New publication – Oxytocin is linked to increased rates of mass gain in seal pups

Newly weaned grey seal pup with a very healthy blubber layer. Pups have to go from about 15kg to over 30kg in just 18 days to stand a chance of surviving their first year of life.

Link to article:

Download the paper for free until the 22nd October here:

Or read the abstract on this site here

It has been a while since the last blog update on this site (for a very good reason, but more on that later), and in the time that has passed the PHATS team has been busy finalising our lab work, analysing our results and of course, writing papers. I’ll be uploading a post soon about two papers on pollutants that the PHATS team have been published since the last update (or you can read one of the papers here, the other one is still in press!). But right now, I’m going to talk about my latest paper on the hormone oxytocin and mother-infant bonding, behaviour and development.

Grey seal mother-pup pair on the Isle of May, Scotland

This paper is the last to come from my NERC funded PhD work with the Sea Mammal Research Unit, University of St Andrews. During my PhD I collected lots of different types of information on the wild grey seals breeding on the Isle of May  and North Rona, two island colonies that I would visit for months at a time to study seals. I have previously published papers showing that the more oxytocin a mother has, the closer she stays to her pup and an experiment that showed that high oxytocin definitely causes seals to seek others out and stay close to them. However, I wanted to investigate what the oxytocin levels in young pups were like when they were still with their mothers, and if there were any interesting dynamics going on, whether there were any developmental consequences for the pups.

What I found out was very exciting. It turns out that mothers with high oxytocin levels produce pups with high oxytocin levels, and I think this is due to positive feedback loops being created in both the mother and pup once they have bonded. This happens because oxytocin is often released when someone interacts with another individual they are bonded to, and the high oxytocin levels created cause the individuals to stay close together, meaning they can be exposed to even more interactions and even more oxytocin release! This has been theorised to exist in mothers and infants before, and there is evidence that it happens in socially bonded individual, even between humans and their pet dogs! However, my study provided the first evidence that these positive loops exist in completely wild animal populations living in natural environments.

Oxytocin causes mothers and pups to stay close together on a breeding colony

The next step was to see if there were links between the oxytocin levels in pups and any of their growth characteristics before they weaned from their mothers. My data showed that there was a strong relationship between oxytocin and the rate that pups gain mass. The higher the level of oxytocin, the more mass a pup gained ever day it was with its mother. While this was exciting, there might have been a straightforward reason for this, that the pups with high oxytocin are motivated to interact more with their mothers, which includes drinking milk from them. Seals have very fat rich milk, so the more a pup drinks, the more mass it will gain per day. However, I had already studied the impacts of oxytocin on the behaviour of mother-pup pairs, and found no relationship between high oxytocin and the frequency or duration of time pups spend drinking milk. Could it be true that the high oxytocin pups were somehow gaining more mass without drinking more milk? Luckily there was another way to test if this was happening, by analysing the mass change rates of the mothers.

Grey seal mothers don’t eat anything while they are on a breeding colony raising their pups. They have to build up a big fatty blubber layer throughout the year and rely on this energy store while they are on the breeding colony. The time that mother grey seals spend rearing their pups is short to help them cope with this, mothers and pups are only together for 18 days before weaning happens and the mother goes back to sea for some well-earned fish. However, this puts an incredible energetic strain on these poor seal mothers, they have to give birth, produce high fat milk and look after their pups all without eating anything. So, as seal mothers will usually loose a large amount of body mass over the 18 days they are rearing their pups, I could use the mass loss rates of the mothers in my study to see if the ones with these high oxytocin pups were losing mass at a faster rates. This would indicate they were producing more milk, or higher fat milk, for their pups to drink, causing increase mass gain in their pups. However, when I analysed the data there was no link between oxytocin levels and mass loss in the mothers. Somehow, the high oxytocin pups are able to gain more mass without any additional strain on their mothers.

Grey seal mothers are usually large and fat when they give birth (left), and newborn pups only have a thin layer of blubber under their skin. By the time pups are approaching weaning on day 18 after birth, they need to have put on a lot of weight, causing the mothers to deplete their blubber stores to generate fat rich milk for them (right).

