Saturday, 7 March 2015

Goats with a Robinson Crusoe—Alexander Selkirk (1704-08)

What happened so far?

1. Joseph Townsend's Dissertation on the Poor Laws (1786) anticipated Malthus's Essay on the Principle of Population (1798) and also recounted a true story of an ecological interaction between goats and dogs on an island of Juan Fernández in the Pacific Ocean (see here).

2. Townsend's source for that dog-goat story was the account of the French Geodesic Mission by Jorge Juan and Antonio de Ulloa translated into English as A Voyage to South America (see here). Given that Townsend's story was based on real observations of real processes on a real island, we can take it as data and interpret it as a case of predator-prey selection without being Whiggish. That is, we do not need to interpret what Townsend or Ulloa thought about that dog-goat story or made of it. The case stands for itself and is, if anything, a case of directional selection exerted by a population of feral dogs on a population of feral goats.

3. Before the dogs were introduced to the island, the goats were easy to catch and served as provision for privateers and buccaneers. One privateer, who has been marooned on the island in 1704, was Alexander Selkirk. He lived there for four years and four months. Sometimes he caught more goats than he needed, and slit their ears before letting them free again. Thirty-two years later, George Anson arrived at the island with his ship Centurion. The goats were no longer easy to catch, because the dogs had already been introduced, but Anson's crew still caught many goats with slit ears (see here). That would be about twice the life expectancy usually given for goats, unless—of course—someone else has been on the island after Selkirk and continued his ear marking habit.

Now, sea captain and privateer Woodes Rogers (1712. A Cruising Voyage Round the World) tells us how they rescued Alexander Selkirk. They began their voyage with two ships, the Duke and Dutchess of Bristol, in 1708 and finished in 1711. They let a boat ashore the island that is now called Robinson Crusoe Island (formerly Más a Tierra). After a while they got concern that their men may have been captured by Spaniards and signalled them to return: The narrative continues:
"Immediately our Pinnace return'd from the shore, and brought abundance of Craw-fish, with a Man cloth'd in Goat-Skins, who look'd wilder than the first Owners of them. He had been on the Island four Years and four Months, being left there by Capt. Stradling in the Cinque-Ports; his name was Alexander Selkirk a Scotch Man, who had been MAster of Cinque-Ports, a Ship that came here last with Capt. Dampier, who told me that this was the best Man in her; so I immediately agreed with him to be a Mate on board our Ship." (Rogers 1712, p. 124-125)
"[...] he kept an Account of 500 [goats] that he kill'd while there, and caught as many more, which he mark'd on the Ear and let go. When his Powder fail'd, he took them by speed of foot;" (Rogers 1712, p. 126-127)
Later sailors had great difficulty in catching any goats (see links to earlier posts given above), and this was not only due to the agility of Selkirk compared with theirs, but also to selective pressure exerted by the released dogs on the goats.

P.S.: Here's the title page from Daniel Defoe's first edition of the novel about Robinson Crusoe. The fact that Crusoe is clothed in fur, which would have been unnecessary on the tropic island in the novel, has sometimes been taken to indicate that the widely publicised story of Selkirk was at least one source of inspiration for Defoe.
Public Domain, copyright expired, also in the US