Intraspecific variation in reproduction

 In my doctoral research, I examined how physiology influences reproductive investment in birds.  Egg size shows substantial variation within many species and yet the proximate and ultimate causes and consequences of this variation remain largely unknown (Christians 2002).  I therefore sought to establish causal links between physiology and phenotype, and to elucidate the costs of producing larger eggs.  To this end, I examined correlations between physiological parameters and reproductive performance (Christians and Williams 2001a; Christians and Williams 2001b), and also experimentally manipulated the reproductive physiology of free-living European starlings (Sturnus vulgaris) and captive zebra finches (Taeniopygia guttata). 

    For instance, to determine whether egg size was determined by the rate or duration of yolk growth, I developed a technique to measure the rate of uptake of yolk proteins from the circulation into the growing oocytes in vivo (Christians and Williams 2001b).  Investment in egg production was manipulated by treating breeding females with estradiol (Christians and Williams 1999b) or follicle-stimulating hormone (Christians and Williams 2002), hormones that are central to the female’s reproductive physiology.  The costs of producing eggs were also investigated by removing eggs from females and thereby inducing them to lay replacement eggs (Christians 2000).  The results of this work yielded insights into the mechanisms by which egg size is controlled, and the physiological nature of trade-offs between egg size and number.  Other research into avian reproductive physiology included manipulation of immune function in the field to test the hypothesis that trade-offs between current reproduction and future survival are mediated by interactions between reproduction and the immune system (Williams et al. 1999). 

 
I’ve never worked with guillemots (pictured), but I haven’t taken any good photos of starlings or zebra finches, and guillemots sure are good looking birds.

 
References

Christians, J.K. (2002) Avian egg size: variation within species and inflexibility within individuals.  Biological Reviews 77: 1-26. [PDF]

Christians, J.K., and Williams, T.D. (2002) Effects of porcine follicle-stimulating hormone on the reproductive performance of female zebra finches (Taeniopygia guttata).  General and Comparative Endocrinology 125: 121-131.

Christians, J.K., and Williams, T.D. (2001a) Intraspecific variation in reproductive physiology and egg quality in the European starling (Sturnus vulgaris).  Journal of Avian Biology 32: 31-37. [PDF]

Christians, J.K., and Williams, T.D. (2001b) Interindividual variation in yolk mass and the rate of growth of ovarian follicles in the zebra finch (Taeniopygia guttata).  Journal of Comparative Physiology - Part B 171: 255-261.  [PDF]

Christians, J.K. (2000) Producing extra eggs does not deplete macronutrient reserves in European Starlings (Sturnus vulgaris).  Journal of Avian Biology 31: 312-318. [PDF]

Christians, J.K., and Williams, T.D. (1999a) Organ mass dynamics in relation to yolk precursor production and egg formation in European starlings (Sturnus vulgaris).  Physiological and Biochemical Zoology 72: 455-461.

Christians, J.K., and Williams, T.D. (1999b) Effects of exogenous 17b-estradiol on the reproductive physiology and reproductive performance of European starlings (Sturnus vulgaris).  Journal of Experimental Biology 202: 2679-2685. [PDF]

Williams, T.D., Christians, J.K., Aiken, J.J., and Evanson, M. (1999) Enhanced immune function does not depress reproductive output.  Proceedings of the Royal Society (London) B. 266: 753-757. [PDF]