Reproductive cycle of Anolis trinitatis, from Licht and Gorman (1970).
All anoles lay only a single egg at a time, but that doesn’t mean that no variation exists among species in reproductive cycles. Still the most comprehensive study of this topic is Licht and Gorman’s (1970) comparison of nine populations of seven species throughout the Caribbean (downloadable as part of AA’s “Classics in Anole Literature” Initiative—pdf contributions welcome!). They found that reproductive activity was most constant through the year in the two southernmost species examined, A. trinitatis and A. griseus from St. Vincent. In the remaining species, both sexes showed some degree of seasonal fluctuations, although reproductive activity by at least some individuals occurred in almost all months. The authors considered rainfall to be the primary factor driving variation in reproductive activity, although in a later paper, they reconsidered and suggested that seasonal temperature cycles were probably more important.
Despite the ease with which such data could be collected, relatively few studies in the past four decades have followed up on this work. One such study on a Colombian anole was featured in AA recently, but for the most part, little work of this sort has been conducted in recent years. Who knows what surprises await an anole comparative reproductive biologist?
Recently AA asked George Gorman for some thoughts on this work, and he provided the summary below of the several papers he and Paul Licht wrote in the first half of the 1970’s on anole reproductive cycles:
Licht and Gorman. 1970. University of California Publications in Zoology 95:1-52.
Our first survey of multiple species included anoles from St. Vincent (13 deg. N), Jamaica, Grand Cayman, and Hispaniola…plus two tropical species introduced into more northern latitudes (A. sagrei on Florida, and Jamaican A. grahami on Bermuda).
We wrote : There is a strong correlation between female reproductivity (e.g. egg production) and rainfall in the Caribbean. Egg production correlates with the severity (e.g. duration) of the dry season. These correlations were particularly striking in Jamaican species. Egg production in the northernmost population (A. grahami on Bermuda) appears independent of rainfall and may be dependent on temperature. Most of the females are probably potentially continuous breeders with seasonal cycles resulting from local climatic stresses (emphasis added).
Gorman and Licht. 1975. Copeia 1975:332-337
Here we were looking at two species on Trinidad that appeared to have been introduced from nearby Lesser Antillean islands, A. trinitatis from St. Vincent and A. aeneus from Grenada.
In the previously cited 1971 study, we noted that A. trinitatis was pretty much reproductively active throughout the year. It was the least cyclical of all populations examined in the 1971 study.
While we did not have baseline data on A. aeneus of from its native island, we surmised that the Trinidad population of aeneus was founded by individuals from the seasonally dry part of Grenada (pale grey lizards…reminiscent of A. cooki coloration; aeneus populations can be quite green in montane wetter forests).
We found that A. trinitatis was continuously reproductive throughout the study. Virtually 100% of females had enlarging follicles or oviducal eggs in all months between June 1972 and December of 1973 (only one animal in the very last sample, December 1973, was scored Class I; the class II animals [enlarging follicle, no oviducal egg] were at higher frequency in certain winter samples, but even in the coldest months/shortest days, generally 75-90% of females sampled had oviducal eggs.
The A. aeneus (which were sympatric and syntopic) were quite seasonal, with 80% to 100% completely non-reproductive in January, February and March.
“The populations of A. trinitatis and A. aenus studied on Trinidad exhibit greater differences in seasonal reproductive patterns than have been observed in any of the several other sets of syntopic Anolis studied.” p.334.
Our interpretation was that A. trinitatis, whose origin was from non-seasonal St. Vincent, was genetically programmed to remain reproductively active throughout the year; and that A. aeneus was programmed to turn off reproduction in the height of the dry season. There’s always an element of “Just SO Stories” in evolutionary interpretations. It appeared to us that A. aeneus was outcompeting and replacing A. trinitatis in areas of overlap. Because the lizards lived in conditions more reminiscent of seasonally dry Grenada than aseasonally wet St. Vincent, we suggested that A. trinitatis was “wasting energy” by not responding to seasonality cues…the assumption being that eggs would be stressed in the dry season. We had no evidence to back this up.
Gorman and Licht. 1974. Ecology 55: 360-369 (Puerto Rico female anole data)
In the first study, we implicated rainfall as determining egg laying cycles…with the observation that in northern latitudes, cool temperatures over-rode rainfall. Introduced populations had cycles reminiscent of temperate zone lizards.
In the second study, we suggested that at tropical latitudes not too different from the native islands, the introduced anoles were locked into seasonal cycling that may have been genetically programmed (i.e. the trinitatis “should have” responded to wet-dry seasonality, and did not).
In the 1974 Ecology paper, the important finding in cristatellus was that the female “upland” populations at Ranger house and Hacienda Roses precipitously declined in the last few months of the calendar year, whereas a significant portion of lowland cristatellus remained reproductively active throughout the year. Also, all the upland populations of evermanni, stratulus, gundlachi and krugi completely closed down female egg production in winter months (Rio Piedras stratulus showed a statistical cycle but a significant percentage of females were reproductively active throughout the year).
But note we did find year-to-year differences between 1971 and 1972… and patterns are not altogether consistent. If you examine Fig. 4… cristatellus…you will note The Ranger House cristatellus declined more precipitously in the second autumn-winter, but the lowland Virgin Island sample declined more steeply in the first year compared with the second year.
Licht and Gorman. 1975. Copeia 1975:496-504 (Puerto Rico, male cycles)
Transplant experiments indicated that male cycles were tuned to temperature, and in most cases, transplanted males responded to local climatic conditions (i.e. upland lizards transported to sea level were less dramatically cyclic; lowland populations moved to cool upland cages cycled like their wild conspecifics. Except for evermanni. “Results with three species, A. krugi, A. stratulus and A. cristatellus indicate that altitudinal differences in reproductive patterns…are probably largely climatically rather than genetically induced. However, the reproductive patterns of the upland species A. evermanni appears to be less plastic. Although its seasonal testis cycle is comparable to those of other upland forms, A. evermanni appears to be either independent of environment (i.e., cycle is endogenous) or it responds differently to environmental cues, because translocation to the lowland does not dampen its typical upland pattern of testis activity; in fact, the translocated males showed more prolonged regression than upland field controls
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alengelk
We’ve certainly had a lot of experience with the seasonal fluctuation of reproductive cycles during our years of husbandry here in the Glor lab. There seems to be a natural tapering off of egg production in the fall, and an adjustment of the temperature to a cooler “winter cycle” and back again to a warmer “summer cycle” in the spring is correlated with the changes in reproduction.
Anthony has also noted a staggering of the reproductive cycles of different subspecies in the lab throughout the progress of his experimental cross this year. While both subspecies do appear to be seasonal, there seems to be an offset of their peak production times. Perhaps this has something to do with their adjustment from their habitat in the wild to their common environment in the lab? Or perhaps there is an underlying genetic mechanism for the offset?