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

Background-Tammar Wallabies

What do they look like?

How long do they live?

~ 14 years

What do they eat? Grass and leaves

Where do they live?

ReproductionNewborn Joey Close-up of newborn 70 Days Old

150 Days Old 200 Days Old

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

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)

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

Materials and Methods

32 adult female tammar wallabies were captured in Tutanning Nature Reserve, Western AustraliaAll were carrying a single young (13 daughters, 19

sons) less than 100 days oldFemales were:

WeighedMeasured (foot length mm)Ear tagged

Materials and Methods Cont.

Young were:WeighedSexed (visually)Micro-chippedAssigned to 1 of 5

treatment groupsRe-attached to teats

MeasuredOffspring survival and body

size at weaning (~300 days) and ~1 year of age

Treatment Groups: (Birthed/Fostered)Group 1: Female/FemaleGroup 2: Female/MaleGroup 3: Male/FemaleGroup 4: Male/MaleGroup 5: Own offspring

after brief removal from teat

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 offspringCross-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

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

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

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!

QUESTIONS?

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,550x550,075,f.jpg• http://www.iayork.com/Images/2009/7-22-09/TammerWallaby.jpg