Dignas, Beate, and Kai Trampedach, eds. 2008. Practitioners of the Divine: Greek Priests and Religious Figures from Homer to Heliodorus. Hellenic Studies Series 30. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_DignasB_and_TrampedachK_eds.Practitioners_of_the_Divine.2008.
9. The Iamidae: A Mantic Family and Its Public Image
One might be tempted to rationalize this explanation by conjecturing that the Crotoniates had offered Callias more money or that he calculated that Sybaris would lose the war. But Herodotus himself takes this explanation at face value, and he apparently expects his readers to do the same. Within the Greek divinatory system of knowledge and belief, in which system Callias was a specialist, unfavorable omens from sacrificial divination were a necessary and sufficient explanation for human action. We see this time and again in the works of Xenophon, particularly in the Anabasis.  The imperfect tense in the phrase “were not turning out favorable” indicates that Callias tried repeatedly to obtain good omens, but that the gods were unwilling to grant them. He thus represented himself as having fully discharged his duties to his original employer; the will of the gods was clear, and there was no need to perish in a doomed cause.
This may not be elegant poetry (and Clytiadae seems to be spelled Clytidae in order to fit the meter), but Eperastus has managed in a single couplet to attribute to himself the qualities of being “prophetic-tongued” and “god-like,” as well as belonging to two illustrious families of seers (since Clytius, the progenitor of the Clytiadae, was in turn a descendant of Melampus). If Eperastus was intending to attract clients (as I presume that he was), then this statue with its inscription would have served as a very effective and conspicuous advertisement of both his martial abilities and his prophetic credentials.
As the text of Herodotus now stands, Tisamenus is called “a Clytiad of the family of the Iamidae.” Most scholars have claimed this to be impossible and have accordingly labeled “Clytiad” as a scholiast’s gloss,  but I see no reason to rule out the possibility of intermarriage between the leading mantic families: that is to say, one of his parents was an Iamid and the other a Clytiad. There was an unimpeachable mythic paradigm for prophetic power passing from mother to son, since Teiresias had a daughter named Manto, who was the mother of the seer Mopsus. And it is easy enough to imagine why someone would have wanted to represent himself as belonging to both of these families. The Clytiadae, as mentioned above, claimed to be descendants of Melampus, the most famous of all legendary seers, and a blood connection with Melampus was an obvious vehicle for mantic self-advertisement and self-promotion.
The association of mantic and warlike abilities was fairly common, and the phrase “good both as a seer and to fight with the spear” had a long history both before and after Pindar.  Perhaps the story of Tisamenus’ near victory in the pentathlon was sufficient to suggest his martial valor, as well as his personal wealth and aristocratic status. Herodotus, moreover, does not as a matter of course tell us all that he knows, and it would not be surprising if a more elaborate version of Tisamenus’ exploits at Plataea circulated orally.