Symbiotic ants as an alternative defense against giraffe herbivory in spinescent Acacia drepanolobium

Abstract

We explore here the occurrence of aggressive ants in an apparently symbiotic relationship with the savanna tree Acacia drepanolobium and their effects on giraffe herbivory on the Athi-Kapiti Plains, Kenya. Trees taller than 1.3 m were more likely to be occupied by aggressive ants in the genus Crematogaster than were shorter trees. Ants wereconcentrated on shoot tips, the plant parts preferred by giraffes. Trees with relatively more foliage had more swarming ants than did trees with less foliage. The feeding behavior of individual freeranging giraffes on Acacia drepanolobium was studied. Giraffe calves exhibited a strong sensitivity to Crematogaster ants inhabiting A. drepanolobium, feeding for significantly shorter periods on trees with a greater number of aggressive ants. Older giraffes were apparently less sensitive to ants, and did not feed for shorter periods on trees with fuller foliage, despite significantly greater ant activity on these plants. The thorns of A. drepanolobium are significantly shorter than are the thorns of A. seyal, a species without symbiotic ants, a pattern that may indicate a trade-off between ants and thorns as defenses.

Temporal variation in extrafloral nectar secretion in different ontogenic stages of the fruits ofAlibertia verrucosaS. Moore (Rubiaceae) in a Neotropical savanna

“…In such cases, the presence of ants foraging on fruits may be nonfunctional, as ants may be inefficient in defending the host plant against some vertebrates (Ho¨lldobler & Wilson 1990;Stapley 1998;Rico-Gray & Oliveira 2007). Moreover, in some cases, the presence of ants may even be undesirable, as they may frighten some possible disperses (Madden & Young 1992;Stapley 1998;Thomas 1988). A recent study performed by Yamawo et al (2012b) showed that the plants can gradually shift from direct to indirect defense against herbivory during leaf aging.…”

Growth, biomass estimates, and charcoal production of Acacia drepanolobium in Laikipia, Kenya

“…Some thorns have swollen bases that con¯uence into large hollow pseudo-galls (hereafter galls), which are red when young and fade to black and grey-white when old. Obligate ants (Crematogaster nigriceps, C. sjostedti, C. mimosae, and Tetraponera penzigi) (Madden and Young, 1992;Young et al, 1997), occupy these swollen thorns.…”

“…However, its utilization by herbivores is hampered by the presence of ants that live in its galls. Together with the thorns (Milewski et al, 1991), the ants are believed to ward off herbivores by their stings and bites and to irritate feeding herbivores (Madden and Young, 1992;Young et al, 1997).…”

The importance of the encounter rate between ants and herbivores and of ant aggressiveness against herbivores in herbivore exclusion by ants on Vicia angustifolia L. (Leguminosae) with extrafloral nectaries

“…The efficiency of herbivore exclusion by ants is mainly influenced by two factors related to ant activities, namely the encounter rate of ants with herbivores on the plant and the ant aggressiveness against the herbivores (Letourneau, 1983;Madden and Young, 1992;Rocha and Bergallo, 1992;Gaume et al, 1997). The encounter rate of ants with herbivores on a plant depends on the number of ants visiting the plant and the foraging patterns of the ants on the plant .…”