Appendix 1

The Phylogeny of Mammalian Testicles

External testicles are owned only by the boreoeutherian land mammals. This large group of mammals includes humans. These testes produce sperm best at temperatures that are lower than core body temperature. For this reason, they are located outside the body in the scrotum. However, there are exceptions. For instance, rhinoceroses, tapirs, bats, and marine mammals such as seals, dolphins, and manatees are all boreoeutherian mammals, but their testes do not hang outside the body.

Non-boreoeutherian mammals don’t have external testicles, either. These include monotremes (echidnas and platypuses), armadillos, sloths, and elephants.

Here’s a chart.

Figure A1.1 Phylogeny of mammalian testicles.

The evolution of the scrotum and testicular descent in mammals: a phylogenetic view.

Source: Werdelin, L, Nilsonne, A. Journal of Theoretical Biology 1999 Jan 7;196(1): 61–72. Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden. werdelin@nrm.se

Abstract

The adaptive significance of the scrotum and the evolution of the descent of the testicles and epididymis have been a focus of interest among biologists for a long time. In this paper we use three anatomical character states of the scrotum and descensus: (1) testicles descended and scrotal; (2) testicles descended but ascrotal; (3) testicles not descended (testicondy). These states are then mapped on an up-to-date phylogeny of the Mammalia. Three main points arise out of this mapping procedure: (1) the presence of a scrotum is either primitive in extant Mammalia or primitive within eutherian mammals except Insectivora; (2) evolution has generally proceeded from a scrotal condition to progressively more ascrotal; (3) loss of testicular descensus is less common in mammalian evolution than is loss of the scrotum. In the light of these findings we discuss some current hypotheses regarding the origin and evolution of the scrotum. We find that these are all incomplete in so far as it is not the presence of the scrotum in various mammal groups that requires explaining. Instead, it is the reverse process, why the scrotum has been lost in so many groups, that should be explained. We suggest that the scrotum may have evolved before the origin of mammals, in concert with the evolution of endothermy in the mammalian lineage, and that the scrotum has been lost in many groups because descensus in many respects is a costly process that will be lost in mammal lineages as soon as an alternative solution to the problem of the temperature sensitivity of spermatogenesis is available.

Copyright 1999 Academic Press.

PMID: 9892556 [PubMed—indexed for MEDLINE]