Baby brown anole

The timing of reproduction strongly influences reproductive success in many organisms. There is a fitness benefit for individuals who can align their reproductive bouts with conditions that positively influence both reproduction and survival of offspring. For species with extended reproductive seasons, like anoles, the quality of the environment often changes throughout the season in ways that impact offspring survival, and, accordingly, aspects of reproductive strategies may shift to maximize fitness. The Warner Lab has now conducted multiple studies of brown anoles (many unpublished, but see Pearson & Warner 2018) that demonstrate that early-produced offspring have a survival advantage over late-produced offspring. This is likely because individuals that hatch late in the reproductive season must compete with older, larger conspecifics and have less time to grow prior to the cool, dry winter months. Life-history theory predicts that when the offspring environment deteriorates through the season, selection should favor females that shift from producing more, smaller offspring early in the season to fewer, better provisioned offspring later in the season. In our recent paper, Tim Mitchell, Dan Warner, and I quantify seasonal changes in reproduction of brown anoles to determine if females seasonally alter their investment in offspring size vs number.

Figure 1. Differences in key reproductive traits between seasonal cohorts (1 – early; 2- mid; 3-late) of female brown anoles

We captured early, mid-, and late-season cohorts of breeding females and bred them in the lab while controlling proximate environmental variables that influence reproduction (e.g. food, temperature, humidity). These breeding colonies varied only by the capture date of the adult animals from the field. We measured  key reproductive traits for each female (fecundity, egg size, egg quality, inter-clutch interval). Our cohorts exhibited variation in key reproductive traits  consistent with seasonal shifts in reproductive effort (Figure 1). Overall, reproductive effort was highest early in the season due to a relatively high rate of egg production. Later season cohorts produced fewer, but larger, offspring. We infer that these results indicate a strategy for differential allocation of resources through the season. Females maximize offspring quantity when environments are favorable (early season), and maximize offspring quality when environments are poor for those offspring (late season). Despite the extra effort allocated to late-produced offspring, early-produced offspring have a significant survival advantage (Pearson & Warner 2018).

Several future directions are worth serious consideration: first, nearly all studies of anole reproduction in the field demonstrate that reproduction is somewhat seasonal. It is quite reasonable to assume that seasonal shifts in offspring size versus number are prevalent throughout the anole radiation. At this point, we simply don’t know (maybe because we have too many people studying male anoles and too few people studying female anoles – just kidding – but seriously – we’re recruiting!). Second, given the major differences in life-history between mainland and island species (e.g., lifespan, time to maturity), seasonal shifts in reproductive allocation likely differ between these groups as well. A robust assessment of how the mainland-island hypothesis (Andrews 1979) applies to reproductive allocation won’t be possible until we have more basic data on reproduction for many species – let’s get busy folks!

Andrews, R. M. (1979) Evolution of life histories: A comparison of Anolis lizards from matched islands and mainland habitat. Breviora, 454, 1–51.

Mitchell, T.S., Hall, J.M. and Warner, D.A., 2018. Female investment in offspring size and number shifts seasonally in a lizard with single-egg clutches. Evolutionary Ecology, pp.1-15.

Pearson, P.R. and Warner, D.A., 2018. Early hatching enhances survival despite beneficial phenotypic effects of late-season developmental environments. Proc. R. Soc. B, 285 (1874), p.20180256.

Joshua Hall
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