Mother's Milk, Literature Sleuths, and Science Fairies

I am currently polishing a manuscript for submission (hence the obvious necessity of working on a blog post, next I will clean my bathroom tile). Delving deep into the literature to precisely contextualize my results, I despair that I will never be able to read ALL the things. For example, entering two key search terms- ‘lactation’ and ‘glucocorticoids’- returns 24,800 results in Google Scholar.

John Medbury (LAZY J Studios)

Reading all that is just not going to happen. Merely identifying the most pertinent papers can be challenging. And with every new literature search, I discover articles that I clearly should have read ages ago. Um especially when I was writing this. 

I had that sinking feeling yesterday as I read a killer review. In the text the authors described an exciting dynamic; high food intake, high cortisol concentrations in mother’s milk, and high activity in kittens covary while low food intake, low cortisol concentrations in mother’s milk, and low activity in kittens covary. 

Neat. Tidy. Compelling.

And there was a gorgeous figure to reinforce this awesomeness! Now, don’t get too excited here, because, spoiler alert, none of this turns out to be an accurate representation of the papers they are citing.

Figure used sans permission. Citation omitted intentionally.

They attributed this conceptual model to a paper by Peaker and Neville in 1991. I said to myself, “Self, that paper looks perfect!” Naturally my institution does not subscribe to that journal and it was behind a paywall. Sidebar: Open Access! Luckily I was able to email lactation biology living treasure Peggy Neville to see if she had a pdf of her article. She sent it directly and I ravenously read what turned out to be an excellent commentary. And at the bottom of the last page, highlighted in yellow, I discovered the passage that lead the unnamed others to generate the conceptual figure above.

Hold the phone, reducing maternal dietary intake increased infant play behavior. And Peaker and Neville make no claims about the directional effect of cortisol. Oh and the reported findings are from Bateson et al. 1990.

Naturally I proceeded to the original empirical article, which was a treat of a “classic” behavioural paper- nuanced, thoughtful, systematic consideration of the data.  Using a within subjects design across two lactations/litters, Bateson and colleagues evaluated the effect of reducing dietary intake from ad libitum levels to 80% of ad libitum levels (a relatively minor reduction). In the nutrient restricted condition cat moms lost more weight because they had to mobilize more of their body fat to sustain milk synthesis. We know they were able to sustain kitten growth because the growth rates were the same in both diet conditions. This ad libitum and 80% ad libitum study design was identical to an earlier study in baboons that found the exact same thing- greater maternal weight loss and sustained infant growth (Roberts et al. 1985).

Science / AAAS

Bateson and colleagues went a step further evaluating the behavioral dynamics among cat mom and kittens. Which is exactly what we would expect from the co-author of the SACRED METHODOLOGICAL TEXT OF ANIMAL BEHAVIOR!

Under nutrient restriction, cat moms were not as generous in the amount of time they reclined on their side offering kittens unfettered access to the milk buffet. Their kittens, however, compensated by nuzzling more when they could access mom’s nipples. And just as reported by Peaker and Neville in their 1991 review, reducing maternal dietary intake increased the amount of time kittens spent engaged in object play. All together total activity budgets in the two diet conditions were the same. It may have been that the kittens' behavioral development, in terms of object play, was accelerated in response to longer inter-nursing intervals, a behavioral cue of earlier weaning. Or maybe they just had more time to kill in between nursing bouts.

And there was no info on cortisol in milk.

Artist’s representation of my scampering through papers & search engines

(alright fine, it’s an illustration of Eloise by Hilary Knight)

After trying unsuccessfully to find a possible phantom paper, I nervously emailed Professor Sir Patrick Bateson, explained the literature sleuthing I had done and asked if I was overlooking a publication on cortisol in cat milk. And he most quickly and kindly said nope, they had never studied cortisol in milk.

So to recap: not specifically “high” food intake vs. “low” food intake, no information about cortisol in milk, no directional predictions of cortisol’s effects, and same activity budgets, with extra object play in the low food condition. Totally different than the conceptual model shown above.

Since the 90s, there's been direct investigations on cortisol in milk and infant behavior. And it could very well be that the conceptual model proposed in the unnamed paper is an accurate reflection of the biological processes operating more generally across species. But it does highlight the pitfalls I warn my students about… Only cite papers that you have read! DO NOT cite papers based on another publication’s report of them. Because every time that happens, a science fairy dies.

by Takeshi Yamada

This due diligence in literature reading and detail checking necessarily belongs to a broader discussion about the head-desk, side-eye, face-palm state of academic publishing. The “tyranny” of high impact journals, the concerns about scientific story-telling, the conspicuous production of least publishable unit papers. Such things do too little to generate, integrate, and disseminate scientific knowledge. Tenure and promotion committees, don’t be so bean-county. Journal editors and manuscript reviewers, be nit-picky about the right things. 

And scientific authors, let’s take to heart and mind the words of Peter Marler, a pioneer in the field of animal communication. He once gave invaluable professional advice to a colleague- “Slow down, reflect more, and publish less.”

Then maybe I'll be able to catch up on my reading.

Literature Cited:

Roberts, S. B., Cole, T. J., & Coward, W. A. (1985). Lactational performance in relation to energy intake in the baboon. The American journal of clinical nutrition, 41(6), 1270-1276.

Peaker, M., & Neville, M. C. (1991). Hormones in milk: chemical signals to the offspring?. Journal of endocrinology, 131(1), 1-3.

Bateson, P., Mendl, M., & Feaver, J. (1990). Play in the domestic cat is enhanced by rationing of the mother during lactation. Animal Behaviour, 40(3), 514-525.