Leptin is made by fat cells and serves as a signal of available energy to lots of systems in the body. Diagram from healthjade.com

When you only have so much money to spend, you have to carefully consider what you’ll use it for. Do you go for instant gratification (dinner at your favorite, but expensive, restaurant!), or do you invest in something with a longer-term return (a needed kitchen appliance that will last years)? Free-living organisms have to make this choice throughout their lives. Of course they don’t cook in a kitchen, but their bodies have to ‘decide’ what to do with precious and limited energy. For our beloved anoles, in what do they invest that hard-earned energy from ingested bugs? Make more and bigger babies right away? Grow more? Invest in their immune system or locomotor performance to survive better?

Animal bodies don’t actually make ‘decisions’ about these things. Instead, hormonal and molecular mechanisms are arranged as networks in the body to make ‘decisions’ under different sets of conditions. In a new paper, Andrew Wang, a recent graduate from Jerry Husak’s lab, was curious how such decisions are made in green anoles. Previous work in the Husak lab showed that when calories are restricted, and lizards are forced to invest in athleticism via exercise training, both reproduction and immune function suffer. Why is that, and is it reversible?

The observation that trained and food-deprived lizards had little to no body fat (imagine elite marathon runners!) suggested that the hormone leptin, produced by fat cells, might be responsible. Leptin affects lots of systems in the body (see figure above), and less fat means less leptin. This means that leptin serves as a direct and convenient signal of energy stores: if you have enough energy, then you can direct organs to get to work. This fact has led to a huge literature on how leptin, as an energy signal, controls tradeoffs among traits. Hopefully you’re seeing a slight paradox here – if more leptin means more energy available, how could it mediate tradeoffs? How do you get more of one trait than another if leptin controls both in the same general direction?

Andrew conducted an experiment to find out. He replicated previous work, training and calorie restricting male and female green anoles to cause suppressed reproduction and immune function. He then gave half supplemental leptin and the other half saline, expecting leptin to ‘rescue’ reproduction, immunity, or both. The results were clear: immunity was ‘rescued,’ but reproduction was not. That is, both sexes were investing in survival-related traits to (hopefully) reproduce later instead of just reproducing right away. These results suggest that either there wasn’t enough energy for reproduction and the signal was moot, or the two traits have different sensitivities to leptin. Future work will help to disentangle these possibilities, but this work gives us more understanding of how anoles allocate energy when it’s limited.

Figure from Wang et al. (2019). Key: U=untrained, T=trained, H=high diet, R=restricted diet, L=leptin injected, S=saline injected. Note here that the swelling response to PHA injection was suppressed with training and caloriee restriction, but it was rescued with leptin (T-R-S vs T-R-L).

Paper: Wang AZ, Husak JF, Lovern M. 2019. Leptin ameliorates the immunity, but not reproduction, trade-off with endurance in lizards. J Comp Physiol B, in press. doi: 10.1007/s00360-019-01202-2