The seasonal sexual cycle of the timber rattlesnake (Crotalus horridus) was studied in the northern part of its range (northeastern New York) to determine when males prepare morphologically for seasonal mating, and to estimate the age of maturity and cost of reproduction in males. Testis and kidney tissues from accidental mortalities obtained over a 14-year period were examined histologically and seminiferous tubular diameters, spermatogenic stage, and sexual segment of the kidney (SSK) diameters were quantified. Males reached sexual maturity in the fourth, fifth, or sixth calendar year of life; mean age of maturity was 5.3 yr (range 4-7 yr, N = 12). Maximum spermatogenesis and seminiferous tubular diameter were reached in July and continued through September. The SSK was hypertrophied throughout the active season. Thus, although mating potentially could begin early in the season, a lack of female reproductive pheromones may account for an absence of spring mating in this population. Crotalus horridus did not begin its annual shedding cycle until June, and females may not become vitellogenic and sexually attractive until mid-July when heterosexual pairs indicated the mating season began. Rattlesnakes were most frequently encountered by humans during the mating season. Sightings and road-killed males significantly outnumbered females (male/female encounter ratio 3.9:1.0) and males had a much higher human-caused mortality than females (male/female mortality ratio 13:1). Among males, mortality was significantly greater during the mating season than during the non-mating season (3.25:1.0). A mating system in pit vipers described previously as ''prolonged mate-searching polygyny'' is associated with a high cost of reproduction in males caused by their increased movements and exposure to predators during the late-summer mating season.
