Myrmecophytes may adjust the investment on ant rewards, depending on tree size and ant defence level. In swollen-thorn acacias (Vachellia collinsii), we tested whether the level of protection provided by the resident ants (defending vs. non-defending) influenced the relation between tree size and ant rewards, or between types of ant rewards (housing and food). We quantified ant rewards in trees occupied by defending and by non-defending ants. We predicted: (1) a positive relation between plant diameter and ant reward investment, with a steeper slope for defending than for non-defending ant species; and (2) that if there is any tradeoff between ant rewards, it should be aggravated (steeper slope) when inhabited by non-defending ants. We found that most structures for ants grew according to plant diameter, but contrary to our first prediction it was independent of the level of ant defence. Most ant rewards did not show a tradeoff between them, besides a weak negative relation between spine length and number of pinnules, which contrary to the prediction occurred when occupied by defending ants. The evidence shows that the interacting ants had a weaker influence on the scaling of defence structures in myrmecophytes than the habitat (location).

Foraging parameters of two ant species, Pseudomyrmex triplarinus and Crematogaster sp., on P. triplarinus-occupied shrubs of Triplaris surinamensis were evaluated in a semi-deciduous forest of Mato Grosso, western Brazil. Live workers of the termite Microcerotermes strunckii, used as baits for ants, were placed on leaves in the lower, medium and upper thirds of the crown of thirty experimental Triplaris (nine baits/plant). Besides attacking more than twice as many baits as Crematogaster (131 against 59), Pseudomyrmex also attacked them signifi cantly faster. Pseudomyrmex patrolled the plant uniformly, while Crematogaster patrolled more intensively the lower portion of the crown of Triplaris. Baits retrieved by Pseudomyrmex were taken to their nests in the stem galleries of Triplaris; those retrieved by Crematogaster were carried to nearby nests in the forest understory. Greater aggressiveness and alertness to foreign objects (i.e. baits), better eyesight, larger size, and an individual foraging technique appear to be responsible for the greater foraging success of Pseudomyrmex when compared with Crematogaster.