Bold, recurring patterns are fascinating, and more so if they are unexplained. The consistency of species-area curves, some population cycles, and allometric rules have kept biologists interested in science for many years and many pages of literature. For biologists living in the tropics, perhaps the most striking of these diversity-defying patterns are the various latitudinal gradients in community and life history metrics. Science excites our curiosity in the search for universal rules, but large-scale, latitudinal changes hint that some things may be fundamentally different in different parts of the world. Could there be really such a thing as "tropical biology"?
One of the best documented of these latitudinal gradients is the tendency for avian clutch size to increase with increasing latitude, both at the intra-specific and interspecific levels, in the old and the new world, in the northern and southern hemispheres. Since there is no evidence that high-latitude populations grow faster than equatorial ones, it has been widely assumed that some other life-history trait must also change with latitude to compensate for the variation in clutch size. Thus, throughout the second half of the 20th century, the idea that adult tropical birds live longer than their temperate counterparts became widely accepted among ornithologists.
The acceptance of a latitudinal gradient in survival, however, was more based on common sense than hard data. So much so, that when Karr et al. (1990, The American Naturalist 136: 277-291) challenged the idea in a paper subtitled “Will the dogma survive?” there were not many voices sounding in defense of the “dogma”. Indeed, two decades later, Corey Tarwater and colleagues suggested that, if there is no latitudinal trend in the survival of adult birds, there could be a trend in the survival of juveniles (2011, Ecology 92: 1271-1281). Their work drew on detailed analysis of age-dependent survival in one Central American passerine species. Specifically, tropical juveniles would survive more than temperate juveniles while adults would show no relationship with latitude. This would imply relatively little difference between adult and juvenile survival in the tropics.
For a paper just published in The Auk (2018, 135: 299-313), Ferraz Lab graduate Alejandra P. Muñoz et al. use bird-banding data to investigate the latitudinal gradient in bird survival on two fronts. First, they quantify the effect of age on survival for forty species of Amazon forest passerines and find that adults of their tropical site have substantially higher survival probabilities than juveniles. Second, they compare their adult survival estimates with 342 survival estimates from 175 species from Peru to Alaska and find that survival does go down with increasing latitude. This latitudinal effect persists even after accounting for effects of migration mode, phylogeny, and time of data collection. The authors conclude that the latitudinal gradient in survival is a fact after all, at least as seen among New World forest passerines.
Muñoz et al.’s work benefitted a great deal from two recent advances. First, there is an ongoing transformation in how tropical ornithologists assess bird age. The Wolfe-Ryder-Pyle molt-cycle system, employed by Muñoz et al., is extremely useful for aging birds in populations that have poorly delimited breeding periods, which is typical of tropical regions. This was central for quantifying the effect of age on survival. Second, Muñoz et al. tapped into the phenomenal database of Vital Rates of North American Birds made available by the Institute of Bird Populations (IBP), which enabled them to analyze estimates from as far North as Alaska. The IBP estimates, combined with a wealth of tropical passerine survival estimates published since 1990, made it possible to take up Karr et al.’s (1990) challenge anew.
From the analytical perspective, Muñoz et al. model their Amazon data with a multi-species Cormack-Jolly-Seber (CJS) approach, a standard method for estimating apparent survival from capture-recapture data on open populations. They treat each of the forty species in their data as a random draw from a wider distribution of species, and the inference about age effects is made at the level of this wider distribution, strengthening the generality of the conclusions. The CJS accounts for the possibility of capture failure, and a "mixture" component of the model, developed by co-author Marc Kéry, makes it possible to incorporate birds of unknown age in the analysis. The work thus addresses a variety of sources of uncertainty before reaching its final conclusions.
The observation of a latitudinal trend in survival probability certainly helps understand the maintenance of a latitudinal trend in clutch size, but it need not be the only explanation. It is also possible, for example, that the number of clutches laid each year also vary with latitude in association with the length of the breeding season. With or without latitudinal change in number of clutches, one cannot tell whether the variation in survival documented by Muñoz et al. is a sufficient explanation for the current variation in clutch size. What's more, even if one explains the maintenance of current variation, there is still the question of how that variation evolved.
There is certainly much to explore in the latitudinal variation of bird life history traits. It is striking that a relatively high survival probability is found in regions where the bird ‘pace of life’ is relatively slow, as measured by their basal metabolic rate (Wiersma et al. 2007 PNAS 104: 9340-9345). Intuitively, ‘slow pace’ is coherent with ‘long-lived’, but why would tropical birds, which inhabit relatively hot environments, have a particularly slow basal metabolic rate? Is it not true that metabolic rate has a positive relationship with temperature (Clark & Fraser 2004 Funct. Ecol. 18: 243-251)? Or could it be that long-lived tropical forest passerines are cooler than their high-latitude counterparts?