There are a few potential explanations for the high mass gain rates I found in the high oxytocin pups. Firstly, we know that oxytocin affects behaviour and seal pups with high oxytocin are likely motivated to stay snuggled up to their mothers. This could help them save energy in two ways, firstly the pups would not be wandering around the colony, burning energy and getting into trouble with neighbouring seals. Secondly, pups that spend more time close to their mothers may be more sheltered from weather conditions, meaning they wouldn’t have to use as much energy keeping warm on the cold, wet Scottish breeding colonies. Both of these potential explanations demonstrate that, rather than putting on more mass per day, these high oxytocin pups may actually be reducing activities that divert energy away from putting on mass.

A heavy rain storm approaches the seal breeding colony on North Rona. Strong winds and wet conditions are common on Scottish seal colonies. All the white dots on the island are seal pups, resting beside their mothers.

There are also potential physiological explanations for the increased rates of mass gain in pups. A few laboratory studies have shown that oxytocin can act on development, with elevated oxytocin increasing fat tissue, impacting on muscle development and bone mass accumulation. Several studies have already proposed the physiological mechanism by which oxytocin can alter mass changes regardless of the food an individual is eating. It is possible then, that the high levels of oxytocin in the pups is altering their physical development and giving the pups a good start in life by helping them put on weight as fast as possible.

Any factor that helps seal pups put on weight is really important. The likelihood a seal pup will survive its first harsh year at sea, learning to survive in the wild, is directly tied to how fat it managed to get while still with its mother. But a seal mother cannot just pour all their resources into one pup, as the energy she uses up during one breeding season negatively impacts on how she will cope with rearing subsequent pups. So this relationship between oxytocin and mass gain rates in pups may be important, as it would enable pups to get as big as possible before weaning without getting seal mothers to invest more energy in rearing them.

Yearling grey seals on the Isle of May in the summer. Surviving their first year at sea is tough for young seals.

It’s not just seals that would benefit from this hormone-development link in infants either. Oxytocin, or oxytocin like peptides, are present in every vertebrate animal group and their function is remarkably similar across all these different species. Infancy is a crucial time, and in all species, gaining mass and developing physically is vital for offspring to survive. In humans alone, problems with infant nutrition and development are estimated to cause 45% of deaths in children under five and if we can better understand the physiology underlying weight gain in infants, we have a better chance at developing ways to help those that are sadly failing to thrive. This work so far has shown that oxytocin release may connect optimal parental or social environments with direct advantages for infant development, which could be important for different fields of biological, medical and veterinary science, and I’m hoping to keep working on this phenomenon in the coming years.

The positive oxytocin feedback loop system in grey seal mother-pup pairs and the energetic dynamics in both individuals (from Robinson et al. 2019, Psychoneuroendocrinology, vol 110).

MEANWHILE I’ve had a baby! This is the reason that there haven’t been any updates for so long, I’ve been busy being a parent. I am still working on the PHATS project and my other research looking at oxytocin and social behaviour in mammals, but everything takes a lot longer now! I did get to go and present some of our findings at the Society of Experimental Biology conference in Seville over the summer, and we all had a great time making new science friends, hearing about amazing research and enjoying the Spanish sun. My daughter took the conference in her stride and I must say that the SEB were wonderfully supportive of me being there with a young baby. I will be writing another blog post soon about my great first experience going to a conference as a mother, especially as there are plenty of other conferences that are not doing such a good job and all the things the SEB did would work in other places. Being able to go to meetings like this makes such a difference to parents trying to get back into their careers after having a baby, so it is important for societies to include things like breastfeeding rooms and childcare options when they are planning conferences.

In the meantime, I will be working on more publications from the PHATS project and my oxytocin work plus grant applications so I can continue my research, so wish me luck and watch this space (or twitter) for updates!

My daughter enjoying her first conference and flying the flag for the PHATS team!

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