This will be our final instalment of the Behaviour of the Day posts and I believe we’ve saved the best for last. Nursing and lactation are some of the most well-studied aspects of phocid seal ecology, outside of their champion diving physiology adaptations. Since we are focusing our sampling efforts exclusively during the lactation period, nursing behaviour obviously plays a very important role in our study.
We only have two formal definitions for these behaviours in our ethogram:
Presenting/Nursing (PN) – Female rolls laterally so that nipples are presented to the pup, usually with the female resting on one flank and is designated as such when the female is within one pup body length at an approximate perpendicular orientation to the pup. This behaviour can also occur with several Comfort Movements (CM) and Pup Checks (PC) to position correctly for nursing and should be specified within this behavioural state. Nursing (N) – Female already in presenting position described with nipples exposed, pup is in direct oral contact with mother and appears to be suckling or moving between nipples using nose to push on mother between attachment. Female often appears to be resting during nursing bouts. For the purposes of this ethogram, presenting and nursing are pooled into the same behavioural state.
Presenting/Nursing Alert (PNAL) – Same as Presenting/Nursing (P/N), but head is raised from a horizontal position with eyes open and is considered Alert (ALHU/ALHS) scanning the environment.
It should be noted that grey seals are what we consider capital breeders, being that they rely solely on the fat reserves (sub-cutaneous blubber layer) that they amass prior to the breeding season to provision both themselves and their pup during the breeding season. Unlike some other pinnipeds, grey seals do not take periodic foraging trips through the lactation period and remain hauled out on land for the entirety of lactation.
Over this lactation period, females can lose anywhere from 40 to 60% of their body mass. The milk that grey seals produce can have a fat content of up to 60%. Comparing this to your average full-fat cow’s milk that one might pour over breakfast cereal (averaging about 3% fat), this stuff is like rocket fuel! This calorie-packed fluid allows pups to rapidly gain weight over this relatively short nursing period. But where do they stack up relative to other phocid seals in the North Atlantic? Grey seals tend to average about 18 days of lactation. While this may seem quite short to us, the Hooded seal (Cystophora cristata) holds the record with a staggering 4-day lactation period from birth to weaning. While our study does not directly measure daily mass gain by the pup and mass loss from the female, we typically use what is known as a mass transfer efficiency, which is typically calculated as the ratio of total mass gain from the pup to mass loss from the female. This gives us a coarse estimate of how much energy is moving between the mother-pup pair.
Behaviourally, nursing bouts seem to come about every 4 hours throughout the day. Early in lactation, the initiation of nursing is often driven by the female, as we see other mom-pup interactions executed as a means to encourage the pup to begin nursing, while generally keeping closer to the pup. Recent work by our colleagues has demonstrated the direct link between oxytocin in physiologically regulating maternal behaviours including nursing and mom-pup proximity. As time progresses and the pups become more mobile, nursing bouts appear to become more often initiated by the pup. In truth, most of the aggression that the females exhibit towards other pups appears to be in the hopes of protecting this valuable resource. Though, we do see the occassional female nursing a pup that is not her own, known as allonursing – sometimes it appears that one female has ‘adopted’ a new pup, while other times it seems as though a weaned pup is trying to cheat the system and has become a bit of a milk theif!
It is our hope that the application of our accelerometers may help us to tease apart this relationship and shed some light on how personality may affect this energetically demanding period for female grey seals. Given that they must rely on the fat stores that they arrive with throughout the lactation period, it is easy to see that small differences in time allocated to each of the behavioural categories we’ve explored so far in our Behaviour of the Day posts may actually lead to larger effects on not only the amount of energy used by the female, but also the time and energy left to transfer to the pup during nursing periods. It is here that we may see the most obvious consequences of personality.