On the Contribution of Reproductive and Offspring Investment on Fertility: Human and Animal Societies

Differences in investment into reproduction or offspring rearing are plentiful throughout the world, from the cells inside our bodies to complex sociological interactions among humans. Such differences can lead to profound impacts on species' fitness, fertility, and reproductive rates, sometimes in startling ways. In this paper, we create a simple game-theoretical model to qualitatively investigate the effects of such differential investment. We focus on fertility in human societies and show that more wealthy individuals produce more offspring within a a mating group. However, when assortative mating mechanisms are introduced, this effectively leads to a speciation event, and a higher reproduction rate for poorer individuals is noticed, capturing what we call the "wealthy-to-poor switch". We discuss extensions and implications of this work to nupital gifts in ecology and to clonal competition in cancer cell lines under the influence of treatment.

Intraclutch eggshell colour variation in birds: are females able to identify their eggs individually?

Background One possibility suggested regarding female post-mating strategies is differential allocation into offspring investment. Female birds produce not only the largest, but also most colourful eggs of all oviparous taxa. Larger eggs provide space for bigger embryos, or more nutrition for their development, but the question why eggs are more colourful and why there is variation in eggshell colouration remains. In this context, the focus of interest has been to explain inter-clutch variation but in many bird species, eggshell colouration also varies within a clutch. Surprisingly, less attention has been paid to this phenomenon. Therefore, we propose the “female egg recognition” hypothesis, suggesting that mothers use colour characteristics to interpret egg attributes and allocate further investment into each egg accordingly. To evaluate the feasibility of the hypothesis, we tested several underlying predictions and examined their suitability using a dataset from our tree sparrow (Passer montanus) study. We predict (i) substantial within-clutch variation in eggshell colouration which, (ii) should be related to laying sequence, (iii) reflect egg quality and, (iv) should stimulate a female response. Methods Eggshell coloration data were obtained via digital photography under standardized conditions, taken after clutch completion. Lightness (L*), representing the achromatic properties of an egg has been chosen as the most important predictor in dark cavities and was related to egg quality and position in the nest. Results In our tree sparrows, first and mainly last eggs were less pigmented, providing information about laying order. Egg volume, which predicts chick quality, positively correlates with eggshell coloration. Finally, we could show that female tree sparrows placed darker, but not bigger, eggs into more central incubation positions. Discussion All basic prerequisites for the “female egg recognition” hypothesis are fulfilled. In this context practicability and feasibility of the hypothesis and alternative explanations are discussed. However future work is necessary to determine a direct effect on offspring condition.

Genomic imprinting is implicated in the psychology of music

Why do we sing to babies? Human infants are relatively altricial and need their parents' attention to survive. Infant-directed song may constitute a signal of that attention. In the rare genomic imprinting disorder Prader-Willi syndrome (PWS), typically paternally-expressed genes from chromosome 15q11–q13 are unexpressed, resulting in exaggeration of traits that reduce offspring investment demands on the mother. PWS may thus be associated with a distinctive musical phenotype. We report unusual responses to music in PWS. Subjects (N = 39) moved more during music listening, exhibited greater reductions in heart rate in response to music listening, and displayed a specific deficit in pitch discrimination ability relative to typically-developing adults and children (N = 589). Paternally-expressed genes from 15q11–q13, unexpressed in PWS, may thus increase demands for music and enhance perceptual sensitivity to music. These results implicate genomic imprinting in the psychology of music, informing theories of music's evolutionary history.