1. Effects of Rearing Condition on Parent Fitness and Offspring Development Alex I. Wiesman, Ashley Bowling, & Rosemary Strasser
Introduction
Parental care is the level of investment provided by parents after birth to offspring at a detriment to their own fitness. Uniparental care (the raising of the offspring by one parent) is the most common pattern of naturally occurring care in the animal kingdom, however biparental care is more common among avian species. Biparental care is the presence of both father and mother in the raising of offspring. Previous research in zebra finch, a biparental species, has found that biparental care can result in sexual conflict between the parents which could subsequently negatively effect the development of offspring (Royle, Hartley, & Parker, 2002, 2006). In comparison, uniparental (mother only) conditions positively effected the growth and development of offspring, while adversely effecting her maternal fitness. It was theorized that this positive effect on offspring s was due to an increased parental investment from the mother resulting from a lack of sexual conflict (Royle, Hartley, & Parker, 2002, 2006). What is unclear from this previous research is what other factors, such as hormones in the mother and offspring, might facilitate these differences in offspring growth and fitness of the mother.
We replicated the Royle et al. experiment, while also collecting additional measures of offspring growth, development of secondary sex traits in offspring, and of hormone activity of both offspring and parents. In addition to the measure of mass used in the Royle study, we used additional measures of offspring quality such as tarsus (bone) length and beak and cheek coloration which are secondary sex traits important for mate selection. We also examined the consequences of rearing condition on parents body mass as an indicator of investment. Finally, we collected hormone samples from both babies and parents to examine if stress hormones could explain the effects on physiology and morphology. We expected to find similar results to the Royle study for mass and parental investment, and predicted that the stress hormone corticosterone is responsible for some of the effects associated with differences in the rearing condition.
Methods
Subjects. Rearing groups were assigned according to number of offspring hatched, where uniparental groups reared two chicks and biparental reared three or four chicks. A total of 24 birds were matched in pairs with rearing method being variable across two sets of clutches.
Procedure. Body mass and fecal samples of parents were collected for a baseline measurement. Parents were assigned to uniparental and biparental conditions at 3-5 days after hatching according to number of offspring. At 48 hours, one week, 20 days and 35 days, parental fecal samples were collected and body mass measured. At these same intervals, baby fecal samples, body mass and tarsus measurements were collected. Beak and cheek ratings of chicks were measured at 20, 25, 30, 35, 40, 45, and 50 days. After 35 days, chicks were housed separately and parents were reassigned to conditions and the procedure was repeated for a second set of clutches.
Results
Uniparental female parents had significantly greater weight loss at one week. Chicks in uniparental and biparental conditions had no significant difference in body mass, however uniparental chicks had increased tarsus growth compared to those in the biparental condition. Hormone samples as well as beak and cheek coloration will be analyzed next to examine if differences in hormone levels can further elucidate the effects of rearing conditions on offspring sexual development and fitness of the parents.
Conclusion
Our data supported the study conducted by Royle, Hartley & Parker (2002), which found that uniparental female parents had increased parental investment suggested by an increase in weight loss when compared to biparental parents, specifically after the first week. However, our results contradicted Royle’s findings regarding differences in offspring weight between the rearing conditions as no significant difference was found. Growth of offspring, measured by bone development was determined to be significantly greater in the uniparental chicks. This could be interpreted as a benefit gained from the increased parental investment of the mother; alternatively, this may be linked to corticosterone levels in the biparetnal condition which could subsequently influence and androgen levels resulting in increased longitudinal bone growth (Clarke, & Khosla, ,2009; Manolagas, Kousteni, & Jilka, 2002).
We intend to perform hormone assays on the remaining samples collected to determine if differences exist between rearing conditions. Preliminary data shows a significant increase in corticosterone levels in offspring and parents in the biparental condition . The Royle study also found an increase in male attractiveness as adults of those reared in the uniparental condition, and future studies may examine the effects of uniparental conditions on the attractiveness of male offspring to females.
Reference
Clarke, B.L. & Khosla, S. (2009). Androgens and bone. Steroids. 74(3): 296–305
Manolagas, S. C., Kousteni, S., & Jilka, R. L. (2002). Sex steroids and bone. Recent Progress in Hormone Research, 57(1),
Royle, N. J., Hartley, I. R., & Parker, G. A. (2006). Consequences of biparental care for begging and growth in zebra finches, taeniopygia guttata. Animal behaviour, 72(1),
Royle, N.J., Hartley, I.R., & Parker, G.A. (2002). Sexual conflict reduces offspring fitness in zebra finches. Nature, 416 (6882),
Figure 1. Parent mass loss. Using an ANOVA, we found that parents in the test (uniparental) group lost more weight than those in the control (biparental) group after the first week, F(1,1) = 4.45, p = .046.
Figure 2. Offspring mass. Using an ANOVA, we found no difference in offspring mass across development between the test and control conditions (contradicting Royle’s findings), and a gradual increase in mass across development (F(1,3) = .619, p = .43; F(3,3) = 451.2, p = <.001, respectively).
Figure 3. Offspring tarsus growth. Using an ANOVA, we found that offspring in the test condition had increased tarsus growth compared to those in the control condition, F(1,3) = 4.278, p = .043.