Limits to sustained energy intake. XVIII. Energy intake and reproductive output during lactation in Swiss mice raising small litters. Zhao ZJ, Song DG, Su ZC, Wei WB, Liu XB, & Speakman JR (2013). The Journal of experimental biology, 216 (Pt 12), 2349-58 PMID: 23720804
Although binging is often attributed to weak human character, a substantial binge can also help a man get in touch with his/her reckless animal roots. Whether it involves a steaming heap of elk intestines or 3 seasons of Arrested Development, there are some treats that evolution has wired animals to consume beyond the point of reasonable satiety. Giving in to these deep urges is one of the many so-called flaws that the Catholic Church utterly failed to eradicate from our animal constitution.
A recent binge was triggered by the current issue of The Journal of Experimental Biology, which contained no less than IV sick papes about mouse lactation from Dr. John Speakman and colleagues. Further research revealed that, over the past decade, Speakman’s lab has published XVIII papers on this subject, each possessing the formulaic title: Limits to sustained energy intake., etc. This linear corpus of papes is ideally suited to sautéing an entire day in thick fatty mouse milk.
Each of these papes poses the same basic question: which factors determine an animal’s physiological limits? Speakman and colleagues study this question in lactating mice, who expend a massive amount of energy to produce milk for their thirsty pups. Two initial proposals were that milk production is limited by (I) the ability of the gut to digest food or (II) the efficiency of the mammary gland itself.
In this pape, the authors test the hypothesis that energy intake is limited by the capacity of an animal to dissipate heat. They increased the ability of lactating female mice to dissipate heat by shaving them bald as porpoises. Shaved mice ate more heartily and produced more milk, which in turn increased the size of their adorable mouse children. This result contradicted the long-held views that nursing performance is limited by the efficiency of the mother mouse’s digestion and subsequent milk production.
Although these initial results suggested that there might be one or a couple limitations to energy expenditure, the most recent papes (XIV - XVIII) show that the story is actually much more complicated. Under different environmental conditions, lactation efficiency and offspring growth are limited by several overlapping factors. There are also important differences across mouse strains. Despite the lack of simplicity in the underlying biology, the narrative organization of these XVIII papes that ask the same, seemingly basic, question, demonstrate an experimental doggedness that you got to respect.