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Mini Symposium
1. Offspring sex varies with
maternal investment ability:
empirical demonstration based on cross-
fostering
Authors: Kylie A. Robert, Lisa E. Schwanz
and Harriet R. Mills
Presented By: Jennifer Driskell
2. Background-Tammar Wallabies
What do they look like? What do they eat?
Grass and leaves
Where do they live?
How long do they live?
~ 14 years
4. Reproduction Continued
After about a 28 day gestation, a single Joey
will be born
The Joey will climb into the pouch and attach
its mouth to a teat to continue developing
If the mother becomes pregnant while a Joey
is in the pouch, the embryo’s development
will pause until the Joey leaves the pouch
5. The Aims of the Study
Examine whether offspring sex is correlated
with maternal investment ability (likelihood of
weaning an offspring)
To test the assumption that maternal
investment has a greater influence on the
fitness of sons than of daughters (survival of
offspring)
6. The Bigger Picture
To determine if maternal investment plays a
role in the sex-ratio of populations
Cost of Maternal Care:
-Time
-Energy
-Immunity
-Increased predation risk
Benefits of Maternal Care:
-Increased survival of young
7. Materials and Methods
32 adult female tammar wallabies were
captured in Tutanning Nature Reserve,
Western Australia
All were carrying a single young (13 daughters, 19
sons) less than 100 days old
Females were:
Weighed
Measured (foot length mm)
Ear tagged
8. Materials and Methods Cont.
Young were: Treatment Groups:
Weighed (Birthed/Fostered)
Sexed (visually) Group 1: Female/Female
Micro-chipped Group 2: Female/Male
Assigned to 1 of 5 Group 3: Male/Female
treatment groups Group 4: Male/Male
Re-attached to teats Group 5: Own offspring
after brief removal from
Measured teat
Offspring survival and body
size at weaning (~300 days)
and ~1 year of age
9. Results
No difference in maternal mass or maternal
condition index at end of study or at weaning
Weaning success was 61.5% for all cross-fostered
offspring and 50% for sham offspring
Cross-fostering did not reduce weaning success
Mass of surviving offspring at weaning was not
influenced by the sex of offspring originally
produced, foster sex, or their interactions
10. Results Continued
Graph A indicates that a mother that originally birthed a son had a significantly
greater probability of weaning her pouch young irrespective of its sex than a mother
that birthed a daughter
Graph B indicates that although weaning success of male offspring was more
strongly influenced than that of a female offspring based upon the sex of the
originally produced offspring, the cross-foster nor foster sex were significant
predictors of weaning success
11. Discussion
This study provides evidence that female tammar
wallabies with greater investment ability give
birth to more sons.
This study failed to provide evidence that the
fitness of sons depended on maternal investment
more than the fitness of daughters.
Small sample size
Offspring success based off of captivity
12. Back to the Bigger Picture
Natural Selection vs. Population Balance
Natural Selection selects for the most “fit”
In this case, the most fit would be the mother capable of providing a greater
maternal investment ability
That mother produces more sons
Thus natural selection inadvertently would be selecting for more male wallabies
This can be a huge problem:
Our guest lecturer stated that in hunting if you kill all but 1 buck he could still
fertilize all of the females
However if there was only one female she would only produce so many offspring
In the case of the wallabies they would most likely be males
This was one of the first studies that looked at sex-allocation so
it will be interesting to see where the research goes from here!
14. References
Article:
• Robert, Kylie A., Lisa E. Schwanz, and Harriet R. Mills.
"Offspring sex varies with maternal investment ability:
empirical demonstration based on cross-fostering." Biology
Letters 618 Nov. (2009): 242-45. Web. 3 Apr. 2012.
Images:
• http://gallery.photo.net/photo/6529679-md.jpg
• http://classes.ansci.illinois.edu/ansc438/lactation/marsupials.
html
• http://ih3.redbubble.net/image.6437941.9531/flat,550x550,0
75,f.jpg
• http://www.iayork.com/Images/2009/7-22-
09/TammerWallaby.jpg
Notes de l'éditeur
What do they look like?-18 inches from head to base of tail-Mostly grayish brownWhere do they live?-Southern and Southwestern Coastal Australia-Some of the nearby islandsWhat do they eat?-Herbivorous-Spend their night’s feeding-Can drink sea water when fresh water is limited
Reproduction-After about a 28 day gestation, a single Joey will be born-The Joey will climb into the pouch and attach its mouth to a teat to continue developing-If the mother becomes pregnant while a Joey is in the pouch, the embryo’s development will pause until the Joey leaves the pouch
Maternal Condition Index- calculated as the residuals from a regression between maternal mass and pes length.
GRAPH A-a mother that originally birthed a son had a significantly greater probability of weaning her pouch young irrespective of its sex than a mother that birthed a daughter. -this indicates that mothers with greater investment ability were more likely to produce sons.GRAPH B-although weaning success of male offspring was more strongly influenced than that of a female offspring based upon the sex of the originally produced offspring, the cross-fostering nor foster sex were significant predictors of weaning successHigher weaning success for mothers that birthed sons occurred in both male and female foster young, thus it is clearly attributed to maternal investment during lactation
The role of maternal investment ability was not reflected in relationships between maternal mass or condition and offspring sex. Maternal condition at fertilization may be more closely linked to offspring sex than the measures at cross-fostering.Survival to weaning in sons appeared to be more strongly influenced than in daughters by maternal investment ability; however this effect was not significant.
Natural Selection selects for the most “fit” -In this case the most fit would be the mother capable of providing a greater maternal investment ability-That mother produces more sons -Thus natural selection inadvertently would be selecting for more male wallabiesThis can be a huge problem:-Guest lecturer stated that in hunting if you kill at but 1 buck he could still fertilize all of the females-However if there was only one female she would only produce so many offspring and they would likely be malesThis was one of the first studies that studied sex-allocation so it will be interesting to see where the research goes from here.
