Resource allocation across the egg laying sequence in the wild zebra finch Taeniopygia guttata

Abstract

Zebra finches have played a central role in the discovery of a variety of maternal effects over the past decade, with females shown to adjust resource allocation to their eggs in response to variables such as the appearance of their partner, their own condition, and the diet on which they are maintained. In addition to being the focus of some of the most high profile individual studies that have influenced maternal effects research in birds, the multitude of zebra finch studies together provide the most comprehensive set of data to illuminate general patterns and compare different maternally derived variables. Surprisingly, to date, virtually all of this work has focused on captive populations of the zebra finch that have been domesticated for many generations, and which are typically held under relatively constant environmental and dietary conditions. Here we report the first data on resource allocation across the egg laying sequence in a free-living wild population. Reassuringly we find that the patterns that have been found in the majority of studies of domesticated populations with respect to investment across the laying sequence were all present in the wild population. The size and mass of eggs increased through the laying sequence whilst the concentration of carotenoids significantly decreased across the laying sequence. Although there was no significant pattern with respect to testosterone across the laying sequence the first two eggs had a higher level of testosterone than the last few eggs in the clutch, which is also consistent with the findings of earlier studies in captive populations. Reassuringly we find that the patterns that have been found in the majority of studies of domesticated populations with respect to investment across the laying sequence were all present in the wild population. The size and mass of eggs increased through the laying sequence whilst the concentration of carotenoids significantly decreased 45 across the laying sequence. Although there was no significant pattern with respect to testosterone across the laying sequence the first two eggs had a higher level of testosterone than the last few eggs in the clutch, which is also consistent with the findings of earlier studies in captive populations.

Egg size is unrelated to ambient temperature in the opportunistically breeding zebra finch

“…These results suggest that although ambient incubation during hot breeding conditions might affect the development time of eggs even before they receive parental incubation (Griffith et al ), females are not adaptively adjusting egg volume in the way we predicted to try and counter the potential head start thus provided to first laid eggs in a clutch by ambient incubation. As with previous studies of this species in captivity (reviewed by Griffith and Buchanan ), and the wild (Griffith et al ), we found intra‐clutch variation in egg size, with eggs generally increasing in size by around 4% with each position in the laying order, and the average difference between the smallest and largest egg within a clutch being 13–23%. In other species this magnitude of variation has been shown to affect offspring development and fitness (Williams , Krist ), and the cause of this intra‐clutch variation in the well‐studied zebra finch remains to be explained.…”

“…The average difference in egg mass between the first and fifth egg in these studies was found to be 12.5% (Griffith and Buchanan ), and laying order biases in investment are also seen with respect to the concentration of testosterone, carotenoids and vitamins (reviewed by Griffith and Buchanan ). Although most of the work on zebra finches (cited above), has focused on domesticated populations of zebra finches in the laboratory, similar patterns of investment have also been reported from wild birds (Griffith et al ).…”

“…We do know that other maternal investment strategies are used in this species, and contribute to systematic variation across the clutch (reviewed by Griffith and Buchanan , Griffith et al ). Perhaps differential investment in the contents of the egg (across a clutch) might be deployed as a better mechanism through which late‐laid eggs can be favoured in hot ambient conditions.…”

“…Perhaps differential investment in the contents of the egg (across a clutch) might be deployed as a better mechanism through which late‐laid eggs can be favoured in hot ambient conditions. For example, differences in androgens across the egg laying sequence have been demonstrated (Rutstein et al , Williams et al , Griffith et al ), and also affect the competitive ability of developing offspring (von Engelhardt et al ). It would be worth exploring variation in the allocation of such parameters in relation to the atmospheric temperature during breeding, using a similar approach to that which we have taken here.…”

When mothers make sons sexy: maternal effects contribute to the increased sexual attractiveness of extra-pair offspring

“…If, for some reason, early laid eggs are more likely to be fertilized by extra-pair partners, as has recently been found [21,22,63], then mothers willindirectly, and not necessarily adaptively-invest more resources in EPO. Indeed, maternal egg investment has been consistently found to vary across the laying sequence, with, for example, levels of yolk androgens and carotenoids decreasing with laying order in zebra finch clutches [64,65]. This scenario could explain why there were no phenotypic differences between social and extra-pair mates in our study, and it illustrates that selection might act on a male's ability to obtain paternity over certain eggs in a female's laying sequence (via sperm competition, for example), rather than on females to choose particular males as extra-pair partners ( [7,9,10], but see [66]).…”