When I was first designing projects for my dissertation, a result from one of my advisor’s papers caught my attention – brown anole males in better body condition (relatively more massive for their body size) sired more offspring and more sons. We didn’t have an explanation for how or why this trend existed but as a wannabe sperm biologist, I was immediately suspicious that it had something to do with sperm quality. I had some preliminary data showing that brown anole males varied in their sperm morphology and sperm count, but I wanted to know if some of this intraspecific variation was due to condition dependence and if there were fitness consequences associated with this variation.
In our recent experiment, we tested whether body condition was correlated with sperm quantity and quality, and whether the variation in sperm traits resulted in differences in a male’s competitive ability. To do this, we placed two groups of males on high-intake and low-intake diet treatments, where males were fed either five crickets three times a week or one cricket three times a week to experimentally alter their body condition. They were fed this diet until the two groups diverged in condition, and then kept on the diet treatments long enough for them to develop a fresh batch of sperm while in this altered body condition. We collected a sperm sample and measured sperm count and the morphology of 25 cells for each male. We focused on measuring the three largest regions of the cell, the head, the midpiece and the tail (see image below). To test for differences in the ‘competitiveness’ of each group’s sperm, we designed reciprocal mating trials so that a pair of males (one male from each group) would compete for fertilization of a female’s brood. Each male pair was mated to two females, and the order in which the males mated with the female was reversed for the second female to account for mate order effects.
To complement this lab study, we collected sperm from a wild population of brown anoles to look for condition dependence of sperm morphology in the wild. We also reanalyzed paternity data from Cox et al. 2011 to test for condition-dependent reproduction in a lab population of brown anoles. It should be noted that the lab population in this study (Cox et al. 2011) differed from our experimental population in a few ways. First, the males from that study did not have experimentally manipulated body condition. They were all fed the same diets, and the pairs of males that contained both a male in naturally high-condition and low-condition were included in this analysis. Secondly, though the mating design in that study was the same as our experimental reciprocal design, in Cox et al. 2011 males were allowed unlimited access to the females for an entire week, where in our experimental study males were limited to a single copulation.
We found that in both the lab and field, males in low body condition or on a low-intake diet treatment had significantly larger and more variable sperm midpieces than males in high body condition. We also found that males on the low-intake diet treatment had significantly lower sperm counts. When we analyzed the paternity data to test for correlations between fertilization success and sperm traits, we found significant negative correlations between sperm head and midpiece length, sperm count and fertilization success (though it should be noted that we only found these correlations for the average proportion of paternity and not when males were analyzed by either the proportion of paternity from their first or their second mating). We tested for condition-dependent fertilization success in our experimentally manipulated population and reanalyzed the data from males who varied naturally in body condition from Cox et al. 2011. We found a significant difference in fertilization success in males who varied naturally in body condition and had unlimited access to females, but found no difference in fertilization success in males who were in the experimental diet treatment groups (though the trend was similar in our experiment). Together, these data suggest that condition-dependent fertilization success is partially mediated by sperm quantity and morphology, and may also be influenced by a male’s ability to mate multiply with the same female.
This is the first paper that is part of my dissertation on the evolution of sperm morphology. I’m using anoles and phrynosomatid lizards to assess the sources and consequences of inter- and intraspecific variation in sperm morphology. Hopefully I’ll have more to share about anole sperm biology soon!
Kahrl, A.F., and R.M. Cox. 2015. Diet affects ejaculate traits in a lizard with condition-dependent fertilization success. Behavioral Ecology (advance access).