Life history strategies in a North American red squirrels

population

 Sébastien Descamps

PhD candidate

Under the supervision of

Dominique Berteaux and Jean-Michel Gaillard

Life history strategies?

Co-adaptation of demographic parameters

Response to evolution

Demographic tactics

« Life histories are the probabilities of survival and the rates of reproduction at each age in the life span »

Partridge & Harvey 1988

The red squirrel at Kluane

• > 15 years of data

• > 1200 males and 1200 females of known age

• Reproduction (litter size, mass, growth rate…)

• Low dispersion

• Food availability index

Questions:

1- Senescence

Age

Survival

Reproduction

Questions:

2- Cost(s) of reproduction

Age

Survival

Reproduction

trade-off

Questions:

3- Cohort effects

Year of birth

trade-offYear of birth

Survival

Reproduction

Age

Questions:

4- Demographic modelisation

Population dynamic

Year of birth

Age

Survival

Reproduction

trade-off

Small mammals:

• Lots of results with mice in the lab

• Very few results in natural conditions, and with small mammals other than mice or voles

Senescence

Red squirrel at Kluane

> 50 individuals older than 5 years old (without the 2003 dataset!)

Preliminary results showing female survival decreasing with age:

Senescence

Females’ survival

Age (years)0 1 2 3 4 5 6 7 8 9

Su

rviv

al

0.0

0.2

0.4

0.6

0.8

1.0

Full-age dependant model : (a) p

FemalesModel Dev np AIC

(j; ]1 yr, 5yr[ ; >5 yr) p 2319 40 2399

(j; >1 yr) p 2349 30 2409

Females’ survival3 age classes: 1 juvenile stage, 1 prime-age stage, 1 senescent stage

2 age classes: 1 juvenile stage, 1 adult stage

Likelihood Ratio Test between

Model with 2 adult classes: ]1 yr, 5yr[; >5 yr

and

Model with 1 adult class: >1 yr

Chi-sq. df Prob.

-------------------------

29.521 7 0.0001

=> Senescence

Females’ survival

=> 2 stages for adults

Females’ survival

1986 1988 1990 1992 1994 1996 1998 2000 2002

Sur

viva

l

0.0

0.2

0.4

0.6

0.8

1.0

Prime age FemalesSenescent Females

Males’ survival

Age (years)0 2 4 6 8 10

Su

rviv

al

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Model with 1 juvenile class and 1 adult class

(j; >1 yr) p(t)

=0.690.21

Males’ survival

•No senescence for males?

•Senescence later in life?

•Strong selection on males => only the best males survive until 5 years and more?

Some theories explain the existence of senescence by the existence of reproduction costs

=> Important to study the reproduction costs to understand the effects of age on reproduction/survival

Previous experimental work:

Koskela et al. 1998 on Bank Vole

=>Manipulation of reproductive effort=>Manipulation of environmental conditions

Systems allowing such manipulations are rare!!!

Expression of the costs of reproduction according to food availability ?

Experimental projectRed squirrel at Kluane:

• Litter size manipulation (Humphries and Boutin 2000)• Food addition (seed buckets)

2 treatments experiment:

Litter size

Increased

+2

Decreased

-2

Food addition

No food addition

Food addition

No food addition

Experimental project

Hypotheses:

Litter size

increased

decreased

Food add

No food

No food

Food add

Group 1

Group 2

Group 3

Group 4

Success Group 1 = Success Group 3

Success Group 2 < Success Group 4

significant cost of reproduction when food scarcity

Experimental projectRed squirrel at Kluane:

Year 2004: low food availability (low cone production since 1999, and very low in 2003)

Red squirrels should respond to experimental food addition

Good opportunity to study the expression of reproductive costs according to food availability

Year of birth

trade-offAge

Survival

Reproduction

Population Dynamic

Conclusion