropolis, and studied the potential causes leading to these individual differences. We studied the behavioural response to human handling in 185 new-born deer over 2 years of study. We quantified different responses, such as their alertness prior to being caught, their docility and heart rate during capture, and finally, how long it took them to leave (latency) once allowed to run away. By recapturing the newborns a few days after the first capture, we could test whether individuals were consistent in their behavioural response.
We found that indeed, there was consistency at the individual level when it came to heart rates and latencies, showing that newborns display innate individuality. We also took a hair sample, from which we could measure cortisol and testosterone, representative of the levels that the deer were exposed to while in the womb. We found that newborns that reacted more actively to human handling were exposed to higher amounts of cortisol, but lower amounts of testosterone prenatally. Our findings show that individual differences are present shortly after birth in a free-ranging population of large mammals, and these differences are influenced by prenatal level of hormones, highlighting a possible mechanism underlying individuality.
Our research opens new scenarios to dissect the drivers of animal personality.