13th Savanna Network Meeting
March 11th 2015
The need for speed
How do elephants mitigate travel time constraints as
the dry season progresses in Hwange NP Zimbabwe ?
[email protected]
Hugo Valls Fox (CEFE, CNRS, Université de Montpellier, France)
Simon Chamaillé-Jammes (CNRS, Montpellier, France)
Hervé Fritz (CNRS, Lyon, France)
Michel De Garine-Wichatitsky (CIRAD, Harare, Zimbabwe)
1
Introduction : Surface water determines forage availability
• In savannas, herbivores must access both water and forage.
2
Introduction : Surface water determines forage availability
• In savannas, herbivores must access both water and forage.
• As the dry season progresses:
- Water sources are scarcer and further apart
3
Introduction : Surface water determines forage availability
• In savannas, herbivores must access both water and forage.
• As the dry season progresses:
- Water sources are scarcer and further apart
- Herbivore aggregate around remaining water sources
4
Introduction : Surface water determines forage availability
• In savannas, herbivores must access both water and forage.
• As the dry season progresses:
- Water sources are scarcer and further apart
- Herbivore aggregate around remaining water sources
- Forage is depleted close to water
5
Introduction : Surface water determines forage availability
• In savannas, herbivores must access both water and forage.
• As the dry season progresses:
- Water sources are scarcer and further apart
- Herbivore aggregate around remaining water sources
- Forage is depleted close to water
How do individuals cope with increasing distances
between food and water ?
6
Introduction : Individual response in space and time
??
7
Introduction : Individual response in space and time
• Space ?
- Reducing spatial constraint -> selecting suboptimal
foraging patches.
8
Introduction : Individual response in space and time
• Space ?
- Reducing spatial constraint -> selecting suboptimal
foraging patches.
• Time ?
- Reduce drinking frequency when temperatures are low.
- Reduce time allocated to foraging and resting?
Eg. Sable antelope Kruger NP (Cain et al 2012)
9
Introduction : Individual response in space and time
• Space ?
- Reducing spatial constraint -> selecting suboptimal
foraging patches.
• Time ?
- Reduce drinking frequency when temperatures are low.
- Reduce time allocated to foraging and resting?
Eg. Sable antelope Kruger NP (Cain et al 2012)
• Reduce traveling time ?
- Increase travelling speed or trip straightness ?
Eg. Birds (Hedenström & Alerstam 1995)
10
Hwange National Park: 15 000 km2 of dystrophic savanna
Study Area
Hwange
National
Park
Zimbabwe
waterholes
11
Introduction: Resource limitation during the dry season
Number of pans in study area
-> Travel further
Temperature (°C)
40
Rain
Cold dry
Hot dry
30
High T°C
Drink
more
often
20
10
0
Apr.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
12
Introduction: GPS collar data and visits to water
Jun - Oct 2013
8 family groups
November 2012
collared 13 adult
elephant cows
901 foraging trips
GPS: 1 loc/30 min
Foraging trip : Portion of movement
track between two visits to a waterhole.
13
Introduction : Why elephants?
• Dominant herbivore Hwange NP
(Fritz et al 2011)
• Major impact on vegetation
(Valeix et al 2007, 2011)
• Elephants spend ±17 hours a day foraging
(Owen-Smith 1989)
• Elephants are water dependent and must
drink regularly (Chamaillé-Jammes 2013)
If we could figure out some
high energy diet so we
would’nt have to spend the
whole day eating !
Introduction : Elephant foraging trips & drinking
Commuting
Change waterhole
Chamaillé-Jammes et al. 2013
looping
Same waterhole
15
Introduction : Elephant foraging trips & drinking
looping
Commuting
Change waterhole
24h
38%
48h
41%
72h
16% Percent of
time budget
Frequency
Short
5%
Same waterhole
Trip duration (hours)
Chamaillé-Jammes et al. 2013
16
Results : outline
How often do elephants drink ?
The
Trade-off How far from water do they travel to forage?
Results : outline
How often do elephants drink ?
The
Trade-off How far from water do they travel to forage?
Minimize How fast do they travel from water and back?
travel
How straight do they go between two patches ?
time
18
Results : outline
How often do elephants drink ?
The
Trade-off How far from water do they travel to forage?
Evolution throughout the dry season
Minimize How fast do they travel from water and back?
travel
How straight do they go between two patches ?
time
19
methods: statistical analysis
Response variables:
- Number of visits
- Maximum distance
to water
- Travelling speed
- Trip straightness
Mixed model framework
Fixed effect
3rd degree polynomial of
Julian date
Random effects
on all parameters
by individual
20
results: Elephants drink more often.
Number of trips
Number of trips over a 10 day period
21
results: Elephants drink more often.
Number of visits
Mean Temperature
22
results: Elephants travel further at the end of the dry season
23
Maximum distance to water (km)
results: Elephants travel further at the end of the dry season
24h loop
24
Maximum distance to water (km)
results: Elephants travel further at the end of the dry season
24h commute
24h loop
25
Maximum distance to water (km)
results: Elephants travel further on 48h trips compared to 24h trips
24h commute
24h loop
48h commute
48h loop
26
results: Changes in speed
Returning
speed
Outgoing
speed
27
starting speed (m/s)
results: 24h trip outgoing speed increases in the late dry season
24h outgoing
commute
24h outgoing
loop
28
Finshing speed (m/s)
starting speed (m/s)
results: returning speed increases earlier on for 24h trips
24h outgoing
24h
outgoing
commute
commute
24h returning
24h
returning
commute
commute
24h outgoing
loop
24h returning
loop
29
Finshing speed (m/s)
starting speed (m/s)
results: 48h trips speed
24h
48houtgoing
outgoing
commute
commute
48h returning
commute
48h outgoing
loop
24h returning
48h
returning
loop
loop
30
results: Measuring trip straightness
Commute
B
𝐴𝐵
𝑆=
𝐿
B
Loop
2 × 𝐴𝐵
𝑆=
𝐿
A
31
results: Commutes become straighter but loops are always straight
24h loop
Trip straightness
24h commute
32
results: straightness is similar 48h trips and 24h trips
Trip straightness
24h commute
48h commute
24h loop
48h loop
33
Synthesis: Do elephant optimize travelling ?
48h
commute
Trip
speed
24h
commute
48h
loop
24h
loop
Trip straightness
34
Synthesis: Do elephant optimize travelling ?
Upper speed limit ?
48h
commute
Trip
speed
24h
commute
48h
loop
Travelling
in straight
lines ?
24h
loop
Trip straightness
35
Conclusion & Discussion
drinking
vs.
foraging
• Elephants travel faster & straighter as the dry season progresses.
• Do other herbivores respond similarly or do these patterns reflect
elephants’ very large body size?
Low energetic cost of accelerating
(Langman et al 1995)
vs.
thermoregulatory constraints
(Rowe et al 2013, Dunkin et al 2013)
36
??
37