Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 240

Although gender in amphibians is genetically determined, temperature and exogenously
applied sex hormones (or hormone mimics) can influence sexual phenotype. Temperature-
induced sex determination does not appear to have any ecological relevance (Hayes 1998)
in the same way as it does in some reptiles, because the low temperatures required to
promote female phenotype is outside the range of temperatures normally encountered
by amphibians during development. Despite this assertion, low temperatures were pro-
posed as the reason for the occurrence of TOs among X. laevis that otherwise had not
been exposed to a hormone or hormone mimic. In a mesocosm study, Jooste et al. (2005)
described a high incidence of TOs among Nieuwkoop and Faber stage 66 X. laevis meta-
morphs after exposure to atrazine (1.0 to 25 μg L -1 ) as well as control animals during larval
development. However, this was not consistent with previous laboratory studies that nota-
bly failed to observe the same (see above), and may have been as a consequence of the low
temperatures (10°C) experienced during the early stages of the study (Hayes 2005; Hayes
et al. 2006). Another explanation for the occurrence of TOs among X. laevis in some studies
(Jooste et al. 2005), but not in others (e.g., Kloas et al. 2009a), is that the test animals were
derived from phylogenitically distinct populations (Du Preez et al. 2009), and that TOs
were a normal phenomenon in some populations.
The effects of hormones and hormone mimics are, of course, the subject of this chapter.
The potential for an environmental contaminant to act as an estrogen mimic is measured
by the extent to which the effects simulate the effects of exogenously applied estrogen.
Generally speaking, exposure to estradiol (usually between 1 and 100 nM) is used as a
positive control, and will result in significantly higher proportions of females and ambigu-
ous males that display TOs or other gonadal abnormalities (Coady et al. 2005; Oka et al.
2008; Kloas et al. 2009a; Langlois et al. 2010a; Storrs-Méndez and Semlitsch 2010).
The main obstacle to the interpretation of gonad morphology as a biomarker of endocrine
disruption in amphibians is the natural occurrence of hermaphroditism during develop-
ment. Developmental hermaphroditism appears to be a common phenomenon among
many amphibians. Storrs-Méndez and Semlitsch (2010) demonstrated that TOs occurred
naturally during gonad development, and frequently persisted beyond metamorphosis
in the testes of several species of native American frogs, including American toads ( Bufo
americanus ), southern leopard frogs ( Rana sphenocephala ), and gray tree frogs ( Rana versi-
color ). Storrs-Méndez and Semlitsch (2010) also demonstrated that among southern leopard
frog larvae ( R. sphenocephala ), ambiguity during development, including a skewed preva-
lence of female phenotypes and the occurrence of TOs and OVT, was normal while they
transitioned through various stages of gonadal development, with no such morphologies
evident in juvenile frogs 120 days after metamorphosis. It is notable, therefore, that the
leopard frogs collected by Hayes et al. (2002b, 2003) from various locations in the USA,
were “small individuals in an attempt to sample newly metamorphosed animals,” and
raises the possibility that field sampling of young animals may promote the observation
of TOs and OVT in field-collected animals. As an aside, this may also be the explanation
for the high prevalence of gonadal TOs observed among newly metamorphosed leopard
frogs that had been exposed during larval development to glyphosate based herbicides
containing polyoxyethylene tallowamine (Howe et al. 2004).
Persistence of hermaphroditic morphologies into adulthood can be expected (Storrs-
Méndez and Semlitsch 2010), and this is supported by both contemporary and histori-
cal literature indicating the occurrence of TOs in a small to medium proportion (0-16%)
of adult, field-collected frogs and represented by numerous species (Witschi 1921, 1929;
Reeder et al. 1998, 2005; Smith et al. 2005; Murphy et al. 2006b; McDaniel et al. 2008; Hyne
et al. 2009; Skelly et al. 2010; Spolyarich et al. 2011). Developmental hermaphroditism helps