Abstract :
Poor maternal care is likely to contribute to neonatal mortality in lambs. We hypothesised
that, as primary caregivers, ewes can modulate their offspring’s sensitivity to pain. We
tested the sensitivity of lambs to thermal stimulus and studied the relationship between
nociceptive thresholds and indicators of mother–young interactions. Eight lambs were subjected
to thermal stimulation at 10 or 11 days of life. A telemetric device was used to
gradually increase the temperature of a heating element placed on the hairless underside
of the tail until a behavioural response such as tail waging or lifting, interpreted as a nociceptive
response, was observed. A safety cut-off was set at 50 ◦C to prevent skin injury. Thermal
stimulation was repeated every 30 min for 3 h. The behaviours of the ewe and the tested
lamb were continuously recorded during the test and instantaneous scan samplings were
conducted every 10 min during the experiment. The distance between the ewe and lamb
was measured as well as their synchrony (performing the same activity). Mean individual
thermal nociceptive thresholds had no measurable effects on social exploration, pawing
another individual, number of vocalisations, walking away from the lambs during nursing.
However, percentage of observations when the ewe and the lamb stayed close to each other
(0–10cm from) and rested synchronously were both positively correlated with the nociceptive
threshold (Pearson correlation, respectively: R = 0.8, P < 0.05 and R = 0.8, P < 0.05).
These results indicate that increased physical proximity and increased behavioural synchrony,
which can be interpreted as indicators of positive mother–young interactions, are
both associated with a decreased sensitivity to thermal stimulus in lambs (i.e. a higher
nociceptive threshold), suggesting that the lambs that follow their mother more closely are
less sensitive to pain.
