Effect of the habitat and tusks on trunk grasping techniques in African savannah elephants

Abstract

Among tetrapods, grasping is an essential function involved in many vital behaviours. The selective pressures that led to this function were widely investigated in species with prehensile hands and feet. Previous studies namely highlighted a strong effect of item properties but also of the species habitat on manual grasping behaviour. African savannah elephants (Loxodonta africana) are known to display various prehensile abilities and use their trunk in a large diversity of habitats. Composed of muscles and without a rigid structure, the trunk is a muscular hydrostat with great freedom of movement. This multitasking organ is particularly recruited for grasping food items while foraging. Yet, the diet of African savannah elephants varies widely between groups living in different habitats. Moreover, they have tusks alongside the trunk which can assist in grasping behaviours, and their tusk morphologies are known to vary considerably between groups. Therefore, in this study, we investigate the food grasping techniques used by the trunk of two elephant groups that live in different habitats: an arid study site in Etosha National Park in Namibia, and an area with consistent water presence in Kruger National Park in South Africa. We characterised the tusks profiles and compared the grasping techniques and their frequencies of use for different foods. Our results show differences in food-grasping techniques between the two groups. These differences are related to the food item property and tusk profile discrepancies highlighted between the two groups. We suggest that habitat heterogeneity, particularly aridity gaps, may induce these differences. This may reveal an optimisation of grasping types depending on habitat, food size and accessibility, as well as tusk profiles.

Keywords: Loxodonta africana; Proboscidea; food manipulation; muscular hydrostat; prehension; skill.

FIGURE 1

Illustrations of the observed elephant tusks presence, breaking, symmetry, opening, curvature and size modalities. The illustrations represent both tusks for better understanding, however each tusk has been studied independently.

FIGURE 2

Illustration of the criteria of the trunk grasping techniques. The first criterion is the grasping type, in particular the trunk posture: the pinch with the fingers of the trunk tip, sometimes helped with a suction (a) and the wrap of the trunk around an item (b). Pictures by R. Cornette and J. Soppelsa. The second criterion is the trunk parts used in the movements represented in schematic form: the base, the shaft and the tip (c).

FIGURE 3

Boxplots of the pinch rate per male individual according to the habitat. The grasped food items were the grass, leaves and small branches. The box at the top left indicates the individuals studied (here, males) and the items considered (here, grass, leaves and small branches). Black diamonds indicate the average pinch rate for each habitat. Chi‐squared test: p < .05.

FIGURE 4

Boxplots of the tip grasp rate per male and female individual according to the habitat. The grasped food items were grass, leaves and small branches. Black diamonds indicate the average trunk tip use rate for each habitat. Chi‐squared test: p < .001.

FIGURE 5

Boxplots of the right grasp rate per male and female individual according to the habitat. The grasped food items were the grass, leaves and small branches. Black diamonds indicate the average right rate for each habitat. Chi‐squared test: p < .05.

FIGURE 6

Correlation matrix between the variables of the tusk profile, for Etosha (N _Individuals = 8) and Kruger specimens (N _Individuals = 31). The darker the square, the stronger the correlation between the variables.

FIGURE 7

Multiple correspondence analysis plots of the tusk profiles for Etosha (N _Individuals = 8) and Kruger (N _Individuals = 31) adult specimens. Dots represent every observed elephant having two tusks and are coloured according to their habitat localisation. Only variables with a contribution greater than or equal to 5% are displayed. Modalities of the tusk profile are displayed in text format.

FIGURE 8

Barplots of adult elephant tusks profiles in the Kruger habitat and Etosha habitat. The different tusk profile criteria significantly different between Kruger and Etosha groups are the tusk break (a), the tusk curvature (b) and the left (c) and right tusk size (d).

FIGURE 9

Boxplots of the lateral grasp rate per male and female individual according to the left tusk opening profile. The grasped food items were the grass, leaves and small branches. Individuals from Kruger and Etosha were all considered. Black diamonds indicate the average lateral rate for each left tusk opening profile. Chi‐squared test: p < .05.

FIGURE 10

Boxplots of the tip grasp rate per male individual according to the right tusk opening profile. The grasped food item was the grass. Individuals from Kruger and Etosha were all considered. Black diamonds indicate the average trunk tip use rate for each right tusk opening profile. Chi‐squared test: p < .05.

FIGURE 11

Boxplots of the pinch rate per male individual according to the tusk presence profile. The grasped food items were grass, leaves and small branches. Individuals from Kruger and Etosha were all considered. Black diamonds indicate the average pinch rate for each tusk‐presence profile. Chi‐squared test: p < .001.

FIGURE 12

Boxplots of the tip grasp rate per male individual according to the tusk presence profile. The grasped food items were grass, leaves and small branches. Individuals from Kruger and Etosha were all considered. Black diamonds indicate the average trunk tip use rate for each tusk‐presence profile. Chi‐squared test: p < .05.

FIGURE 13

Boxplots of the tip grasp rate per male individual according to the tusk‐breaking profile. The grasped food items were grass, leaves and small branches. Individuals from Kruger and Etosha were all considered. Black diamonds indicate the average trunk tip use rate for each tusk‐breaking profile. Chi‐squared test: p < .01.