Life history variation in Galliformes across elevational gradients: A test for fecundity/offspring quality trade-off in precocial species

Background/Question/Methods

Elevational gradients offer an excellent opportunity to understand how environmental variation shapes life history variation. Patterns from altricial passerines show that higher elevation species and populations have reduced fecundity through laying smaller clutches and raising fewer broods, have longer developmental periods, and exhibit increased parental investment in the form of nestling and fledgling care. This may result from a trade-off with survival and/or a trade-off with offspring quality resulting from increased parental investment in the form of feeding young, an energetically expensive behavior. In this study, I use phylogenetically controlled methods to test the generality of the fecundity/offspring quality trade-off by analyzing patterns of life history variation (female body mass, clutch size, egg mass, egg volume, clutch mass and  incubation period) in 181 species of Galliformes across elevational gradients after also controlling for allometry and breeding latitude. I test the predictions:

1.If nestling and fledgling care is a means of parental investment in higher elevation altricial species and influences trade-offs with clutch size, then precocial species lacking such care should not show variation in clutch size across elevational gradients.

2. Higher elevation precocial species should increase parental investment and this may occur through investment in egg mass. 

Results/Conclusions

As predicted, clutch size did not vary with elevation whereas egg mass increased after controlling for female body mass and latitude. Incubation periods were longer for high elevation species. Female body mass and egg volume did not vary with elevation. 

At higher elevations, limited breeding time in conjunction with harsher environmental conditions selects for higher offspring quality over offspring number. Parental care in altricial species is energetically intensive; lack of such care may free resources to allow precocial species to lay larger clutches than altricial species. Altricial nestling care may thus drive a trade-off with clutch size found in higher elevations. Fecundity in precocial species that lack this care do not trade-off, and thus clutch size does not vary across elevational gradients. Egg mass is positively related to offspring weight and survival in altricial and precocial species and thus influences offspring quality. Although egg mass is a significant means of parental investment in precocial species, nestling care can override effects of egg mass in altricial species. Thus precocial, but not altricial, species show a positive egg mass/elevation correlation. Both types of species show a fecundity/offspring quality trade-off along an elevational gradient, but manifest it in different ways.