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

3. Reproduction
Newborn Joey Close-up of newborn 70 Days Old
150 Days Old 200 Days Old

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!

13. QUESTIONS?

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