Can where an organism lives on the planet influence something as seemingly unrelated as how much steroid hormone they have circulating in their bodies? It turns out, yes, it can! The reason for this is that in the tropics there is a longer amount of time to potentially breed, and further away from the Tropics (toward the poles) the time available to breed (the breeding season) gets shorter and shorter. During the breeding season, males of many vertebrate species compete intensely over access to females that are ready to reproduce. The steroid hormone testosterone has an important role in ramping up males for reproduction and is typically elevated during the breeding season. Corticosterone, another steroid hormone, typically associated with “stress” and the response to stress, is also moderately elevated during this period, probably to facilitate the uptake, storage, and mobilization of energy needed for breeding. In places where the breeding season is short, for example at high latitudes, competition over females may be more intense than at places where breeding is extended, such as in the Tropics. In a recent study, my colleagues and I sought to test whether species having a shorter breeding season, and presumably more intense competition for mates during this short window of time, would have higher steroid hormones levels compared to species living in areas closer to the Tropics where there is a larger window of time for breeding. To do this, we examined variation in circulating levels of testosterone and corticosterone, as well as latitude and length of the breeding season in two major vertebrate classes: amphibians and reptiles.
The southeast Asian River Toad (left) is a tropical, low-latitude amphibian that has circulating testosterone levels 300 times lower than the high-latitude American Toad (right). American Toads can be found in Minnesota, and they have a very short breeding season, so male competition is intense, requiring lots of testosterone!
For both groups we found that male testosterone levels are higher when breeding seasons are shorter. This makes sense, because the short window of time to breed leads to intense competition among males, competition that is governed by testosterone production. In amphibians, male corticosterone levels are also higher when breeding seasons are shorter, but no such relationship was observed in reptiles. These patterns most likely reflect more intense reproductive competition experienced by males when the time to reproduce is short. In both groups testosterone and corticosterone levels are positively correlated, which suggests an energetic demand for testosterone-regulated behavior that is met with increased baseline corticosterone concentrations. Similar studies have shown some of these trends also exist across bird species. Our study shows that examining large-scale patterns of hormone levels in free-living vertebrates can offer important insights into how even very large-scale factors such as geography may influence physiology and life history traits beyond local factors, such as density, that are traditionally studied.